WASHINGTON (AP) — For President Barack Obama to turn his Supreme Court victory into a clear-cut political win this fall, he must do something other candidates have failed to do: make voters care that GOP opponent Mitt Romney once embraced the health care policies he now fiercely criticizes.

If Obama can't do that, then Romney may find it easy to fire up conservative activists who despise what they label "Obamacare," while also attracting moderate voters who simply dislike it.

And the Republican will have a new anti-tax argument, thanks to the high court.

On the policy front, Thursday's Supreme Court decision was a huge victory for Obama. It's trickier politically, however, because the court upheld the health care law under Congress' power to tax people, a rationale that surprised many.

In essence, the five-justice majority said, the penalty that Americans will start paying in 2014 if they refuse to obtain health insurance amounts to a tax.

Obama has insisted the fee is not a tax. He also notes that poor people would receive subsidies to buy insurance, which amounts to a tax cut. But his lawyers cited the tax-powers argument in their Supreme Court appearance.

Romney and fellow Republicans immediately launched a line of criticism that often has been potent: Democrats — in this case, Obama and his allies in Congress — are too eager to hike taxes.

"Obamacare raises taxes on the American people by approximately $500 billion," Romney said in his brief remarks.

Sen. Lindsey Graham, R-S.C., told Democratic lawmakers to "stand by your tax increase or stand with us to repeal and replace Obamacare."

Senate Minority Leader Mitch McConnell, R-Ky., said: "The Supreme Court has spoken. This law is a tax."

There's a political problem with that argument, Democrats quickly noted. Romney, while governor of Massachusetts, also required almost every resident to obtain insurance or pay a fee. As former Obama aide Neera Tanden put it, "If you call the mandate a tax increase, then Mitt Romney increased taxes in Massachusetts."

That might inoculate Obama from Romney's tax-hike accusations, if Democrats can make it stick. But Romney's GOP primary opponents repeatedly failed when they tried to wrap "Romneycare" around his neck.

Former Minnesota Gov. Tim Pawlenty reminded Republican voters that Obama "said that he designed Obamacare after Romneycare." Texas Gov. Rick Perry said, "I think Mitt is finally recognizing that the Massachusetts health care plan he passed is a huge problem for him."

Former Sen. Rick Santorum tried hardest of all. He said Romney's health care legacy made him "the worst Republican in the country to put up against Barack Obama."

Romney simply stuck to his criticisms of Obama's health law and avoided detailed discussions of its similarities to his Massachusetts initiative.

Some Democrats think Obama and his well-financed ally groups will do a much better job of painting Romney as a hypocrite or flip-flopper on health care.

Romney's GOP opponents "weren't exactly the most formidable of politicians, and they lacked the resources to really make it an issue on TV," Democratic strategist Doug Thornell said. Romney "is the godfather of the individual mandate and health reform," Thornell added, and thus ill-positioned to lead a fight on the issue.

Jim Kessler, co-founder of the Democratic-leaning group Third Way, said: "The individual mandate that Romney invented in Massachusetts is now a tax. That's a real pickle for Romney."

Republicans have options. They can have surrogates and lawmakers carry the health care battle against Obama, leaving Romney as far removed as possible.

Or it may turn out that voters don't care much about what Romney did as a one-term governor several years ago. That would free him to lead assaults on "Obamacare" with minimum damage.

If that happens, Thursday's court ruling could work against Obama's re-election hopes. Recent AP-GfK polls have found that more Americans oppose the 2010 health care law than support it. Opposition to the "individual mandate" — it would require most people to get insurance or pay a fee — was even deeper in a March poll.

Such findings delight Republican operatives. For Obama, Thursday's ruling was "probably the most damaging of all possible outcomes," GOP strategist Mike McKenna said.

"Identifying the mandate as a tax shears away all of the pretense," McKenna said. "It will energize everyone on the right, even those with deep reservations about Romney."

Another Republican campaign veteran, Terry Holt, said: "Obama might have his law, but the GOP has a cause."

Obama seemed eager to avoid the tax debate Thursday. He did not mention taxes in his 1,200-word speech before cameras.

The president's allies might be more willing to engage. If Republicans insist on calling the health insurance fee a tax, said Tanden, who now heads the Center for American Progress, "then the only person in America who has implemented that policy is Mitt Romney."