Despite only holding a two-seat majority, their opposition is arguably in a tougher spot. Republicans are defending eight seats in 2018, while Democrats are defending 24 plus the two independents who caucus with them.
But don’t forget the likely Democratic difficulties in holding states including West Virginia and Indiana, where Vice President Mike Pence is promising to be extensively involved.
Here are the Senate races to watch.
Tennessee's Sen. Bob Corker is retiring and the race to replace him is on
BY CHRISTOPHER DONATO
Currently representing Tennessee’s 7th Congressional District
Age: 65 (June 6, 1952)
Former Representative for Tennessee’s 8th Congressional District (2011 to 2017)
Age: 44 (Feb. 7, 1973)
Truck driver for Big G express. Right now his campaign is a one-man-show; if you call him, he’ll likely be driving his truck.
Age: 42 (June 23, 1975)
This is his first time running as a Republican; In 2010 he ran for the Tennessee House of Representatives, rand for US Senate in 2012 and 2014, and in 2016 ran to represent Tennessee’s 9th district in the US House as a Democrat.
Age: 62 (Sept. 28, 1955)
Former mayor of Nashville (1991 to 1999), former governor of Tennessee (2003 to 2011), swept every county in the state in his 2006 re-election bid
Age: 74 (Nov. 21, 1943)
When Sen. Bob Corker, R-Tenn., announced he would not seek re-election seven candidates -- six Republicans and one Democrat -- jumped in the race and to replace him.
Corker’s retirement announcement has left the GOP defending a deep-red seat, which hasn’t been held by a Democrat since 1995. There’s a crowded field of Republicans vying for the Senate seat: Rep. Marsha Blackburn, Larry Crim, former Rep. Stephen Fincher, Steven Hughes, Aaron Pettigrew and Dr. Rolando Toyos,
Democrats got their top recruit in coaxing former Gov. Phil Bredesen off the sidelines.
The race to replace Corker is important for Republicans; they need to keep it red as protection against losing a majority in the Senate. Meanwhile, Democrats are looking to become the majority party in the Senate and are looking to ride a wave of momentum after deep-red state Alabama elected a Democrat to the Senate in a strongly contested special election in December 2017.
Primary: Aug. 2
Election: Nov. 6
A senator stepping down in a critical border state leads to a fiesty Republican primary
By MEGHAN KENEALLY
Age: 49 (Jan. 25, 1969)
A controversial figure who touted himself as America’s toughest sheriff
Age: 85 (June 14, 1932)
A current congresswoman from a district along the border, who was the first female Air Force fighter pilot.
Age: 51, (March 22, 1966)
He previously volunteered for his now-opponent Ward and also used to run a revenge porn website.
Age: 33 (Nov. 9. 1984)
Christian ‘C.J.’ Diegel
A certified financial planner and former Air Force special operations intelligence officer.
Age: 39 (Sept. 27, 1978)
A doctor who calls for changes in health care, English as the national language, and improvements in veteran’s care.
A current congresswoman who has a relatively moderate voting record and already made history by becoming the first bisexual member of Congress.
Age: 41 (July 12, 1976)
A Muslim-American attorney who is originally from Arkansas but who has lived in Arizona for 20 years.
Age: 46 (Nov. 11, 1971)
A lawyer who is driven by outrage over the lack of bipartisan work to solve problems.
Age: 49 (May 19, 1968)
An indigenous woman and mother of four, she’s venturing into politics for the first time.
Age: 42 (Sept. 8, 1975)
He ran an unsuccessful congressional race in Illinois before successful running for city council and is now trying again in his new home state.
A lively cast of characters and the center seat to the debate over immigration policy will keep the Arizona Senate race bustling through November.
Sen. Jeff Flake’s decision not to seek re-election opened up the seat -- and Republican field, allowing the warring factions of the party to vie for the spot. The Democrats, led by Rep. Kyrsten Sinema, are hoping that their opponents' infighting could lead them to help make the state a deeper shade of purple than it already is.
