WASHINGTON – The Supreme Court should uphold a law requiring most Americans to have health insurance if the justices follow legal precedent, according to 19 of 21 constitutional law professors who ventured an opinion on the most-anticipated ruling in years.

Only eight of them predicted the court would do so.

“The precedent makes this a very easy case,” said Christina Whitman, a University of Michigan law professor. “But the oral argument indicated that the more conservative justices are striving to find a way to strike down the mandate.”

A ruling on the constitutionality of the Patient Protection and Affordable Care Act’s individual mandate is among the last pieces of business heading into the final week of the Supreme Court’s term. Bloomberg News last week emailed questionnaires to constitutional law experts at the top 12 U.S. law schools in U.S. News & World Report magazine’s 2012 college rankings.

Five of the 21 professors who responded, including Whitman, said the court is likely to strike down the coverage requirement. Underscoring the high stakes and complexity of the debate, eight described the outcome as a toss-up.

During arguments in March, four justices appointed by Republican presidents questioned Congress’ constitutional power to enact the mandate, including Chief Justice John Roberts and Justice Anthony Kennedy, who had been viewed as potential swing votes. A fifth, Justice Clarence Thomas, rarely speaks during courtroom sessions. Questioning by four Democratic appointees was more sympathetic to the provision, a centerpiece of President Obama’s health care law.

“There was certainly a lot of hostile questioning by the more conservative members of the court,” said Jesse Choper, a law professor at the University of California at Berkeley who described the court as likely to support the mandate. “It’s relatively straightforward — if they adhere to existing doctrine, it seemed to me they’re likely to uphold it.”

There was broad agreement that the ruling, barely four months before November’s presidential election, has the potential to hurt the Supreme Court’s reputation as an impartial institution.

Eighteen of the 21 professors said the court’s credibility will be damaged if the insurance requirement — which passed Congress without a single Republican vote — is ruled unconstitutional by a 5-4 majority of justices appointed by Republican presidents.

“When you take the fact of a high-profile, enormously controversial and politically salient case — to have it decided by the narrowest majority with a party-line split looks very bad, it looks like the court is simply an arm of one political party,” University of Chicago Law Professor Dennis Hutchinson said.

Nine of the law professors said if the coverage mandate is invalidated the justices are likely or very likely to throw out several related provisions, such as requiring insurance companies to offer policies without regard to pre-existing medical conditions. Five respondents said the justices will leave those provisions in place.

By a large margin, 15 of the 21 professors predicted the Supreme Court won’t kill the entire law, even if justices throw out the insurance mandate and related provisions. Only three said the rest of the statute is likely to be voided and three called it a toss-up.

The law, the biggest overhaul of the U.S. health-care system since the enactment of Medicare and Medicaid in 1965, is designed to extend coverage to at least 30 million uninsured Americans and would reshape an industry that makes up about 18 percent of the U.S. economy.

The law professors split over whether they expect Republican and Democratic court appointees to line up on opposing sides: Eight said a partisan divide is likely, eight said it isn’t likely, and five called it a toss-up.

“I continue to find it extremely unlikely that Justices Roberts and Kennedy will support a 5-4 decision that has such an insubstantial basis in 75 years of Supreme Court case law,” said Yale University Professor Bruce Ackerman, the only respondent who said the court is very likely to uphold the insurance-coverage requirement.

Choper, who was a clerk for Chief Justice Earl Warren in the 1960s, said in a telephone interview that questioning by justices doesn’t always predict how they’ll vote.

“You can be misled by oral arguments,” he said. “You can’t rely on their tone.”

A party-line vote would prompt questions about political motives because most constitutional scholars thought courts would uphold a mandate, before small-government tea party activists rallied Republicans around complaints that Congress exceeded its authority to regulate interstate commerce, said Harvard University Law Professor Charles Fried.

“It’s become just a very partisan battle cry on behalf of an argument which a few years ago was thought to be completely bogus,” Fried, who represented Republican President Ronald Reagan’s administration at the Supreme Court as U.S. solicitor general from 1985 to 1989, said in a telephone interview. “For objective observers on all sides, this was thought to be a lousy argument and the only people who were making it were sort of the wing nuts.”

In a separate Bloomberg National Poll of 1,002 adults released this week, 71 percent of Americans said politics will influence the Supreme Court’s ruling, compared with 20 percent who said the court will decide the case solely on legal merits.

Requiring Americans to have health coverage was proposed more than two decades ago by the Washington-based Heritage Foundation, which describes itself as a conservative public policy research institution. Before it was incorporated into Obama’s health-care law, the notion of an individual mandate had Republican support as an alternative to Democratic proposals for a single government-run health system or mandatory coverage by employers.

Obama’s Republican re-election opponent, Mitt Romney, has opposed the nationwide insurance requirement. While a Massachusetts state law enacted when he was governor includes a coverage mandate, Romney has said states may adopt policies that aren’t appropriate for the country as a whole.

University of Pennsylvania Professor Kermit Roosevelt said Kennedy is a key vote and that, “whatever happens,” Roberts is likely to be in the majority.

“If Kennedy joins the four Democratic appointees to uphold, I think Roberts will vote that way, too,” Roosevelt said in an email. “If Kennedy goes the other way, Roberts will probably vote to strike it down.”

Hutchinson, who has edited the University of Chicago’s Supreme Court Review journal since 1981, said it’s important for the public to view the high court as staying above the political fray and providing “a disinterested interpretation” of the Constitution.

“We believe in something called the rule of law,” he said. “That’s why we have faith in courts, that they’re not just another arm of a political party, so it’s worth respecting their judgments.”