WASHINGTON (Reuters) - Riding a wave of resentment against Republican President George W. Bush, Democrats headed toward winning possibly their biggest majorities in the U.S. Congress in decades in Tuesday’s elections, polls showed.

If Democratic presidential nominee Barack Obama also prevails, the new Congress that convenes in January should be able to take action on a long list campaign promises, within the constraints of the current financial crisis.

Democrats, who now control the Senate 51-49 and the House of Representatives 235-199, have pledged to withdraw U.S. troops from Bush’s Iraq war, roll back Bush’s tax cuts for the rich and move to end the worst economic crisis since the Great Depression.

They hope to win a big enough Senate majority, 60, to overcome Republican procedural hurdles and pass legislation.

All 435 House seats and 35 of the Senate’s 100 seats are up for election.

Regardless of how many seats Democrats pick up, record federal deficits and the poor state of the economy will limit what they can do. Democrats would likely have to limit or postpone any big new spending programs, such as ones to expand health care, upgrade education and advance renewable energy technology.

“None of this will be easy. It won’t happen overnight. But I believe we can do all this because I believe in America,” Obama said in his party’s weekly radio address on Saturday.

Democrats have a plausible chance to gain nine Senate seats and amass for the first time in three decades a majority big enough to keep Republicans from using parliamentary rules to kill legislation and presidential nominations.

But even if they fail to reach the 60 votes needed to end procedural roadblocks, or filibusters, Democrats figure they will be able to win over enough moderate Republicans to pass legislation.

“With 56, 57, 58 (Democratic Senate seats), we’ll be able to get a lot done,” said Sen. Charles Schumer of New York, chairman of the Senate Democratic Campaign Committee.

On the other side of the U.S. Capitol, Democrats could gain upward of 30 seats in the House, analysts say. That could give them their biggest House majority since the mid-1980s.

UNPOPULAR BUSH

If Republican presidential nominee John McCain defies the polls and wins, Democrats will have to compromise with him or face McCain vetoes on issues like Bush’s tax cuts and wars.

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Democrats won control of Congress from Republicans in 2006. But Republicans blocked much of the Democrats’ agenda the past two years -- on matters from expanding health care to withdrawing troops from Iraq -- with Senate filibusters.

Republicans running for Congress have been hurt by Bush’s poor public standing and McCain’s inability to rally support in states that have traditionally backed Republicans.

They also have been damaged by the unpopular Iraq war and the fact that Democratic candidates have been able to raise far more money than Republican contenders.

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“The face of our party is a president with a 25 percent approval rating,” said a senior Republican Party aide. “We have two wars and the economy is tanking.”

“When people think of our party they think of George Bush and they are very angry and that is a drag on all of our candidates,” the aide said. “If I had to bet my life on it, I’d bet Obama wins and Democrats end up with 57 or 58 seats in the Senate.”

A number of big-name congressional Republicans are in jeopardy of being unseated.

They include Senate Minority Leader Mitch McConnell of Kentucky and Sen. Elizabeth Dole of North Carolina, whose husband, Bob Dole, was the party’s 1996 presidential nominee.

Sen. Ted Stevens of Alaska, the longest serving Senate Republican in history, more than four decades, is also in trouble. He was convicted last week of political corruption.

Mary Landrieu of Louisiana appears to be the only Senate Democrat in any jeopardy of being unseated, but polls show her with a slight advantage over her challenger, state Treasurer John Kennedy.