WASHINGTON (AP) - Barely four months before the nation votes, one of the biggest factors in the fight for the White House still is a mystery. That will change on Thursday.

The Supreme Court's expected ruling on President Barack Obama's sweeping federal health care law will shape the contours of the presidential campaign through the summer and fall. Both Obama and Republican rival Mitt Romney are primed to use the ruling - whatever it is - for political gain.

Obama expresses 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.

Neither candidate has any direct influence over the decision. The court may uphold the health care law, strike it down or deem the requirement that most Americans carry health insurance unconstitutional while keeping other aspects in place.

(AP) A view of the Supreme Court in Washington, Wednesday, June 27, 2012. Saving its biggest case for... Full Image

The ruling is expected to be followed almost immediately by a barrage of advertisements and fundraising appeals from Democrats and Republicans, with both sides trying to cast the decision in the most advantageous light for its candidate.

Romney, running on a pledge to repeal Obama's overhaul as a costly federal power grab, 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.

"My guess is they're not sleeping real well at the White House tonight," a confident Romney told cheering supporters gathered Wednesday evening at a Sterling, Va., electronics manufacturer.

The night before, he told donors in New Jersey that if the Supreme Court lets the law stand, "it will make it very clear to the American people that they must elect someone who will stop it." If the high court overturns the law, "then the first three and a half years of the Obama administration will have been entirely wasted, because that's where he devoted his energy and passion," the Republican said.

Romney's campaign also is running new ads this week in Virginia, North Carolina and Iowa promising he would move to "repeal Obamacare" on his first day in office.

(AP) President Barack Obama speaks during an campaign fundrasising concert at the Fillmore Miami Beach... Full Image

Obama, while recently avoiding mentioning the impending court ruling directly, has vigorously defended the overhaul as critical to the public's health and well-being in his own 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.

The White House also published a blog post Wednesday touting the benefits of the overhaul, including free preventive services for people on Medicare and health insurance rebates for nearly 13 million Americans.

Both Obama and Romney were scheduled to be in Washington on Thursday. Romney planned to comment on the ruling during an event on Capitol Hill, and Obama was certain to address the decision as well.

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.

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