Trump said early on Wednesday that House investigations would be countered by investigations of Democrats by the Senate, which remains in Republican hands after Tuesday's congressional elections. For former President Barack Obama, midterm elections in 2010 and 2014 were catastrophic compared to Trump's moderate losses.
Internal Revenue Service law allows House and Senate tax-writing committees to request federal returns, and the ranking Ways and Means Democrat, Rep. Richard Neal of MA, has signalled publicly he would ask for them - a move the White House might fight.
Democrats needed to pick up two dozen seats to seize the House majority and two seats to control the Senate. Party leaders on Capitol Hill and in the White House soon turned on one another and against their candidates with growing intensity.
Trump offered an olive branch to House Democrats as an alternative to an investigative face-off.
But in Georgia, Democrat Stacey Abrams - who was hoping to become the first black female governor in the USA - refused to concede as her Republican opponent Brian Kemp took a commanding lead after a bitter campaign. And she lost. Too bad.
Democrats themselves sought to root their campaigns in more bread-and-butter issues, such as health-care and what they portrayed as Republican assaults on the popular aspects of Obamacare.
With majority subpoena power, it's expected that Democrats are prepared to launch investigations into Trump, his policies and his Cabinet members.
"I think Mainers sent a pretty clear message in that they want a change in how our state is run", Sen.
Joining Davids as one of the first Indigenous women elected to Congress was fellow Democrat Deb Haaland, a member of the Laguna Pueblo tribe who will represent New Mexico's first district.
The mixed verdict in the first nationwide election of Trump's young presidency underscored the limits of his hardline immigration rhetoric in America's evolving political landscape, where college-educated voters in the nation's suburbs rejected his warnings of a migrant "invasion". In 1962, the then president John F Kennedy lost four seats (the fewest ever).
"This election marks the largest Senate gains for a president's party in a first midterm election since at least President Kennedy's in 1962", Trump said.
"Peter Roskam didn't want the embrace", Trump said.
"There's so much to the Trump administration that could be investigated, it's an unprecedented situation of major business entanglements around the world", said Dana Allin, senior fellow with the International Institute for Strategic Studies.
Russian President Vladimir Putin denies any involvement in Trump's election victory, and the Kremlin shrugged off concerns that a Democratic-controlled House would increase pressure on Russia. "The president has energized a staggering number of Americans at packed arenas and in overflow crowds at rallies across the country".
The Democrats, in turn, benefited from extraordinary voter enthusiasm, robust fundraising and unusually fresh candidates.
They argue that such favours to Trump include Chinese government trademarks for his companies, payments for hotel room stays and event-space rentals by representatives of Saudi Arabia and Kuwait, and proceeds from Chinese or Emirati-linked government purchases of office space in Trump Tower. Tea Party candidates were even able to displace traditional Republicans. "It's about restoring the constitution's checks and balances to the Trump administration", Pelosi told a news conference.
Interested in Midterm Elections?
President Donald Trump initially tried to act gracious, calling Democratic House Leader Nancy Pelosi to laud her victory.
Ms Pelosi - who is set to become speaker, a position she held from 2007 to 2011 - told supporters: "Today is more than about Democrats and Republicans".
Trump said Wednesday that "if they give her a hard time, perhaps we will add some Republican votes". "Do not let their scare tactics frighten you away from the polls". About a quarter of Republican women, and 2 in 10 Republican men, said they were very concerned about this.