UPDATE:

WASHINGTON (AP) - The Supreme Court has upheld the individual insurance requirement at the heart of President Barack Obama's health care overhaul.

The court on Thursday handed Obama a campaign-season victory in rejecting arguments that Congress went too far in requiring most Americans to have health insurance or pay a penalty.

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WASHINGTON (AP) - The Supreme Court ruling on President Barack Obama's health care overhaul, now just hours away, marks a key pivot point in the presidential race. But neither side knows which direction the high court's decision will turn the contest.

"My guess is they're not sleeping real well at the White House tonight," presumptive Republican presidential nominee Mitt Romney told supporters Wednesday in northern Virginia.

Anticipation of Thursday's decision could be equally unnerving for Romney, whose opposition to the law has become a central pillar of his campaign.

Neither candidate has any direct influence over the ruling. The court could uphold the health care law, strike it down or deem the requirement that most Americans carry health insurance unconstitutional while retaining other aspects of the law.

The announcement is expected to be followed almost immediately by a barrage of advertisements and fundraising appeals from Democrats and Republicans, all trying to cast the decision in the most advantageous light for their candidates.

The Obama campaign began trying to raise money off the ruling even before it was announced. In a Thursday morning fundraising email with the subject line "Today's Decision", Obama campaign manager Jim Messina told supporters "no matter what, today is an important day to have Barack Obama's back."

The Democratic Congressional Campaign Committee also issued a fundraising appeal for a "health care rapid response fund," telling supporters in an email Wednesday that however the court rules, "Democrats are in for a tough fight."

Secrecy has marked each campaign's planning for the critical moment.

Obama will be in Washington and is expected to respond, but his specific plans to do so were unclear. Romney's campaign has refused to disclose the location of a Capitol Hill venue where he will face reporters shortly after the announcement.

It is clear, however, that the Supreme Court's ruling on Obama's sweeping federal health care law will shape the contours of the presidential campaign through the summer and fall. Both Obama and his Republican rival are primed to use the outcome - whatever it is - for political gain.

Obama has expressed confidence the court will uphold his signature legislative initiative. But he won't be shocked if a conservative majority overturns the most controversial provision, those familiar with his thinking say. Romney aides say the Republican candidate will get a political boost if the court strikes down the measure. But they don't want celebrations that could alienate voters who could lose health care benefits through the decision.

The court's ruling could have a far-reaching impact on the nation's health care system. If the law is upheld, about 30 million of the 50 million uninsured Americans would get coverage in 2014 when a big expansion begins.

Overturning all or part of the law could leave as uninsured the more than 3 million young adults who gained coverage through a provision allowing them to stay on their parents' insurance up to age 26, according to the Health and Human Services Department. Another 60,000 people who gained coverage through a plan for those with pre-existing conditions may not be able to get coverage elsewhere if the entire law is struck down.

Obama recently has avoided mentioning the impending court ruling directly, but he has vigorously defended the health care overhaul as critical to the public's health and well-being in campaign events this week.

"I think it was the right thing to do. I know it was the right thing to do," he told supporters in Boston.

Romney, who as Massachusetts governor signed a health care law on which the Obama's federal law was modeled, has focused more than usual on the Supreme Court ruling this week. In campaign appearances in Virginia, New Jersey and New York, he offered supporters and donors a preview of his likely response to the decision and said Obama's first term would be essentially wasted if the law is overturned.

If the court upholds the law, Romney told supporters at a northern Virginia electronics manufacturer Wednesday, it's still bad policy. "And that'll mean if I'm elected president we're going to repeal it and replace it," he said.

And if the court strikes down the law, Romney said, "They're going to be doing some of my work for me. "

Obama advisers say the Supreme Court showed reasonableness earlier this week in a ruling on an Arizona immigration case, and they see it as a hopeful sign for how the court might rule on health care.

If the court upholds the law, Obama could get an election-year gust of wind at his back, with his vision and leadership validated. If the court strikes down the overhaul, the White House would seek to cast the decision as detrimental to millions of Americans by highlighting popular elements of the law that would disappear, such as preventive care and coverage for young adults on parents' plans.

The Romney campaign has coordinated its response directly with the Republican National Committee and House Republicans, who have agreed not to "spike the ball" - as one Republican put it - should the law be struck down. His campaign worries that an over-celebratory tone may turn off voters affected by the decision.

Indeed, the stakes are high for both candidates. Polling suggests that most Americans oppose the law, but an overwhelming majority want Congress and the president to find a new remedy if it's struck down.

Romney so far has spent little time crafting a comprehensive plan to replace the overhaul. And the Obama campaign already has seized on Romney's opposition to the most popular provisions in the law. For example, Romney would not prevent health care companies from denying coverage to new customers with medical conditions. Nor would he force them to cover young adults on their parents' plans through age 26.

Still, both sides will use it to raise money and motivate supporters. And outside groups are ready to unleash a flood of advertisements following the ruling, including a 16-state, $7 million ad buy from the conservative political action group Americans for Prosperity.