The Republican primary will not only be a competition of differing ideas on the border wall and immigration, but also show the competition between the candidates as they tout their respective ties to President Donald Trump. Kelli Ward, who unsuccessfully went up against Sen. John McCain in his 2016 re-election, had an early lead in the race that was helped by the backing of former White House chief strategist Steve Bannon and financial support from conservative fundraiser Rebekah Mercer.
But two key opponents could threaten her hold on her base.
Sheriff Joe Arpaio threw his name in the race in January, months after being pardoned by Trump, and ex-fighter pilot Rep. Martha McSally flew into the competition touting her tendency for tough talk. While these three candidates are vying for the more conservative voters in the state, there are four others who will be nipping at their heels until the Aug. 28 primary.
The primary is on Tuesday Aug. 28.
The general election is on Nov. 6.
Heitkamp plays defense in deep red North Dakota
BY MARIAM KHAN
Sen. Heidi Heitkamp
Democrat – incumbent
Currently representing North Dakota in the U.S. Senate
Age: 62 (Oct. 30, 1955)
Rep. Kevin Cramer
Representative from North Dakota’s At-Large Congressional District (2013 to present)
Age: 57 (Jan. 21, 1961)
The Senate race in North Dakota between incumbent Democrat Heidi Heitkamp and at-large Republican Rep. Kevin Cramer is all but a toss-up.
Heitkamp is among the most vulnerable senators up for re-election in November. President Trump won North Dakota in the 2016 presidential election by roughly 36 percentage points over Democratic nominee Hillary Clinton.
This puts Heitkamp in a uniquely challenging place. She’ll have to peel off Republicans and independent voters and keep her liberal, democratic base happy at the same time.
She’s also running in a state where President Trump remains fairly popular, and thus it works in her benefit to stay on friendly terms with the president.
She has voted in favor for almost all of Trump’s cabinet nominees. Last month she snagged a prime spot at the White House for a bill-singing event. And last year, she hitched a ride with the president on Air Force One to an event in North Dakota.
At one point, Trump told the crowd Heitkamp was a “good woman.”
But Heitkamp has her work cut out for her. She’s running against a highly popular candidate who has just as much name recognition as she does.”
Nove. 6, 2018: General election
Mississippi prepares for Senate special election
By NIA PHILLIPS
Chris McDanielRepublicanMississippi state Senator, lawyer, 2014 U.S. Senate GOP Candidate Age 46 (Feb. 28, 1971)
Cindy Hyde-SmithRepublicanMississippi Agricultural secretary, former Mississippi state Senator, beef cattle farmer Age 58 (May 10, 1959) **need to confirm this**
Mike Espy DemocratFormer U.S. representative, former U.S. Agricultural secretary, lawyer Age 64 (Nov. 30, 1953)
In Mississippi, Republicans are scrambling to maintain both of their U.S. Senate seats after Sen. Thad Cochran announced his retirement beginning April 1. Now, they look toward a November special election, strategizing on which of two non-traditional candidates they will rally behind to send to Washington.
Complicating the plan, the Magnolia State’s “jungle primary” format where candidates battle in an open election regardless of party affiliation. If one person is unable to win 50 percent of the vote, Mississippians will return to the polls once again three weeks later, deciding between the top-two contenders.
Republican Chris McDaniel, a lawyer and former conservative talk-show host with a large social media presence is working to fire up his conservative support base, after gaining wide recognition in a narrowly-lost primary fight against Cochran in 2014.
The Tea Party candidate who originally declared his run against incumbent Sen. Roger Wicker is instead focusing on the special election. McDaniel’s path to Capitol Hill, however, is now seemingly more difficult after an early endorsement by political outsider Steve Bannon. Despite President Trump giving his early support for Wicker, McDaniel continues to align himself with the commander-in-chief’s political values.
Possibly standing in McDaniel’s way is Gov. Phil Bryant’s interim Senate pick: Cindy Hyde-Smith.
The beef-cattle farmer who served as Mississippi’s commissioner of Agriculture and Commerce since 2011, launched her fall campaign the same day as Bryant’s announcement. Hyde-Smith could potentially make history if she were to become the first woman to represent the Mississippi in U.S. history.
