WASHINGTON (AP) — Democrats sought to pad their majority power in the House of Representatives at the polls Tuesday.

Long lines of voters queued up — before daybreak in some cases — as the polls opened across the country. Democrats were counting on heavy turnouts to capture more than 20 GOP seats, although the man who heads their campaign committee cautioned that the gains might not be in the range that some pundits had envisioned.

Rep. Chris Van Hollen, D-Md., did say Tuesday that a high turnout for Barack Obama should help Democratic down-ballot candidates. But over the past few days, he said, "we saw actually a tightening in a lot of races. That is why I've been careful ... about these huge numbers people are talking about."

There was no question about voter interest, however.

"I knew the lines were going to be really long," Jennifer Howard, 51, of Herndon, Va., told reporters as she got ready to vote. "I'm a nurse and I had to be at work on time," said Howard, who showed up 55 minutes before her polling place in Virginia opened.

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. In 2006, they won 30 seats and control of Congress in a surge powered by voter anger over the Iraq war.

This year it's the sour economy and public antipathy for President Bush that posed the biggest challenges for Republican candidates. A wave of GOP retirements and huge financial and organizational disadvantages compared with Democrats made a grim fight even tougher.

Democrats now control the House by a 235-199 margin, with one vacancy.

GOP lawmakers at risk include Alaska's Rep. Don Young, Rep. Marilyn Musgrave of Colorado, Rep. Tom Feeney of Florida, and Michigan 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.

Early election night bellwether seats include a GOP-held seat in Indiana, where polls close at 7 p.m. EST and Rep. Mark Souder is encountering trouble that few expected just weeks ago. Whether Virginia GOP Reps. Thelma Drake and Virgil Goode hold their seats will offer further clues, as will the fate of Democratic freshman Rep. Carol Shea-Porter of New Hampshire.

Republican Party strategists expect 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 aren'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, is 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 shouldn't have had a chance.

A crop of freshman Democrats in conservative-leaning districts quickly 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 Election Day.

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.