By David A. Fahrenthold and N.C. Aizenman, Published: March 27

Political Washington spent Tuesday engaged in an odd parlor game that combined the guesswork of charades with the messaging confusion of “telephone.” Inside the Supreme Court, 500 people stared at nine justices, trying to divine the meaning behind questions, quips and even smiles.

Outside, a peculiar subset of the capital’s denizens were on their various electronic devices, eagerly trying to decode what those inside thought they saw.

By the end of Tuesday’s long-awaited oral arguments, the individual mandate — a crucial piece of President Obama’s health-care law — seemed to be in trouble. The solicitor general, charged with guarding the measure, had a rough outing under sharp questioning from the justices.

The result was that the two factions that had built their identities around the controversial law — one promising to defend it, the other to end it — were forced to consider a future without it. Proponents were hopeful; critics were encouraged. Neither had a Plan B.

“If they don’t uphold it, I suspect it will be a major, major issue in the elections — congressional and presidential — this fall,” said Sen. Patrick J. Leahy (D-Vt.). “The debate will be with the American people.”

Virginia Attorney General Ken Cuccinelli II (R), an opponent of the law, said he was buoyed by the arguments: “I went in cautiously optimistic about our prospects on the individual mandate, and I came out happier than when I went in,” he told reporters after the two-hour hearing.

Tuesday was the second of three days of oral arguments before the Supreme Court on the measure. But it was the one that counted most. The mandate that virtually every American must obtain health insurance by 2014 or pay a penalty is one of the linchpins of the legislation. Opponents say the mandate is unconstitutional, an improper extension of federal authority.

And so Washington’s attention was focused on a single room with the capacity of a high school auditorium.

The crowd included Attorney General Eric H. Holder Jr., Health and Human Services Secretary Kathleen Sebelius, lawmakers, state attorneys general, and dozens of journalists and lawyers. BlackBerrys were banned. It was a rare kind of moment in a town where senators tune out in hearings and House members speak to empty chambers.

There was nothing to do but listen.

“It reminded me [of] ‘Mr. Smith Goes to Washington,’ ” said Hans von Spakovsky, a lawyer at the conservative Heritage Foundation, recalling the scene in that movie when Jimmy Stewart’s character speaks to a rapt, and full, Senate chamber. “There are hardly ever any real debates . . .but that does happen in the Supreme Court.”

Lawyers in the crowd were most attentive to Justice Anthony M. Kennedy, who is expected to be a key swing vote in the case. In the first hour of arguments, Kennedy gave the law’s opponents hope by asking skeptical questions.

“Do you not have a heavy burden of justification,” he asked Solicitor General Donald B. Verrilli Jr., “to show authorization under the Constitution?”

But as the arguments went on, Kennedy asked other questions that encouraged the measure’s supporters. He seemed at one point to accept an argument key to the Obama administration’s case: that people who don’t buy health insurance are still in the health-care market, because they will need care at some point.

“They are in the market in the sense that they are creating a risk that the market must account for,” Kennedy said.

So what does that all mean? Advocates on both sides of the case took Kennedy’s statements as a signal that he and the court will rule in their favor.

“I am still hopeful that we have a majority on the court to uphold the act,” said Elizabeth B. Wydra of the Constitutional Accountability Center. She supports the law. “You know, hopefully, 6 to 3.”

Said Von Spakovsky, who opposes the measure: “I think it’s at least 50-50” that the mandate will be struck down.

“Oh, I think it’s better,” said his Heritage Foundation colleague Todd Gaziano, sitting across a conference table at the group’s headquarters. He had taken extra encouragement from a particular smile from Justice Samuel A. Alito Jr. Gaziano thought that was a sign that the Obama administration’s case was slipping away. “I think it’s an 80 percent chance.”

“I’d probably go to 60,” Von Spakovsky said, encouraged by his colleague’s optimism.

Outside the courtroom, things looked even more muddled.

“It’s just the damnedest thing in the 21st century,” said Russell Wheeler, a legal expert at the Brookings Institution. He was hitting a “refresh” button every three to four minutes, scanning law blogs. His heart sank and then rose as he heard secondhand reports about Kennedy’s questions. “With all this technology, we’re . . . basically looking for puffs of smoke to come out of the court.”

As Tuesday went on, a consensus began to emerge: It had been a good day for the law’s opponents, raising the odds that some or all of it will be overturned.

“Overall, there were four justices up there who articulated every aspect of our argument as well as we did, if not better,” said Randy Barnett, a Georgetown University law professor who has led the fight against the law. In an interview on the court steps, Barnett said he was counting Chief Justice John G. Roberts Jr., as well as Alito and Justices Antonin Scalia and Clarence Thomas. “I don’t see how we can ask for anything better than that.”

There also was criticism for Verrilli, who seemed to struggle at times in the face of skeptical queries.

The real importance of Tuesday’s arguments won’t be known until June, when the court is expected to announce its decision. Leading Democrats said there are no plans for a legislative fallback position if the mandate is overthrown, saying they could not decide how to respond until after reading such an opinion.

“You cannot base what the court is going to do based on an oral argument. It’s nice, it’s good to speculate as to what might happen, but believe me, those nine men and women are extremely smart, and a lot of times they probe with those questions, not in any way to tip their mitt as to how they’re going to vote on it,” Senate Majority Leader Harry M. Reid (Nev.) told reporters Tuesday afternoon.

Republicans were fuzzy on how they would proceed should the law be struck down. Senate Minority Leader Mitch McConnell (Ky.) on Tuesday would say only that if Republicans won control of the Senate in the fall, they would seek to replace the legislation with “something that makes more sense and is targeted at the problems that we actually have in American health care.”

At the end of the day, the parlor game wound up with this: On CNN’s “Situation Room With Wolf Blitzer,” Democratic strategist James Carville was trying to spin a Supreme Court defeat that hadn’t happened.

“I think this will be the best thing that has ever happened to the Democratic Party,” he said. If the law is struck down, and the health-care system reverts to its old problems, Carville said, that would be a good thing: “Then the Republican Party will own the health-care system for the foreseeable future. And I really believe that. That is not spin.”

Staff writer Paul Kane contributed to this report.