But one potential wrinkle for the longtime lawmaker is her political past.
The former state Senator’s also a former Democrat, switching to the Republican party in 2010. And her past affiliation’s already fuel to the fire for McDaniel, immediately noting Hyde-Smith’s Democratic past just moments after the governor announced his pick.
But, the Republicans will also have to wait and see who the Democrats bring to the table in the special election as they hope to have a similar outcome as their neighbors in Alabama this fall.
Former Washington insider Mike Espy telling ABC News he has a “strong intention” to run for Cochran’s vacant seat. The attorney was elected as Mississippi's first African American Representative in U.S. Congress since the Reconstruction era. Espy additionally worked as the U.S. Agricultural Secretary during the Clinton administration.
SPECIAL ELECTION: Nov. 6, 2018
In Vice President Mike Pence's home state of Indiana, a showdown over a senate seat
BY ALISA WIERSEMA
Sen. Joe Donnelly
Age: 62 (Sept. 29, 1955)
Rep. Luke Messer
Age: 48 (Feb. 27, 1969)
Rep. Todd Rokita
Age: 47 (Feb. 9, 1970)
This year’s Indiana Senate race is sure to feature showdowns in the primary and general election cycles, putting Vice President Mike Pence’s home state in the spotlight for battles within his own party, as well as across the aisle.
The matchup is set in a state President Donald Trump won by nearly 20 points in 2016 -- meaning incumbent Sen. Joe Donnelly is looking at a tough race against a slew of Republicans eager to take his seat.
Indiana businessman Mike Braun launched into the field with the first ads of the election cycle. Candidates on the right also include House Representatives Todd Rokita and Luke Messer, who are duking it out in an intra-party war marked with personal jabs.
Such heated exchanges set the stage well ahead of this year’s midterm cycle, meaning whichever Republican candidate comes out on top will have already fought hard just to get the party’s nomination.
As for the incumbent, Donnelly will have to fight a bitter battle to defend his seat at home and on a national level. While Donnelly aligns with Republicans on issues like abortion, he has a target on his back for voting against tax reform that was painted on by the president himself during a tax event last fall in Indianapolis.
“If Sen. Donnelly doesn't approve it, because you know, he's on the other side, we will come here. We will campaign against him like you wouldn't believe,” Trump said at the time.
Filing deadline: Feb. 9, 2018
Primary: May 8, 2018
Swing state senate candidates running neck and neck
BY MATTHEW FUHRMAN
Republican - incumbent
Appointed to the Senate by Gov. Brian Sandoval in 2011, elected in 2012
Currently represents Nevada’s 3rd District in the House of Representatives
Nevada is one of the few states where Democrats have a chance to pick up a senate seat in 2018. In fact, it’s the only state with a Republican senator up for reelection in a state won by Hillary Clinton. Nevada is a quintessential swing state; it has sided with the winner of the presidency for all but two elections in the last 100 years, voting for Gerald Ford in 1976 when Jimmy Carter was elected, and going for Hillary Clinton in 2016 when Donald Trump was elected.
Incumbent Republican Dean Heller was appointed to the Senate by Governor Brian Sandoval in 2011, and ran to keep the job one year later. Heller squeaked out a win, beating challenger Shelley Berkley by less than 12,000 votes statewide. Will a Republican who won by such a small margin be able to hold on if a blue wave sweeps across the United States this November?
Expect health care to play a major role in the Nevada election. Sen. Heller opposed a repeal of the Affordable Care Act without a replacement, but decided to support repeal after President Trump suggested he was open to supporting another Republican in the Nevada Senate race. Immigration also will play a major role in the race with Nevada’s large immigrant population, as well as gun rights following the October 1st shooting in Las Vegas.
Election day: Nov. 6, 2018
18 For 18’ is ABC News’ powerhouse political coverage of the 2018 midterm elections. To stay up to date, visit ABCNews.com and the ABC News app, and follow our midterm elections alerts.