"In a certain way I am on the ballot", Trump told supporters during a tele-town hall organized by his re-election campaign.
Incumbent Democratic Senator Joe Manchin won a hotly contested race in conservative West Virginia, and Marsha Blackburn held a Senate seat for Republicans.
Democratic candidates had won or were leading in a net dozen or so House of Representative seats held by the Republicans by about 8:30 p.m., apparently on target for gaining the 23 extra districts needed to seize a majority in the House. Ballot counting could drag in tight races, leaving some races undecided long after Election Day.
Even so, Democrats were highly confident, with Nancy Pelosi, the party's top leader in the House, saying "it's just a question of the size of the victory".
Trump spent the evening watching returns in the White House with family and friends. Democratic Senate incumbents are up for re-election, for example, in North Dakota, Indiana, and Missouri - states Trump carried by nearly 25 percentage points on average two years ago. The results match his approval in recent public opinion polls, which have shown him to be under water nationally for months.
Voters go to a polling place to cast their ballot during the midterm elections on November 6, 2018 in Phoenix, Arizona. This time, they're sitting up.
The Republicans control both houses of Congress - the midterms provide the Democrats with a chance of clawing back some political control.
And on the streets there was a palpable buzz all day.
REUTERS/Michelle McLoughlin Voters line up to vote as polls opened in the U.S. midterm congressional and gubernatorial elections in Deerfield Beach, Florida, U.S. November 6, 2018.
Vecinos compran donas todos los días para apoyar a vendedor
John Chhan y su esposa han hecho donas desde hace 30 años en Seal Beach , California . Esto la motivó a iniciar una solidaria campaña con todos los vecinos.
At a polling station in Arlington, Virginia, head election officer William Harkins said: "it's a very good turnout".
Trump himself noted the energy as he wrapped up a punishing schedule of rallies around the country that were meant to boost Republican candidates - and his own brand heading towards reelection in 2020.
"If the radical Democrats take power they will take a wrecking ball to our economy and our future", Trump declared in Cleveland, using the same heated rhetoric that has defined much of his presidency. "Now it's like the hottest thing".
Tuesday's result was a bitter outcome for Trump, a 72-year-old former reality TV star and businessman-turned-politician, after a campaign that became a referendum on his leadership.
While Trump is not on the ballot, the most polarizing USA president for decades has put himself at the center of every issue. More than 39 million Americans have already voted, either by mail or in person, breaking early voting records across 37 states, according to an AP analysis.
GOP Whip Steve Scalise said the president's rallies were building momentum and with the economy a selling point, he predicted his party would retain a slim majority. Individual races "will be close", she said, but because of the "quality of our candidates" and emphasis on preserving health care, "I feel confident we will win".
Mr Manchin was the only member of his party to vote to confirm Supreme Court Justice Brett Kavanaugh, whose nomination narrowly survived sexual assault claims.
At Trump's pre-election rallies, large crowds have consistently cheered his rhetoric about strengthening the southern border, chanting "build the wall!" Democratic favorite Conor Lamb stunned Washington by winning a special election in the state and faced Republican Rep. Keith Rothfus in a new district that was among four that could flip from red to blue.
"This is our source of strength", O'Rourke said as he voted in El Paso.
If Republicans maintain control, it will put the GOP in position to continue driving forward the President's agenda in at least one chamber of Congress.