Here is the state of play so far:
Democrats take the house
The House of Representatives has flipped from Republican control into the hands of the Democrats, US networks projected. Early results showed Democrats winning pick-ups in congressional districts in Virginia, Florida, Pennsylvania, Colorado and New Jersey, as voters there sent a message of repudiation against Trump.
The non-partisan Cook Political Report is forecasting a Democratic gain of at least 30 seats in the 435-member House. Democrats needed to gain 23 seats to reclaim control.
With a House majority, Democrats will have the power to investigate Trump’s tax returns and possible conflicts of interest and challenge his overtures to Saudi Arabia, Russia and North Korea.
They also could force Trump to scale back his legislative ambitions, possibly dooming his promises to fund a border wall with Mexico, pass a second major tax-cut package or carry out his hardline policies on trade.
A simple House majority would be enough to impeach Trump if evidence surfaces that he obstructed justice or that his 2016 campaign colluded with Russia. But Congress could not remove him from office without a conviction by a two-thirds majority in the Republican-controlled Senate.
Democrats in the House could be banking on launching an investigation using the results of U.S. Special Counsel Robert Mueller’s already 18-month-old probe of allegations of Russian interference on Trump’s behalf in the 2016 presidential election. Moscow denies meddling and Trump denies any collusion.
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.
Republicans retain control of Senate
It was a different story in the 100-member US Senate, where Republicans retained their control, US networks projected.
The party drew blood in Indiana, ousting one-term Democrat Joe Donnelly in a state that had been seen as a must-win for Democrats seeking to gain control of the upper chamber.
Doubling the Democratic pain, North Dakota Senator Heidi Heitkamp crashed to defeat against Republican state lawmaker Kevin Cramer.
Democrats were defending a whopping 10 seats in states won by Trump in 2016. A few had already won re-election Tuesday, including in Ohio, Pennsylvania, West Virginia and Wisconsin.
But in addition to Donnelly and Heitkamp, Missouri’s Claire McCaskill faces a serious risk of defeat.
Their party had put high hopes in flipping the traditionally Republican Texas, but candidate Beto O’Rourke came up just short against incumbent Senator Ted Cruz.
The election results mean Democrats will resume House control in January for the first time since the 2010 election, beginning a split-power arrangement with the Republican-led Senate that may force Trump to scale back his legislative ambitions and focus on issues with bipartisan support, such as an infrastructure improvement package or protections against prescription drug price increases.
It also will test Trump’s ability to compromise, something he has shown little interest in over the last two years with Republicans controlling both chambers of Congress.
The loss of power will test Trump’s political hold on House Republicans, most of whom had pledged their support for him lest they face the wrath of the party’s core supporters, who remain in his corner.
Most Democratic candidates in tight races stayed away from harsh criticism of Trump during the campaign’s final stretch, focusing instead on bread-and-butter issues like keeping down health care costs, maintaining insurance protections for people with pre-existing medical conditions and safeguarding the Social Security retirement and Medicare healthcare programs for senior citizens.
The final weeks before the election was marked by the mailing of pipe bombs to his top political rivals, with a political fan of Trump arrested and charged, and the mass shooting at a Pittsburgh synagogue in which 11 people died, sparking a debate about Trump’s biting rhetoric and whether it encouraged extremists.