Several Republican senators and attorneys general, whose states filed the lawsuit against President Barack Obama's health care law, expressed confidence that the US Supreme Court would overturn the legislation.
The court on Tuesday heard a second day of arguments on Obama's signature legislative achievement. A third day was scheduled for Wednesday.
While the nine justices appeared divided over the constitutionality of reforms, attorneys from Florida and other states joined senators in saying their side could be gaining the upper hand.
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"You never know. These are only arguments... but I would say today the government had a tough day," Republican Senator Mike Johanns told reporters outside the US Capitol after listening to the case in the Supreme Court.
Attorney General Pam Bondi of Florida agreed.
"We've been in the courtroom now for two days. We feel very confident. We feel very pleased with the questions that the justices have been asking," she said.
Americans are sharply divided over the two-year-old health care law, derided by critics as "Obamacare" -- particularly the so-called individual mandate, which requires that all Americans buy health insurance.
Attorney General Jon Bruning of South Dakota said the 13 original states who filed grievances over the law -- there are now 26 involved in the case -- were "mocked roundly across the country by law professors who said this case will never end up in the Supreme Court."
But "here we stand... and I can tell you I feel a lot better after witnessing two hours of argument than I felt when we started," Bruning said.
All eyes were on Justice Anthony Kennedy, regarded as a swing vote in the court.
"Justice Kennedy was very skeptical of the mandate and the constitutionality of the mandate, and that makes me feel better about our chances," Bruning said.
Alan Wilson, attorney general for South Carolina, said Kennedy grilled Solicitor General Donald Verrilli about the most controversial part of the law.
Kennedy, according to Wilson, said the mandate "fundamentally changes the relationship between the people and the government."
Johanns said Kennedy "pointed out that if this really is going farther than any other case, which I believe all sides acknowledge that it is, that there is a heavy burden on the part of the government to justify their action -- to find authority within the constitution to do what they're doing."
Democratic senators, however, were optimistic the court would see things their way.
"I expect the court will uphold it," Senator Patrick Leahy said. "If we don't... I suspect it will be a major, major issue in the elections -- congressional and presidential -- this fall."
