Against the backdrop of a frenetic Democratic presidential primary contest, control over the U.S. Senate will once again be in play in 2020, with Republicans defending over 20 seats to maintain their majority and Democrats eyeing several pickup opportunities to expand their political map.
After voters delivered a split judgment on the Trump presidency in 2018 -- rebuking his agenda by letting Democrats regain control of the House, yet keeping the Senate within Republican hands -- both parties are navigating on different terrain in 2020.
Despite holding onto the majority, buttressed by an advantageous map, Republicans still lost six seats in states Trump won in 2016 -- Arizona, Michigan, Montana, Ohio, Pennsylvania and West Virginia -- and they also failed to win in Nevada.
In 2020, the battlefield won’t be a safe harbor for the GOP, since it includes a series of competitive races, and some that are still taking shape, that will put the GOP on defense as Democrats prepare for an aggressive offensive strategy.
"Even though it's early, we're well-positioned given the impressive candidates who are breaking fundraising records, outpacing Republicans, holding their opponents accountable, and building strong grassroots support for their campaigns," said Democratic Senatorial Campaign Committee spokesperson Stewart Boss.
With Republicans holding the majority by a 53-47 margin, Democrats need a net gain of four seats to flip the chamber (but if a Democrat wins the presidential race, they will only need a net gain of three).
Of the 12 seats up for Democrats, only Alabama’s Doug Jones and Michigan’s Gary Peters are competing in states Trump won in 2016, and two Republican incumbents, Colorado’s Cory Gardner and Maine’s Susan Collins, are facing tough roads ahead in Clinton-won states.
The true toss-ups
Many of the GOP incumbents up for re-election are in deep red states; therefore, Democrats looking to make inroads this cycle are competing on a contracted battlefield that includes: Arizona, Alabama, Colorado and Maine, which are all currently rated as toss ups by Cook Political Report.
In Alabama, the endangered Jones will need to replicate his 2017 victory in a special election -- when he squared off against Judge Roy Moore, the conservative firebrand who is running again in 2020 in a crowded primary of six candidates -- to win again in a deep red state that Trump carried by nearly 30 percentage points over Clinton in 2016.
Republicans are eager to unseat the the first-time senator in the ruby red state, with some appearing bullish about the race -- and one GOP strategist even arguing that there is "little hope for Doug Jones."
"There's not enough ticket splitters in Alabama to ever account for what the margin will be at the top," the strategist told ABC News. "Alabama is not a blue state. It's not a purple state. It’s not a light red state. It is a deep red state."
But Doug Jones has so far out-raised the entire GOP field, raising nearly $3.7 million and has $4.3 million in cash on hand.
In Arizona, the GOP is betting on first-time senator, Martha McSally, to hold the seat in one of the most highly-contested races of the cycle. McSally, who was appointed to the seat by Gov. Doug Ducey late last year after the death of Sen. John McCain, is running as an incumbent after she lost to Democrat Kyrsten Sinema in the 2018 Senate race.
The former congresswoman could potentially square off against Mark Kelly, a former NASA astronaut and husband of former Congresswoman Gabby Giffords, who is proving to be a prolific fundraiser -- raking in $8.4 million so far this year.
While Trump won Arizona in 2016, a Democratic official believes Trump is potentially in danger of losing the state, particularly after the GOP’s losses in 2018.
Colorado’s Gardner is the most vulnerable Republican, and will ultimately face a challenge from one of 12 Democrats currently competing for the nomination, including former-governor-turned presidential candidate-turned senate candidate, John Hickenlooper.
The Democratic official argued that the entrance of Hickenlooper, who secured the endorsement of the DSCC a day after he announced his Senate candidacy, is a "big win" in a state that is trending more blue over the last few elections, before adding that Gardner does not have "a personal brand in this state to outperform."
But in a state carried by Clinton, Gardner has already publicly broke with the president, arguing in January that Congress should only push for more money for border security after the government reopens amid the longest government shutdown stalemate .
In Maine, Collins is up against a tough reelection road in 2020 -- with both a formidable opponent in Maine House Speaker’s Sara Gideon and the weight of her vote to confirm Justice Brett Kavanaugh to the Supreme Court.
In her announcement video, Gideon highlighted the incumbent Republican's record as an ally of the president, including her votes for the GOP tax cuts and for Kavanaugh.
"When Collins announced that she would vote to confirm Brett Kavanaugh to the Supreme Court, money came flowing in for her 2020 Democratic opponent — who, at the time, did not exist. Only in the last few months have credible Democratic contenders started to pop up," writes Leah Askarinam, a political analyst at Inside Elections.
While Maine has voted for a Democrat in every presidential election since 1988, Collins has withstood those outsize challenges. But as she faces her first primary challenger since first being elected in 1996 in Derek Lavasseur, a pro-Trump Republican, Collins told Bloomberg she won’t make a decision on whether she’ll run until early fall.
Democrats hope to expand the map
Democrats’ strategy to win the majority also runs through Iowa, Georgia, Kentucky, North Carolina and Texas, which might not necessarily be thought of as competitive races this early (they are rated either as likely Republican or solid Republican by Cook Political Report), but could become crucial later in the game if battle for control comes down to the wire.
Incumbent Sen. Joni Ernst, in the crucial early caucus state of Iowa, is already seeing four challengers stack up on the Democratic side, including Theresa Greenfield, a former candidate for Iowa's 3rd Congressional District in 2018, who secured the endorsement of the DSCC.
Despite former President Obama winning the state in both 2008 and 2012, Trump reclaimed the Hawkeye state for Republicans, winning by 10 percentage points over Clinton in 2016.
While the Republican strategist asserts that Democrats have struggled to land "top recruits" this cycle, Democrats see Ernst’s support in the swing state as "fairly soft," claiming she is "somewhat out of step with where the state is."
