"It's the night we have been waiting for," said House Speaker Nancy Pelosi, D-Calif.
The head of the Democrats' campaign committee counseled caution.
"It's still early," said Rep. Chris Van Hollen Maryland. He said a high turnout for Barack Obama should help Democratic down-ballot candidates.
First-term Democratic Reps. John Yarmuth of Kentucky and Joe Donnelly of Indiana won easy re-election. They were part of a crop of freshman Democrats in conservative-leaning districts who began compiling campaign war chests and moderate voting records almost from the moment they were elected two years ago, leaving only a few of them endangered on Tuesday.
Former five-term Republican Rep. Anne Northup was unable to mount a comeback in Louisville, Ky., against Yarmuth despite GOP presidential nominee John McCain's decisive victory in Kentucky.
Yet another Democratic freshman, Indiana Rep. Brad Ellsworth, jumped to a large lead in early returns.
There were early signs of trouble for Republicans in Florida, where Rep. Ric Keller of Florida was badly trailing his Democratic challenger, attorney Alan Grayson, with more than three-quarters of the vote tallied. Democrat Suzanne Kosmas, a former state legislator, had opened a huge lead over GOP Rep. Tom Feeney in early returns.
It could be the first time in more than 75 years that Democrats would ride large waves of victory to bigger congressional margins in back-to-back elections.
"This will be a wave upon a wave," Pelosi said.
In 2006, they won 30 seats and control of Congress in a surge powered by voter anger over the Iraq war.
This year the sour economy and public antipathy for President Bush posed the biggest challenges for Republican candidates. The Democrats were aided by a wave of GOP retirements and huge financial and organizational advantages over Republicans.
That's despite voter hostility toward the Democratic-controlled Congress. Just one in five voters Tuesday approved of the job Congress was doing, about as poorly as Bush fared, according to AP exit polling.
Six in 10 voters cited the economy as the most important issue facing the nation. About half said the economy is poor and nearly all the rest said it's not good. The results were based on a preliminary partial sample of nearly 10,000 voters in Election Day polls and in telephone interviews over the past week for early voters.
Democrats now control the House by a 235-199 margin, with one vacancy.
GOP lawmakers at risk included Alaska's Rep. Don Young, Colorado's Rep. Marilyn Musgrave, and Michigan's Reps. Tim Walberg and Joe Knollenberg. Rep. Lee Terry of Nebraska, once considered a safe bet for re-election, is also in major trouble in a state Obama is actively contesting.
In one of the first states to report, it appeared Republican Rep. Rep. Mark Souder, who encountered trouble that few expected just weeks ago, was holding off his Democratic challenger in early returns. Rep. Virgil Goode appeared to be holding his own in conservative Southside Virginia with more than a third of the vote in.
Republican Party strategists expected to lose several GOP-held seats left open by Republican retirements or departures, including in Arizona, Illinois, Maryland, Ohio, Virginia, and two each in New Mexico and New York.
Democrats weren't expecting a clean sweep. Rep. Tim Mahoney, D-Fla., who is under investigation by the FBI and a House panel after admitting to two adulterous affairs, is all but certain to lose his re-election race. Other Democrats most at risk of losing include Reps. Paul E. Kanjorski in Pennsylvania and Nick Lampson in Texas.
Rep. John P. Murtha, D-Pa., who chairs a subcommittee with the most influence on the Pentagon's spending, was also in an unexpectedly tight race to keep the seat he's held for 34 years, after calling his district south of Pittsburgh "racist."
Democrats, who came out of their huge 2006 victory girding for losses this year, instead were able to aggressively spread their considerable wealth to campaigns around the country, including to traditionally Republican districts where their candidates normally wouldn't have had a chance.
Republicans, meanwhile, were fighting on a playing field skewed by the departure of 29 of their members, leaving lesser-known GOP contenders to battle better-financed Democrats in races shaped in large part by antipathy toward Bush.
Both parties took in huge amounts of campaign cash in House races, although Democrats had a clear edge. Democratic candidates raised $436 million, compared with Republicans' $328 million, according to federal data compiled by the nonpartisan Center for Responsive Politics.
The parties' campaign committees also bankrolled the most competitive races, with the Democratic Congressional Campaign Committee pouring in $76 million and the National Republican Congressional Committee spending $24 million.
Because of hurricanes that delayed October primaries, the winners of two Louisiana seats - one that belonged to retiring Republican Rep. Jim McCrery and another now held by indicted Democratic Rep. William Jefferson - won't be known until December. Those districts held primaries Tuesday.