But the liberals are outnumbered on the court, 5-4.
The law's supporters had pinned their hopes on Justice Anthony Kennedy, who has often cast the swing vote in decisions that split on ideological lines.
But Kennedy seemed just as skeptical of the mandate as the other conservatives. He described it as "unprecedented" and saw a "heavy burden of justification" to uphold it under the Constitution.
Without Kennedy's vote, the law's supporters face long odds.
Verrilli seemed ill at ease from the start of his hour-long argument. As he began his prepared remarks, he hesitated, coughed, and then repeated his first sentence. He never seemed to gain a full stride.
The liberal justices used several occasions to make Verrilli's arguments for him. Justice Stephen Breyer asked whether Supreme Court precedents didn't already uphold Congress' power to create new forms of commerce, like a health insurance market. Kagan observed that healthy people who subsidize insurance for the sick will one day become the subsidized themselves.
Verrilli struggled most with repeated questioning from conservatives over the limits on Congress' power. Chief Justice John Roberts asked whether a mandate to purchase health insurance you may need some day was any different from a requirement to buy a cellphone for use in a possible emergency. Alito asked whether the government could force Americans to buy burial coverage, since it is something everyone will eventually need.
Verrilli responded that Congress can regulate the method of paying for something, like health care, in advance, if entry into the market is virtually certain. But the conservatives seemed unconvinced.
The justices seemed more receptive to the possibility of upholding the mandate on a different ground. The government argued that the penalty for failing to have insurance is really just a tax, and Congress has broad power to impose taxes.
However, even on this point, Verrilli faced considerable skepticism.
Former Solicitor General Paul D. Clement, arguing against the law for the 26 states that brought the challenge, faced an opposite constellation of questions. The four liberals asked the most, led by Breyer, who asked him repeatedly to respond to different justifications for the mandate. The conservatives were much quieter, with Scalia and Alito, in particular, asking Clement very little.
The same pattern held for Michael A. Carvin, who followed Clement in opposing the mandate on behalf of the National Federation of Independent Businesses.
While the justices were playing their hands carefully inside the courtroom, outside there was little question where people stood. A stream of protesters paraded in front of the courthouse steps all morning.
Some sang, some chanted, and some stood quietly holding signs. A group of African American physicians from the National Medical Association posed for pictures in white lab coats with buttons supporting the law. A tea party supporter stood next to them.
"You don't really practice, do you?" she asked. "Don't you mostly teach?"
"We certainly do practice," one of them replied.
The next question before the court is what it will do if it strikes the mandate down. Will it keep the rest of the law intact, or throw the whole thing out? Arguments on that question are slated for tomorrow.
Court watchers continue to look for clues from all justices but one. Clarence Thomas is widely expected to rule against the mandate based on past comments. But you couldn't read that from his response to the oral arguments. True to his long-standing practice, he said nothing.
Robert I. Field is professor of law at Drexel University Earle Mack School of Law and professor of health management and policy at Drexel University School of Public Health.