Despite the president’s signature, the legislative work on the bill is not over, nor is the partisan tussle over it. Republicans on Tuesday renewed their vow to repeal the measure, albeit with a fresh slogan, “repeal and replace,” in a nod to the political difficulties of campaigning to overturn a measure that includes popular new benefits, like allowing young people to stay on their parents’ plans until age 26.

Attorneys general in more than a dozen states, most Republican, filed lawsuits contending that the measure is unconstitutional. In the Capitol, the Senate opened what is expected to be a contentious debate on a measure that contains the final revisions to the health bill. Democrats and Republicans are bracing for a fierce fight that is expected to last the balance of the week. The Republicans have said they will try to block the measure, or at least use procedural weapons to punch as many holes in it as possible by striking out key provisions.

Democrats urged Republicans to stand down, given that the measure is already law.

“Now it is a fact,” declared Senator Max Baucus, Democrat of Montana. “Now it is law. Now it is history. Indeed, it’s historic.”

Image Republican lawmakers plan to put up roadblocks to the health reconciliation bill. At a news conference on Tuesday, from left, were Senators Jon Kyl, Judd Gregg and Mitch McConnell. Credit... Luke Sharrett/The New York Times

Tuesday’s signing ceremony was the beginning of what will be an intense sales pitch by the White House and leading Democrats to convince Americans of the benefits of the health bill. As soon as it was over, Mr. Obama went into campaign mode, traveling to the Interior Department — the federal building with the biggest auditorium the White House could find — to address a crowd of more than 500 cheering doctors, nurses, patients and federal employees.

It was a remarkable turnabout from just two months ago, when many Democrats thought the bill was dead after Scott Brown, a Massachusetts Republican, won Mr. Kennedy’s old Senate seat. His victory deprived Mr. Obama of his 60-vote supermajority and left Democrats deeply nervous. At the White House on Tuesday, they seemed jubilant, even giddy.

They chanted “Nancy! Nancy! Nancy!” as the House speaker, Nancy Pelosi, who had kept her skittish caucus together, entered the East Room. They posed for pictures in front of the president’s podium as they waited for Mr. Obama to arrive. When he did, accompanied by Vice President Joseph R. Biden Jr., the audience broke into his standard call-and-response campaign chant: “Fired Up! Ready to go!”