He said in a tweet Wednesday that if they do, the Republican-controlled Senate may investigate Democrats.
The US President who headlined an unprecedented 50 rallies - 30 in the last two months alone - and has campaigned for dozens of candidates at all levels of government, according to White House Press Secretary Sarah Sanders, watched the results come in with friends and family in the White House residence.
The Republican president also clashed repeatedly with reporters at the White House, ordering them to be quiet.
The verdicts in the House and Senate were based on incomplete results as vote counting continued across the country and some states were still voting in a congressional election cast as an unofficial referendum on Trump.
He also asked a question about developments in a federal investigation into Russian interference in the 2016 presidential election and any coordination between Moscow and the Trump campaign.
Those measures, they believe, will be broadly popular.
"Today is more than about Democrats and Republicans". The ties between the two men, who are said to communicate frequently, could come under increased scrutiny by Democrats. "There's an expectation that we're a check and a balance so that means a stalemate".
In a suburban battleground in Atlanta, Republican Rep. Karen Handel won a costly special election earlier this cycle but faced an upstart challenge from Lucy McBath, whose 17-year-old son was shot and killed at a gas station.
Republicans entered Tuesday with strong prospects of keeping their Senate majority but the victory by GOP challenger Mike Braun over Sen.
In the one of the most eagerly-anticipated U.S. midterm elections in recent memory, the Democrats were on course to gain the 23 seats they needed to capture the Republican-held House.
Many Democratic candidates in tight races shied away from harsh criticism of Trump, focusing instead on bread-and-butter issues like maintaining health insurance protections for people with pre-existing medical conditions and safeguarding the Social Security retirement and Medicare healthcare programs for senior citizens.
In Texas, Democrat Colin Allred, a former National Football League player and civil rights lawyer, defeated the incumbent Republican, Pete Sessions.
Republicans have sought to capitalize on Pelosi's low favorability numbers for several campaign cycles. Republicans in this midterm benefited from having only nine seats up for grabs in the senate, compared to 26 for Democrats and the Independents who caucus with them.
Haaland will replace New Mexico Democratic Rep. Michelle Lujan Grisham, who vacated the seat to run for governor. Many are from the party's progressive wing. Newcomers like Alexandria Ocasio-Cortez, a self-described democratic socialist from NY, and Ayanna Pressley of MA will nearly certainly take the lead in pressing for a liberal agenda.
"Mainers have entrusted you with the future of our state and to represent all of the people", he said.
MA passed a referendum to protect trans right.
At the same time, Democrats must be careful not to overreach.
In victory, The Washington Post said Democrats regained some of the confidence - although less of the power - they lost in 2016, when Trump won a surprise victory over Hillary Clinton. But she added, "Where we can not, we must stand our ground".
"The one issue that Leader Pelosi and I discussed this morning where there could be a possible bipartisan agreement would be something on infrastructure, but there could be a lot of other things", he said.
Voters delivered divided government to Washington, ousting the Republican majority in the House while reinforcing the GOP majority in the Senate.
Democrats seized at least 26 seats to gain a majority that could stymie his agenda and lead to investigation of his administration.
And then there's a lawsuit against Trump by almost 200 Democratic senators and representatives who accuse Trump of constantly violating the Constitution's emoluments provision banning the acceptance of gifts from foreign and domestic interests.