"Things are pretty good, yet we have all this division and we have this president who's relatively unpopular, so we have this odd juxtaposition", said Kent State politics professor Michael Ensley, citing Trump's poor approval ratings despite a rollicking USA economy and the absence of any major foreign-policy challenges.
Some Democrats have already vowed to force the release of his tax returns.
Democratic candidates for the House flipped seats in several suburban districts outside of Washington, Philadelphia, Miami, Chicago and Denver that were considered prime targets for turnover because they were won by Hillary Clinton in 2016. "Two can play that game!"
Facebook also blocked about 115 user accounts after USA authorities tipped it off to suspicious behaviour that may be linked to a foreign entity, the company said hours voting began.
Tuesday's midterms were among the most bitter in years, with Trump stoking conservatives' fears of caravans of immigrants traversing Mexico and inaccurately painting Democrats as broadly striving to bring socialism to the U.S. He acknowledged the contest would be a referendum on his presidency.
Still, he celebrated GOP success in expanding the party's majority in the Senate and seemed to blame losing candidates for distancing themselves from him and his unorthodox methods.
"I believe he values immigration, but he wants to make sure we're safe", said Tina Newby of Wetland, Michigan, a GOP voter.
"Yesterday was such a very Big Win, and all under the pressure of a Nasty and Hostile Media!" he added.
Trump had 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.
"Tomorrow will be a new day in America", declared House Minority Leader Nancy Pelosi, who would be in line to become the next House speaker. "She has earned this great honor!"
Health care and immigration were high on voters' minds as they cast ballots, according to a wide-ranging survey of the American electorate conducted by AP. Now, Democrats are in control once again.
White House aides called on Democrats to work with Republicans in the next Congress. More women than ever were running, along with veterans and minorities, many of them motivated by Trump's rise.
After knocking off a couple of Democratic incumbents so far, Republicans could not only defend but potentially increase their 51-seat majority as additional races are called throughout the night. An open seat that included Atlantic City was also ripe for Democratic pickup by state lawmaker Jeff Van Drew after the GOP campaign committee abandoned Republican Seth Grossman over racially charged comments.
It could have been a much bigger night for Democrats, who suffered stinging losses in OH and in Florida, where Trump-backed Republican Ron DeSantis ended Democrat Andrew Gillum's bid to become the state's first African-American governor.
The Democrats' narrow path to the Senate was slammed shut after setbacks in Indiana, Tennessee, North Dakota and Texas.
The White House for days has stressed the historical headwinds it faced: In the last three decades, 2002 was the only midterm election when the party holding the White House gained Senate seats. A record number of women were running for Senate, House, governorships and state legislative seats.
The significance of the Republican victory in the Senate, which the party has dominated for the past four years, was magnified because Democrats wrested House control from the GOP.
He and the first lady were to host an evening watch party for family and friends. Among those expected were Trump's adult children, White House aides, Republican officials and presidential friends.
The Republican Party maintained control of the Senate.
The results were more mixed deeper into Trump country. But the president received negative marks from voters on temperament and trustworthiness.
Almost two-thirds said Trump was a reason for their vote.
Trump's scorched-earth campaigning came to define the 2018 campaign. In the final days, he sought to motivate supporters with the battle over the confirmation of Supreme Court Justice Brett Kavanaugh.
"Donald Trump went out and worked his tail off", Sen.
Despite Trump's deeply polarizing effect on voters, the GOP harnessed his rock-solid popularity with hard-right voters in rural, deep-red states.