Jim Young/Reuters

The smile on his face said it all.

President Obama arrived Wednesday morning to formally repeal the military’s “don’t ask, don’t tell” policy with an ear-to-ear grin. He strode across the stage, stopping to hug his Democratic allies. He gave a peck on the cheek to Senator Susan Collins, Republican of Maine.

And when he got to the microphone, the “shellacking” his party took just seven weeks ago seemed a distant memory.

“This is a good day,” Mr. Obama said as one of the 500 people in attendance yelled, “You rock, President Obama!”

“You know,” the president continued, letting the applause linger a bit, “I am just overwhelmed. This is a very good day.”

A man whose election shattered one longstanding barrier appeared to take particular pleasure in signing a law that will end another. In an impassioned speech, the president traced the long history of gay soldiers’ being shunned, from the War of Independence through World War II.

“You’re not the first to have carried this burden,” Mr. Obama said. “For while today marks the end of a particular struggle that has lasted almost two decades, this is a moment more than two centuries in the making.”

The day got even better for the president a few hours later, as the Senate gave final approval to the New Start nuclear treaty with Russia, which the administration had negotiated, and passed a medical benefits bill for 9/11 first responders, which Democrats had pushed. The votes capped one of the busiest lame-duck sessions in decades.

Mr. Obama is scheduled to hold a news conference at the White House on Wednesday afternoon, a final opportunity to shape the political conversation before lawmakers leave for the year and the president heads to Hawaii for a 10-day vacation.

It may be the last free shot Mr. Obama gets for a while. Republicans are just days away from taking control of the House from Speaker Nancy Pelosi. And in the Senate, Republicans are adding to their numbers, making it even harder for Mr. Obama to peel off enough senators like Ms. Collins to pass his agenda.

In an interview with Politico on Tuesday, Senator Mitch McConnell of Kentucky, the Republican minority leader, issued a bold warning to Democrats.

“There’s much for them to be angst-ridden about,” Mr. McConnell said. “If they think it’s bad now, wait till next year.”

White House officials are mindful of those political threats. Mr. Obama’s aides have said that they expected the president to work more closely with Republicans like Mr. McConnell in the coming months. The agreement on the tax deal this month was worked out largely between those two men.

“As we’ve said before, we’re going to have to, and we will, work with Republican leaders in Congress more closely than we have in the past couple of years,” Robert Gibbs, the president’s press secretary, said on Wednesday. “As I said earlier, they have a responsibility to the governing of this country unlike they have had in the previous two years.”

But as Mr. Obama arrived at the Interior Department for the “don’t ask, don’t tell” bill signing Wednesday morning, compromise with his adversaries was the last thing on anyone’s mind.

Repealing the longstanding ban on gays’ serving openly in the military had been a major promise of Mr. Obama to gay Americans. For the audience, and for the president’s broader liberal constituency, making good on the promise offered new reassurance of his commitment to their causes.

The decision to push for an end to the policy puts new pressure on Mr. Obama to move on to other issues important to gay Americans, including repeal of the Defense of Marriage Act. Those issues are particularly thorny for the president, who is on record as opposing gay marriage.

In the meantime, however, the president and the people in the audience were clearly savoring a moment that had been a long time coming.

“We’re here, Mr. President. Enlist us now,” someone yelled as Mr. Obama took several pens in hand to sign the legislation.

After it was over, Mr. Obama waded into the crowd, working the front rows like the rope lines at the biggest rallies of his presidential campaign. Rally music played in the background.

But first, he had to sign the bill.

As he stood up, he smiled and said, “This is done.”