WASHINGTON — Congressional Democrats on Tuesday wrestled over whether to abandon President Obama’s tax-cut plan, with some House moderates joining Republicans in calling for an extension of Bush-era breaks for the wealthy as well as middle-income earners.

But Senate Majority Leader Harry Reid of Nevada and House Speaker Nancy Pelosi remained solidly behind Obama’s proposal to allow tax cuts for upper-income people to expire as scheduled Dec. 31.

The divisions among Democrats contrasted with strong unity among Republicans in supporting a full renewal of all tax cuts, regardless of income, despite a 10-year cost to the government of about $700 billion above Obama’s plan.

Some House Democrats, particularly moderates facing difficult re-election battles in districts carried by GOP presidential nominee John McCain two years ago, agree with a proposal offered by Republicans for a short-term renewal of all of the Bush-era tax cuts.

“We look forward to working with you to extend all income-tax rates,” a small group of conservative-to-moderate House Democrats wrote in a draft letter to party leaders as lawmakers trickled back into town Tuesday from their summer break.

Democratic Reps. Jim Matheson of Utah, Melissa Bean of Illinois and Glenn Nye of Virginia were circulating the letter for more signatures on Tuesday.

There also was nervousness among Democrats over casting a difficult vote on taxes before Election Day. Some endangered lawmakers would prefer to punt the issue to a lame duck session after the election.

Sen. Dick Durbin of Illinois, the No. 2 Democrat in the Senate, said most Democrats support Obama’s plan to allow income tax rates on family income exceeding $250,000 to rise to as high as 39.6 percent. But he also said some want to raise the amount of income exempted from the higher rates above the $250,000 figure advocated by Obama – while not advocating a full renewal for, say, millionaires.

“Some people think it should go beyond $250,000, but how much and for what period of time is still being debated,” Durbin told reporters.