They are always waves, tsunamis, earthquakes, landslides. Or hurricanes. Whenever U.S. voters move en masse toward either party in an election, it's time for a metaphor alert.

For Democrats in 2010, the year of midterm discontent, the appropriate image might be a scythe.

Projections show Republicans poised to net anywhere from 48 to 65 seats in the U.S. House on Tuesday, well more than the 39 required to take control there. Election Day will be a "historic bloodbath" for Democrats, analyst Stuart Rothenberg said.

The party is in a sense a victim of its own recent success, having won House seats in more conservative districts in 2006 and 2008, when the wave of public opinion pushed Republicans out of power. At least 100 Democrat-held seats are in play, including 50 in districts carried two years ago by GOP presidential candidate John McCain.

Now, the economy remains mired in recession, and the "hope and change" aura surrounding President Obama has faded amid public worry over federal budget deficits, an expensive stimulus program, and a divisive overhaul of health care.

And, as usual in national elections, Pennsylvania is one of the states that could tip the balance of power. Races for seven U.S. House seats held by Pennsylvania Democrats are competitive; the GOP has a realistic shot to capture four of them, even if the party underperforms projections, polls and interviews with strategists on both sides suggest. It could also pick up a seat in South Jersey.

"This election was cooked and done well before Election Day, just like 2008," said Larry Sabato, director of the Center for Politics at the University of Virginia, who in August predicted a GOP House takeover. The economy was weak then and has not turned around, he said.

As for the Senate, Democrats, who hold 19 of the 37 seats at stake around the nation Tuesday, are expected to see their majority shrink. Polls suggest the GOP could pick up eight seats, which would just barely keep the Democrats in control of the chamber.

One seat that could flip is in Pennsylvania, which has one of the most closely watched Senate races in the nation. Recent polls show Democratic Rep. Joe Sestak of Delaware County within striking distance of Republican former Rep. Pat Toomey of the Lehigh Valley, who has led for months in the battle for the seat now occupied by Republican-turned-Democrat Arlen Specter.

An array of interest groups and unions, freed up by a Supreme Court ruling this year that overturned a ban on corporate money in elections, have poured nearly $24 million into the Sestak-Toomey race. Only Colorado's Senate race has drawn more outside money, according to the Sunlight Foundation, a Washington group that promotes transparency in elections and government.

The shower of ads has not always had the intended effect. "I don't think anyone should be in Congress this year," said Bob Steffan, 75, a retired upholsterer in Sellersville, Bucks County, and a registered Republican. "There are lies on one commercial after another."

It is not only in Congress that the GOP is poised to make big gains. Republicans also are favored to have a majority of the nation's governorships when balloting is done, and their pickups seem likely to include Pennsylvania, where the GOP candidate, Attorney General Tom Corbett, leads Democrat Dan Onorato, the Allegheny County executive.

Democrats are racing to try to energize their base, urging sporadic voters and those who cast their first ballot, for Obama, in 2008 to get to the polls. The party, through its Organizing for America arm, has poured more than $50 million nationwide into get-out-the-vote efforts, including phone banks and door-to-door canvassing.

A massive operation is also under way by labor unions and other groups allied with the Democrats.

Capping off the party's efforts locally, first lady Michelle Obama is due at a University of Pennsylvania rally Monday evening, after her husband's appearance Saturday at Temple University - the second time the president has campaigned in the city this month. Philadelphia's corner of the state is filled with competitive races, and its media market also blankets voters in New Jersey's Third District, where Democratic Rep. John Adler faces former Eagle Jon Runyan, the Republican; and Delaware, where Democrat Chris Coons, the New Castle County executive, is battling tea-party-backed Republican Christine O'Donnell.

Obama's Philadelphia visit is part of a string of events he has been headlining in staunch Democratic areas where he won 60 percent of the vote or more in 2008.

Most polls show independents breaking more heavily to Republican candidates. But as there are more registered Democrats than Republicans, the party believes it can win some races, or at least minimize losses, by closing the so-called enthusiasm gap - polling that shows Republicans more intent on voting than Democrats.

"There are a number of states where closing the enthusiasm gap is the best way to get a candidate to 50 percent plus one," Jeremy Bird, executive director of Organizing for America, said Friday.

Penn Action, a state liberal group, has targeted the competitive Eighth District House race, where Democratic Rep. Patrick J. Murphy faces a stiff challenge from the Republican he beat four years ago, former Rep. Michael Fitzpatrick. It was spending $50,000 to contact 20,000 infrequent Democratic voters in person and by phone.

Republicans have their own efforts, bolstered by activists from the tea party movement, who are motivated by anger at federal spending and the expansion of government power, such as the health-care law.

Though Democratic officials say they have done as well as or better than the GOP in states where early voting is allowed, the absentee-ballot count in Pennsylvania favors the GOP.

According to the Secretary of State's Office, 53,226 absentee ballots have been returned by registered Republicans in Pennsylvania, compared with 37,631 by registered Democrats. Republicans have returned 67 percent of the absentee ballots they requested, compared with 55 percent for Democrats.

The state of play is a turnaround for Pennsylvania, which has trended blue over the last decade. Since 2002, state voters have gone with the Democrat in all the major races, picking Gov. Rendell and Sen. Bob Casey, and backing Democratic presidential nominees John Kerry in 2004 and Obama in 2008.

In last week's Franklin and Marshall College poll, 62 percent of likely voters who supported Toomey said they were motivated by opposition to Obama, while 48 percent of Sestak backers said they wanted to register support for the president.

"It's a completely negative election - people are motivated to vote against rather than for," said Lara Brown, political science professor at Villanova University. "The reason polls in a lot of races are widening and then narrowing and widening is because any time somebody gets a lead, the partisans on the opposite side get all riled up."

The Gallup Poll, a reliable predictor of midterm results for 60 years, found last week that Republicans had at least an 8-percentage-point enthusiasm advantage. It was 6 percent in 1994, when Republicans took 52 seats to seize control of the U.S. House for the first time in four decades.

"It seems ludicrous, but Gallup hasn't been wrong," Brown said. "If it's right, we're looking at something none of us have ever seen, and the country hasn't seen since the 1890s."

In 1890, the Republicans lost 93 seats in the House, giving power to the Democrats. Four years later, the Democrats lost 125.

If the GOP takes the House this year, it would be the third "change" election in a row - after 2006, when voters reacted against the Iraq war and GOP corruption scandals, and Democrats took back the House; and 2008, when voters elected the first African American president.

In a brutal irony of an election season marked by opposition to the policies of liberal Democrats, it is the moderate-to-conservative "Blue Dog" caucus of the party that stands to take the hit Tuesday.

Experts say that as many as half the 54 House Blue Dogs could lose, mostly because they won in marginal districts during the Democratic wave elections of 2006 and 2008 - districts that swing back to the right when the national mood is headed that way.

That would mean a smaller Democratic caucus of determined liberals. The Republicans would have their own problems, with up to 50 new tea party-style conservatives not inclined to compromise with the opposition.

"It will be interesting to see how conflicts between the tea partyers, with their libertarian strain, and the Republican leadership work out," said Jeff Brauer, political science professor at Keystone College. "The Republicans are going to have a coalition-type government."

And with probable rough parity in the Senate and the White House in Democratic hands, Brauer said, "there's not going to be a lot of legislation done in the next two years."

That, he said, may set up Obama well for his 2012 reelection bid.

Contact staff writer Thomas Fitzgerald at 215-854-2718 or
Inquirer staff writer Katie Eder contributed to this article, which also contains information from the Associated Press.