But the GOP gained ground in the Senate and preserved key governorships, beating back a "blue wave" that never fully materialized.
Curbelo, a moderate and critic of President Donald Trump, was trying to defy the political winds against Trump.
Republicans are expected to retain their slight majority in the Senate, now at two seats, which would let them retain the power to approve US Supreme Court and other judicial nominations.
"Tomorrow will be a new day in America", Democratic Leader Nancy Pelosi said at a victory party in Washington.
But the Democrats' edge is narrow.
"This election will decide whether we build on this extraordinary prosperity we have created", Trump told a cheering crowd in Macon, Georgia, warning that Democrats would "take a giant wrecking ball to our economy".
A Democratic House represents a likely threat to Trump's legislative agenda for the next two years, and provides Democrats subpoena power to investigate the president - his long-elusive tax returns are high on their wish list - or even trigger impeachment proceedings, although that seems unlikely with a Republican Senate.
Faced with the possibility of keeping the Senate but losing the House, aides in recent days were laying out the political reality to Trump, who could face an onslaught of Democratic-run investigations and paralysis of his policy agenda. It wasn't clear what "leaks" he was referring to.
Control of the House of Representatives is expected to be determined by a few dozen districts, many of them in the nation's suburbs, and will offer clues to where Americans stand in 2018 on immigration, guns, healthcare and gender equality in the #MeToo era.
Minnesota voters braved a chilly rain-snow mix Tuesday to cast ballots in high-profile elections for governor and attorney general, two U.S. Senate seats and at least four hotly contested congressional races around the state.
As for Republicans retaining control of the Senate, she called it "a huge moment and victory for the president". And a status-quo election would actually set up Democrats very well to take over the chamber when they have much better opportunities in 2020 and 2022.
Democrats also boast record diversity on the ballot. She was among the record number of women running this year. The House was also getting its first two Muslim women, MA elected its first black congresswoman, and Tennessee got its first female senator.
Three states could elect their first African-American governors, while several others are running LGBT candidates and Muslims.
Almost 40 percent of voters cast their ballots to express opposition to the president, according to AP VoteCast, a national survey of the electorate, while about 25 percent said they voted to express support for Trump. Not all were endangered, and those winning re-election included senators in Michigan, Ohio, Pennsylvania, West Virginia and Wisconsin.
Republicans rolled into the election with a 77-57 House majority.
Boca va por Independiente, River por Estudiantes
La primera final de Copa Libertadores se jugará este sábado 10 de noviembre en el estadio 'La Bombonera' de Buenos Aires , desde las 3:00 p.m., hora peruana.
Former President Barack Obama, whom Trump frequently criticizes and compares himself to, had a 46% approval rating heading into his first midterm elections.
Trump, as he did throughout the campaign, also blamed the media for sowing division in the country and insisted they were to blame for the scene unfolding in the East Room.
Retired fighter pilot Amy McGrath has given Republican incumbent Andy Barr his toughest test yet as he seeks a fourth term outside Lexington.
After the surprising results in the 2016 presidential election, pundits are reluctant to make predictions.
Democratic Senate incumbents were up for re-election, for example, in North Dakota, Indiana, and Missouri - states Trump carried by nearly 25% on average two years ago.
Some hurt worse than others.
The Grand Old Party (GOP) also controls the US Senate, with 51 of 100 seats.
More than 60 percent of eligible voters took part in the 2016 vote, when the presidential race was on the ballot.
AP VoteCast, a national survey of the electorate conducted by The Associated Press, highlighted the effect Trump was having on voters.
Still, about one-third of voters said Trump was not a factor in their votes.
Several television networks, including the president's favorite Fox News Channel, yanked a Trump campaign advertisement off the air on the eve of the election, determining its portrayal of a murderous immigrant went too far. They are the same five where Trump won by double digits in 2016. Voters scored Trump positively on the economy and for standing up "for what he believes in".
Based on data from Gallup, no other president in modern U.S. history had such a low approval rating heading into their first midterm election.
Democratic former Vice President Joe Biden called on Americans to use their votes to reject Trump.
Senators Bernie Sanders of Vermont and Elizabeth Warren of MA easily won re-election as they consider bids for the Democratic presidential nomination.
With more than half of Trump's presidency still to come, how the market will perform over his whole term is unknown. If they win, those new representatives will be hard-pressed to go back on their word.