WASHINGTON — An already difficult situation for Democrats in Congress is worsening as the 2010 political season opens.

To minimize expected losses in next fall’s election, President Barack Obama’s party is testing a line of attack that resurrects George W. Bush as a boogeyman and castigates Republicans as cozy with Wall Street.

Four House Democrats from swing districts have recently chosen not to seek re-election, bringing to 11 the number of retirements that could leave Democratic-held seats vulnerable to Republicans. More Democratic retirements are expected.

Over the holiday break, another Democrat, freshman Rep. Parker Griffith of Alabama, defected to the GOP. “I can no longer align myself with a party that continues to pursue legislation that is bad for our country, hurts our economy, and drives us further and further into debt,” said Griffith, who voted against Democrats’ three biggest initiatives in 2009: health care, financial regulation and reducing global warming.

In the Senate, at least four Democrats — including Majority Leader Harry Reid of Nevada and five-term Connecticut Sen. Chris Dodd — are in serious trouble. The party could also lose its grip on seats Obama held in Illinois and Vice President Joe Biden long occupied in Delaware.

Going into 2010, Democrats held a 257-178 majority in the House and an effective 60-40 majority in the Senate, including two independents who align themselves with Democrats.

But they face an incumbent-hostile electorate worried about a 10 percent unemployment rate, weary of wars and angry at politicians of all stripes. Many independents who backed Democrats in 2006 and 2008 have turned away. Republicans, meanwhile, are energized and united in opposing Obama’s policies.

The one thing that heartens Democrats is that voters also don’t think much of the GOP, which is bleeding backers, lacking a leader and facing a conservative revolt.

House Democrats began an ad campaign in December assailing Republicans for opposing legislation restructuring federal financial rules and recalling the final days of the Bush presidency, when the economy tanked.

“Remember? We all know we should never let this happen again,” the ad says. It lays into Republicans for voting “to let Wall Street continue the same risky practices that crippled retirement accounts and left taxpayers on the hook for $700 billion.”

Maryland Rep. Chris Van Hollen, who heads the House Democrats’ campaign arm, said his party wants to remind voters who was on their side at a difficult time. “The Republican Party in Washington today is no different than the Republican Party that ran the Congress before,” he said.

But that was three years ago. Democrats have been in control since, and Bush is long gone. This is Obama’s country now. Democrats tried to use Bush against Republican Chris Christie in the New Jersey governor’s race in November — and Democratic Gov. Jon Corzine still lost.

A top Democratic priority is minimizing losses among nearly four dozen seats the party now holds in moderate-to-conservative districts that Republican John McCain won in the 2008 presidential race. The most vulnerable in that group include Democratic Reps. Mary Jo Kilroy in Ohio, Harry Teague in New Mexico, Frank Kratovil in Maryland, Tom Perriello in Virginia and Travis Childers in Mississippi.

Reps. Bart Gordon and John Tanner, both of Tennessee, were in that group until they chose to retire. So was Griffith, before he switched to the GOP. Retirement announcements from Reps. Dennis Moore of Kansas and Brian Baird of Washington put two more Democratic seats in swing-voting districts on the GOP’s target list.

Democrats insist that Gordon, Tanner, Moore and Baird are leaving for personal reasons and are not the first ripple in a wave of retirements akin to 1994 when 28 Democrats chose not to run, and Republicans won control in part by winning 22 of those seats.

Republicans don’t agree.

“Democrats are beginning to see the writing on the wall, and instead of choosing to fight in a difficult political environment, they are taking a pass and opting for retirement,” said Ken Spain, a spokesman for the House GOP’s campaign arm.

The GOP will be defending at least a dozen open seats because of retirements, with several lawmakers leaving the House to run for higher office.

The situation for Democrats in the Senate is nearly as grim as it is for them in the House.

Democrats crowed after six Senate Republicans — four from swing states Florida, Ohio, Missouri and New Hampshire and two from GOP-leaning Kansas and Kentucky — announced retirements.

Spirited GOP challenges are now expected in all six states, and Republicans say they are optimistic they can retain the seats. An emboldened GOP also is looking to put a pair of senior Senate Democrats out of office.

Reid, who is seeking a fifth term, is faring poorly in surveys in a hypothetical matchup with Nevada GOP chairwoman Sue Lowden, one of several Republicans competing for a chance to challenge him.

Dodd, the Banking, Housing and Urban Affairs Committee chairman who has taken heat for a discounted VIP mortgage loan he got from a subprime lender, has been consistently behind potential GOP challenger Rob Simmons in Connecticut polls. Simmons, a former House member, has his own challenger in World Wrestling Entertainment co-founder Linda McMahon, who also is seeking the Republican nomination for Dodd’s seat.

Also vulnerable are Sen. Blanche Lincoln, a moderate Democrat in GOP-leaning Arkansas, and Sen. Michael Bennet in Colorado, who was appointed when Ken Salazar became Obama’s interior secretary.

Republicans have high hopes for picking up Senate seats in Illinois and Delaware that were held by the president and vice president, respectively. Neither of their appointed successors is seeking election to the seats.

Early polling shows GOP Rep. Mark Kirk leading among Republican candidates in Illinois. Veteran GOP Rep. Mike Castle, a former two-term governor, is running for the Senate in Delaware. Biden’s son, Democratic state Attorney General Beau Biden, is considering whether to challenge Castle.