The party needed a net gain of 23 seats to break the Republicans' eight-year hold on the House that began with the tea party revolt of 2010. By late evening, as the polls on the West Coast closed, the Democrats had picked up at least 18 seats GOP-held seats in one of the most volatile midterm elections in modern U.S. history.
While the Republican Party maintained control of the Senate, a win for the Democrats in the House would end the GOP monopoly on power in Washington and open a new era of divided government.
"Tomorrow will be a new day in America," Democratic Leader Nancy Pelosi said at a victory party in Washington.
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. They also made inroads into Trump country as Democrats tried to win back white working-class voters.
Midterm elections are typically difficult for the party in power, and GOP incumbents were on the defensive in many races across the country.
As Election Day unfolded, Democrats were increasingly confident, predicting a House majority on the strength of voter enthusiasm, robust fundraising and unusually fresh candidates. More women than ever were running, along with veterans and minorities, many of them motivated by Trump's rise.
The campaign unfolded against a backdrop of heated rhetoric and angry debates on immigration, health care and the role of Congress in overseeing the president.
To stem Republican losses, Trump sprinted through mostly white regions of the country, interjecting dark and foreboding warnings about what Democratic power would mean for the nation.
The debate was dominated not by the GOP's $1.5 trillion tax cuts but by Trump's dire prediction of "invasion" from the migrant caravan and what he called the "radical" agenda of speaker-in-waiting Pelosi.
GOP Whip Steve Scalise of Louisiana predicted his party would retain a slim majority, saying on election eve: "In the end, we hold the House because of the strong economy."
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. AP VoteCast also showed a majority of voters considered Trump a factor in their votes.
For Democrats, the road to the 218-seat majority ran through the two dozen suburban districts Clinton won and through swaths of Trump country in the Rust Belt and heartland where voters backed the president two years ago. How women and independent voters cast their ballots was likely to determine the outcome.
In the suburbs outside the nation's capital, Virginia Rep. Barbara Comstock — among the most endangered GOP incumbents, branded Barbara "Trumpstock" by Democrats — lost to Jennifer Wexton, a prosecutor and state legislator. Wexton was among the record number of women running this year.
In the Miami area, former Clinton administration Cabinet member Donna Shalala won an open seat, while GOP Rep. Carlos Curbelo lost his bid for a third term in another district.
Pennsylvania looked particularly daunting for Republicans after court-imposed redistricting and a rash of retirements put several seats in play. Democratic favorite Conor Lamb, who stunned Washington by winning a special election in the state, beat Republican Rep. Keith Rothfus in a new district. At least three other red districts flipped to blue.
In Kansas, Democrat Sharice Davids beat a GOP incumbent to become the first Native American and gay woman elected to the House.
But in Kentucky, one of the top Democratic recruits, retired Marine fighter pilot Amy McGrath, lost her bid to oust to three-term Rep. Andy Barr in the Lexington-area district.
Outside Richmond, Virginia, one-time tea party favorite Rep. Dave Brat faced an unusually strong challenge from Democrat Abigail Spanberger, a former CIA operative motivated to run for office after the GOP vote to gut the Affordable Care Act. Like other Democrats across the country, Spanberger emphasized protecting people with pre-existing conditions from being denied coverage or charged more by insurers.
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.
The GOP's hold on the majority was complicated by an unusually large number of retirements as well as persistent infighting between conservatives and centrists, with much of the conflict centered on the question of allegiance to Trump.
Republicans had expected the GOP tax plan would be the cornerstone of their election agenda this year, but it became a potential liability in key states along the East and West coasts where residents could face higher tax bills because of limits on property and sales tax deductions.
The tax law has been particularly problematic for Republicans in New Jersey, where four of five GOP-held seats were being seriously contested. Democrat Mikie Sherrill, a former Navy pilot and federal prosecutor, was favored for a suburban Newark seat.
The GOP campaign committee distanced itself from eight-term Rep. Steve King of Iowa after racial remarks, and his seat was unexpectedly contested in the final week of the campaign.
In California, four GOP seats in the one-time Republican stronghold of Orange County were in play, along with three other seats to the north beyond Los Angeles and into the Central Valley.
"We always knew these races are going to be close," said Rep. Katherine Clark of Massachusetts, co-chair of House Democrats' recruitment efforts. "It's just a very robust class of candidates that really reflects who we are as a country."