WASHINGTON — Marking a pivotal point in the presidential campaign, the Supreme Court's decision to uphold President Barack Obama's sweeping federal health care law handed the Democratic incumbent crucial election-year vindication for his signature legislative accomplishment.

Republican rival Mitt Romney, an ardent opponent of the law, prepared to use the decision for his own political gain and planned to cast himself as the next best hope for the millions of Americans who favor the law's repeal.

The decision put an end to what had been one of the biggest unknowns in the presidential race. Four months from Election Day, both Obama and Romney will seek to use the high court ruling to bolster their vision for the country, as well as raise money for their campaigns.

Both were expected to comment on the decision later Thursday from Washington, with Obama scheduled to speak at the White House around midday.

The high court announced Thursday, in a 5-4 decision, that it was upholding the requirement at the heart of the health care law: that most individuals must buy health insurance or pay a penalty.

The decision means the historic overhaul will continue to go into effect over the next several years, affecting the way people receive and pay for personal medical care. The ruling also handed Obama a campaign-season victory in rejecting arguments that Congress went too far in requiring most Americans to have health insurance.

The Obama and Romney campaigns have been quietly preparing for months how they would respond to the ruling.

While the White House publically expressed confidence that the overhaul would be upheld, Obama aides feared the political ramifications for the president if the law were to be overturned.

In anticipation of the law being upheld, Romney aides cautioned against excessive celebration, fearing that could alienate voters who could lose health care benefits as a result of the decision.

Romney, who as Massachusetts governor signed a health care law on which the Obama's federal law was modeled, previewed his likely response to the decision during a campaign event earlier this week.

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.

• Click here to see video and a detailed infographic that breaks down what this ruling means to you

The court's ruling will have a far-reaching impact on the nation's health care system. About 30 million of the 50 million uninsured Americans would get coverage in 2014 when a big expansion begins.

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.

The court's announcement was 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.

Obama's 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 by email Wednesday that, however the court rules, "Democrats are in for a tough fight."

Outside groups also are ready to unleash a flood of advertising, including a 16-state, $7 million ad buy from the conservative political action group Americans for Prosperity.

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AP White House Correspondent Ben Feller contributed to this report.

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