But in Georgia, where Democratic top recruit and former gubernatorial nominee Stacey Abrams passed on the opportunity to take on GOP Sen. David Perdue, a field of three challengers started to form in her absence.
Among the Democrats seeking to oust the former business executive and Trump ally is Sarah Riggs Amico, the latest candidate to enter the primary less than a year after she lost the race for lieutenant governor, alongside Abrams.
The GOP strategist said Abrams’ decision not to run put Democrats "in a hole" because "the candidate pool they have their right now is lackluster at best and and problematic at worst."
But a Democratic source, who says Perdue is in a "weak position" and thinks there is a possibility that former congressional candidate Jon Ossoff could get into the race "in the near future," also suggested that the "floor is rising" in Georgia -- meaning the state is becoming more fundamentally competitive.
On Wednesday, the other Senate seat in the Peach State opened up for 2020, after Sen. Johnny Isakson announced he’s resigning on December 31 due to health concerns. According to Georgia state law, Republican Gov. Brian Kemp will appoint a replacement to the seat that will serve until the next statewide election, which will take place in November 2020.
But an Abrams spokesperson told ABC News she will not run for either Senate seat next year, at least for now.
The GOP on defense
One of the toughest battles for Democrats will be against the Senate’s top Republican in Kentucky, where two are pursuing the challenge: retired Marine fighter pilot Amy McGrath and health care professional Steven Cox.
McGrath, a high-profile rival for McConnell after narrowly losing to Congressman Andy Barr in 2018, brought in $2.5 million in the first 24 hours of launching her campaign, which her campaign claims is the highest amount raised in the first day by any Senate candidate.
Despite her fundraising haul, McGrath saw some early stumbles after she first told a reporter she would have voted to confirm Kavanaugh, only to concede days later that she made a mistake and said she would vote no.
And McConnell, who has been in the Senate since 1985, is a prolific fundraiser, with more than $5 million raised so far this year and nearly $7.9 million cash on hand, which doesn't include his many outside group allies.
In North Carolina, amid a tough re-election battle with one declared GOP primary challenger and a handful Democrats lining up to replace him, Tillis secured a much needed boost from Trump early in the cycle.
The president endorsed the incumbent in a tweet -- even as he faces an upstart opponent in businessman Garland Tucker, who has repeatedly criticized Tillis for flip-flopping.
"When the emergency powers issue came along, he wrote the famous Washington Post op-ed and then when he got a lot of pressure from conservatives back home, he flip-flopped on that issue, but I think on immigration, he’s been very, very weak," Tucker told Sean Hannity on his radio show in May.
During the interview, Hannity said Tillis "has not exactly been the most conservative for the people of North Carolina."
In the key battleground state, Tillis is also a top target for Democrats in 2020, who see him as "one of the weakest incumbents."
Among three Democrats competing in the primary to unseat him, Cal Cunningham, a former state senator and veteran, is the fundraising front-runner. But the president carried the state over Clinton, and is already slated to hold a second rally in the state so far this year in Fayetteville on Sept. 9, a day before the special re-do election in North Carolina’s 9th congressional district.
After former Texas Congressman Beto O’Rourke’s near-miss in his 2018 Senate bid against GOP Sen. Ted Cruz -- even raising an astonishing $80 million in his upstart campaign -- a crowded Democratic primary is vying for the seat held by GOP Sen. John Cornyn.
The seven-way primary features a diverse crowd in a year in which the Lone Star state is expected to be in play -- including MJ Hegar, a retired Air Force major and former candidate who lost in Texas’ 31st congressional district.
Cornyn is "sort of widely liked but not passionately liked, with relatively few people who dislike him," said Cal Jillson, a political science professor at Southern Methodist University, earlier this month.
The Democratic contenders already seeking to oust Cornyn in 2020 are picking up right where O’Rourke left off in 2018, fueled by the party’s nearly quarter-century-held hopes of turning Texas blue.
O’Rourke received more votes than any Democrat has in the history of Texas -- but the state has not elected a Democrat to statewide office since 1994.
The potential Bay State brawl
The dynamics of the fight for the Senate are still developing this early in the election, even within the parties themselves.
In Massachusetts, the prospect of a Kennedy primarying a long-standing Democratic senator is teeing up an intraparty clash between veteran Democrats and the next class of young leaders.
Congressman Joe Kennedy confirmed Monday he is mulling a Senate bid in 2020, even filing with the FEC to create a Senate campaign committee, against Sen. Ed Markey.
As competitive primary matchups continue to plague Democratic incumbents (in 2018, Ayanna Pressley secured an upset victory over longtime incumbent Michael Capuano in the Bay State’s 7th congressional district), Kennedy would be launching one of the highest profile primary challenges in 2020, and with a sizable war chest. By the end of June, Kennedy had $4.2 million cash on hand compared to Markey, who has $4 million.
But Markey has been preparing for a potential primary challenge for over a year, and beefing up his progressive credentials -- introducing the Green New Deal, along with freshman Congresswoman Alexandria Ocasio-Cortez. The DSCC also has a strict policy of supporting incumbents.
Kennedy told reporters in Massachusetts Tuesday that his time frame for a decision was "as long as it takes" to come to the "right" decision, before adding, "the sooner the better ... I don't intend to prolong that any longer than is necessary."
The landscape in 2020 will also feature one dominating factor on the ballot: Trump.
The president is already hitting the campaign trail, with some key senators by his side since the beginning of the year, including Cornyn and Tillis, to rally his base ahead of some of the most critical races to come in less than a year and a half.
But the effect of the president in 2020 remains to be seen.
ABC News' Mariam Khan, Soo Rin Kim, John Verhovek and Benjamin Siegel contributed to this report.