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The court will hear two hours of arguments over the individual mandate on Tuesday, while the expansion of Medicaid and whether the mandate can be severed from the rest of the Affordable Care Act is on Wednesday’s docket.

A CNN poll released Monday found that 73 percent of Americans want the court to overturn at least some of the health care reform’s provisions, while 23 percent want to keep the law as is.

Just across the street, congressional lawmakers continued highlighting their differences over the law, with Senate Minority Leader Mitch McConnell, Kentucky Republican, blasting it from the Senate floor.

“We’re basing our opinion on something simpler than the legal arguments we’ll hear this week,” Mr. McConnell said. “We’re looking at whether this law helped or hurt. And on that question, the verdict’s already in. Just like so much else this president has done over the past few years.”

In a rare moment where both sides agreed in Monday’s hearings, plaintiffs’ attorney Gregory Katsas also argued against applying the AIA to the case. He represents the National Federation of Independent Businesses and 26 states bringing the lawsuit.

Both sides said that if Congress had viewed the penalty as a tax, it would have clarified that. But Mr. Long argued that by directing the penalty to be “assessed and collected in the same manner as a tax,” Congress was intentionally placing it in the same category as a tax.

Justice Breyer seemed to disagree. “It being collected in the same manner as a tax doesn’t make it a tax,” he said.

Justice Breyer said that if the court decides the penalty isn’t a tax, it could give future courts wide latitude to determine what counts as a tax and what doesn’t. Giving future judges too much wiggle room “plays a significant role in my mind,” he said.

“We don’t want 500 federal judges substituting their opinion,” he said.