WILLIAMSBURG, Va. — President Obama veered away from his repeated calls for bipartisanship on Thursday night, laying into Republican opponents of his massive stimulus package before a wildly enthusiastic audience of House Democrats.

“This package is not going to be absolutely perfect,” the president told more than 200 House members gathered here for their annual policy conference. “You can nit it. You can pick. That’s the game we play here. But we can’t afford to play that game now,” he said to loud cheers.

Coming into the first day of the three-day policy conference, some House members fretted that Obama and Democratic congressional leaders weren’t doing enough to combat Republicans who claim the $819 billion package (HR 1) passed by the House two weeks ago contains too much wasteful federal spending and not enough in job-stimulating tax cuts.

Laying into the critics, Obama said, “They say this isn’t a stimulus bill. It’s a spending bill. Well, that’s the whole point.” He rarely used the word “Republican.”

Even before he took office on Jan. 20, Obama said he wanted to work with the GOP on stimulating the recession-hit economy. He went so far as to take the unusual step of coming up to the Capitol a few days after his inauguration to meet with House and Senate Republicans to solicit their ideas.

But not one House Republican voted for the package, and Senate Democrats are having a hard time finding GOP support as the negotiations enter their end stage.

“I believe it’s important for us to set aside some of the gamesmanship and get something done,” the president told the House caucus, which greeted his arrival like the coming of a rock star. He spent almost an hour in the ballroom at the exclusive Kingsmill Resort shaking members’ hands, embracing them and posing for pictures.

Democratic leaders said they were glad Obama had started to fire back at the Republicans.

“I think the president decided it was time to lay out the facts, and take control of the debate,” said Majority Whip James E. Clyburn of South Carolina.

“You can’t sit back and let them define the debate,” he added.

Chris Van Hollen of Maryland, head of the Democratic Congressional Campaign Committee, said Obama has decided the stimulus bill needs to move quickly.

“What we heard tonight from the president was a determination to get this done on an urgent basis,” Van Hollen said. “What we’re seeing is a determination by some of the Republicans to say “no,” no matter what.”

‘We Have His Back’