If conservatives needed any more motivation to unseat President Obama, they got it Thursday from the Supreme Court, which provided fresh political opportunities for Mitt Romney even as it handed the president a legal victory.

Chief among those new lines of attack is the court’s determination that the law’s individual mandate — the penalty it would impose on people who refuse to buy insurance — amounts to a tax. Obama had previously insisted it was “absolutely not a tax increase.”

“The most effective argument for Romney is that this is a massive tax increase that will impact hardworking Americans,” said GOP strategist Ron Bonjean, who worked for a number of Republican leaders on Capitol Hill. “By adding the health-care tax to economic hardships people are feeling, it will quickly become a rallying cry to win over independents in battleground states.”

Romney’s history, however, may make it difficult for him to capi­tal­ize on that argument.

The health-care system that he put into place as Massachusetts governor — which was a model for the federal law — included a mandate with a similar penalty for noncompliance.

At the time, Romney also denied that it was a tax, preferring instead to refer to it as a “fee” or an “incentive.”

Nonetheless, the court’s ruling will encourage Republicans to rally around their presumed nominee, despite their misgivings about some of the more moderate aspects of his record, including the Massachusetts health-care law.

“Politically, this was not a bad result for Governor Romney,” said Steve Schmidt, who was a top strategist for 2008 GOP nominee John McCain (R-Ariz.). “There is no chance there will be a dissipation of intensity in the Republican base. It guarantees a united and intense Republican base.”

Rep. Phil Gingrey (R-Ga.) went so far as to predict on CNN: “This is going to elect Mitt Romney the 45th president of the United States.”

Romney moved quickly Thursday to try to seize the moment, arguing that if voters want to get rid of the unpopular law, their only recourse is to replace the occupant of the Oval Office.

“What the court did not do on its last day in session, I will do on my first day if elected president of the United States,” Romney said at a rooftop news conference in downtown Washington, with the Capitol as his backdrop. “And that is, I will act to repeal Obamacare.”

One of the first indications of the fervor unleashed by the Supreme Court decision was the effect it had on Romney’s campaign coffers. Barely three hours after the ruling, Romney spokeswoman Andrea Saul said that more than $1 million had come in from nearly 10,000 donors.

Whether that enthusiasm will continue into the fall — and resonate beyond the conservative base to swing voters — is likely to depend on how well Romney can incorporate the health-care issue into his larger message about jobs and the economy. Those remain at the top of most Americans’ concerns.

“The debate is going to be reframed around: Does [the health-care law] really stop job creation? Is it taxing people who are already in a dire situation?” said Robert J. Blendon, a professor at Harvard University who directs its research program on public attitudes toward major social policy issues.

Republican pollster David Winston agreed.

“The challenge for Governor Romney is not just in the health-care context, but also putting it into the broader economic context,” Winston said, adding that Romney should stress that the best way for most Americans to obtain health coverage is to have a job.

In his statement after the ruling, Romney called the health-care law “a job killer” and said, “Three-quarters of those surveyed by the Chamber of Commerce said Obamacare makes it less likely for them to hire people.”

Polls have consistently shown that fewer than half of Americans approve of the new health-care law. Their views also diverge sharply among party lines: Almost nine out of 10 Democrats overwhelmingly support it, while about two-thirds of Republicans disapprove, and only about 40 percent of independents have a favorable view.

But individual elements are quite popular — among them closing a gap in Medicare prescription drug coverage known as the “doughnut hole,” allowing dependents to stay on their parents’ health policies until age 26, and requiring that insurance companies offer coverage to those with preexisting illnesses.

Whether public acceptance of the overall law will grow as it is more fully implemented between now and 2014 remains to be seen.

In vowing to introduce a bill repealing the health-care law on his first day in office, Romney has promised to replace it with a system in which states can tailor their own programs.

He has also promised that people with preexisting conditions will not lose coverage — so long as they continuously maintain insurance. Romney’s proposal would not, however, guarantee insurance for ill people who currently lack it.