The Trump administration now holds power in all branches of government - the presidency, the Congress and the Supreme Court - and if Democrats were to take back the House, they could put a pin in much of Trump's agenda.
As for the Senate race, despite Trump's deeply polarizing effect on voters, the GOP harnessed his rock-solid popularity with hard-right voters in rural, deep-red states.
They've also already reportedly threatened to subpoena Mr Trump's tax returns.
Midterm elections are typically hard for the party in power, and GOP incumbents were on the defensive in many races across the country.
Democrats also started to cut into the GOP's massive advantage at the gubernatorial level, flipping seats in Illinois, Kansas, Michigan and New Mexico.
A debate about whether Trump's rhetoric encouraged extremists erupted in the campaign's final weeks after pipe bombs were mailed to his top political rivals allegedly by a Trump supporter who was arrested and charged, and 11 people were killed in a shooting at a Pittsburgh synagogue.
The most recent record in voter turnout during congressional midterm elections, when the White House was not on the line, came in 1982, when 49 percent of the voting-age population showed up to vote, according to the US Census Bureau.
Americans voted for all 435 seats in the House, and female candidates stole the spotlight in an election cycle that had been billed as the Year of the Woman. In many cases they're running against career politicians.
The S&P 500 has risen 28 per cent since Trump's election in November 2016 to the eve of congressional midterm elections on Tuesday.
Polls are also closed in parts of in, where Democrat Joe Donnelly, a top target of the President's frenetic final midterm election campaign swing, is fighting an uphill battle for re-election against Republican Mike Braun. Basically, what happened was the same slight error - the marginal underrepresentation of white working class voters favouring Trump - was replicated in a number of key districts located in states with large numbers of electoral college votes.
Numerous most competitive races for the House of Representatives will be fought in the suburbs, where anecdotally we have been hearing that white married women could be moving away from the GOP.
"On the other hand, several factors favor Republicans". Democrats would need a swing of at least 23 votes to gain control of the House. At least three other red districts flipped to blue.
The midterms also featured some potentially historic candidates, including Stacey Abrams, who would be the first black woman to be governor of Georgia if she won.
He was due to be joined by Republican megadonor Sheldon Adelson, Blackstone CEO Stephen Schwarzman and the President's ex-campaign aides, David Bossie and Corey Lewandowski, a source familiar with the guest list said.
Outside Richmond, Virginia, one-time tea party favorite Rep. Dave Brat lost to Democrat Abigail Spanberger, a former Central Intelligence Agency operative motivated to run for office after the GOP vote to gut the Affordable Care Act. And I think that some of the more radical leftist members of the House on the Democratic side are going to become committee chairs. Finally, Democrats have performed well in off-year elections ("special elections"). In Texas, House Homeland Security Chairman Michael McCaul's race and retiring Republican Joe Barton's open seat were previously considered "Solid Republican", but now are rated "Likely Republican". Rep. Marsha Blackburn was able to hold Tennessee's seat for Republicans, as current Sen.
Early results began to trickle in from states such as in, site of a vital US Senate race, and Kentucky, home of a pivotal US House of Representatives race, but it could be hours before the winners are determined. "It's just a very robust class of candidates that really reflects who we are as a country".