In Oklahoma, a state Trump owned by 36 points in 2016, Democrat Kendra Horn flipped Oklahoma's 5th district seat, which has been represented by a Republican for 44 years. The president won the district by double digits and GOP Rep. Steve Russell won his first election there in 2014 by 23 points.
Horn edged Russell out of the seat by just two points, according to ABC projections, based on exit polling. Russell's campaign raised less than $1 million and got almost no money from outside groups. Comparatively, Horn was able to attract some national attention and pulled in about half a million dollars from billionaire Democratic donor Michael Bloomberg's Super PAC, Independence USA.
Ahead of the election, ABC News partner FiveThirtyEight forecasted Horn had a one in seven chance of winning.
The 5th district is now the lone congressional seat in the state represented by a Democrat. Horn told ABC affiliate KOCO-TV she planned to focus on health care and education, which was where she staked most of her campaign.
"We talked about health care, we talked about education and all of these people are the ones who helped make it happen," Horn said. "It's a historic election moment for a number of reasons. I am the third woman ever elected to represent Oklahoma in Congress, and we are talking about issues that are so important."
In Pennsylvania, two incumbents -- one a three-term Republican and the other a Democrat who won a special election earlier this year -- battled for the recently redistricted 17th congressional district. Rep. Conor Lamb successfully unseated three-term Republican Rep. Keith Rothfus in the race.
The race grew more complex after the state’s Supreme Court found the old congressional map violated the state's constitution through a partisan gerrymander. As a result, experts predicted that over the long term, Democrats could expect to pick up one or two seats with the state Supreme Court's newly redrawn map.
Lamb's success in the district may also have been a result of his more conservative views. He has sided with Trump -- who won the district by nearly 20 points in 2016 -- on issues involving steel and aluminum tariffs, and personally opposes abortion, but backs the Supreme Court's decision to legalize it.
The Marine Corps veteran and former federal prosecutor defeated Republican Rick Saccone in a March 2018 special election in Pennsylvania's 18th Congressional District. The seat was left open after GOP Rep. Tim Murphy resigned after a report that he had asked an extramarital lover to end her pregnancy. The area had not been represented by a Democrat since 2002, when Murphy won the seat with more than a 20-point lead.
Republicans tried during the special election to tie Lamb to House Minority Leader Nancy Pelosi, but he came out strong against those claims, saying in an ad that he wouldn't support her as the House speaker.
In South Carolina’s 1st Congressional District, Democrat Joe Cunningham defeated Republican Katie Arrington, in a district Trump won by 13 points in 2016. Cunningham’s win is the first time Democrats will represent the district in 40 years.
The district was represented by Republican Rep. Mark Sanford, who lost his primary to Arrington in June. His loss may have been tied to his willingness to criticize the president. Ahead of the primary, Trump tweeted that Sanford was "nothing but trouble" and then he endorsed Arrington. After Sanford lost, Trump wrote on Twitter that he'd "never been a fan of his."
Arrington is one of 21 candidates who Trump endorsed that did not win their races Tuesday. There were still eight races undecided Wednesday.
In Virginia’s 2nd Congressional District, Navy veteran Elaine Luria managed to unseat GOP Rep. Scott Taylor in the Republican-leaning district.
The district has been reliably red since 2000, with the exception of Democrat Glenn Nye's single term from 2009-2011. It is also worth noting that Republicans in that period have not won in the Virginia Beach-area district by large margins. Former Republican Rep. Scott Rigell won reelection in 2014 with the largest margin, defeating his Democratic challenger by 17 percentage points.
Complicating this year's race was a lawsuit brought by state Democrats to remove Independent candidate Shaun Brown from the ballot when her qualifying petition appeared to have forged signatures. According to news reports, many of the signatures were gathered by staffers working for Taylor. While there were names listed of people who supported Taylor, the news reports indicated that those people didn't actually sign the petition. Brown told The Virginian-Pilot she didn't know Taylor staffers were collecting signatures for her. Taylor acknowledged to the newspaper that he knew his campaign workers were collecting signatures for Brown, but said it was because they thought Brown had been treated unfairly by Democrats.
A judge later ordered Brown's name removed from the ballot, citing the forged signatures. Taylor had fired his campaign manager before the petition signatures came into question, The Virginian-Pilot reported, and cut ties with his campaign consultant afterward.
With only the two Navy veterans on the ballot, Luria squeezed out Taylor with a two-point lead.
Luria was one of the first woman to serve her entire career on a Navy combat ship and campaigned on how her military experience could help her get things done in Washington.
“Having that experience of working together with people of all perspectives, all backgrounds, and accomplishing a mission is something that I think we as female veterans collectively feel we can take to Washington,” she said.
ABC News has not yet been able to project the outcomes in two House districts, Maine’s 2nd and New Jersey’s 3rd, which were among 21 that voted for former President Barack Obama and then for Trump. As of Thursday afternoon, ABC News could project that Democrats currently have a net gain of 31 seats in the House, with 10 races still outstanding. For updates, go to: https://abcnews.go.com/Politics
ABC News' Johnny Verhovek and MaryAlice Parks contributed to this report.