WASHINGTON (Reuters) - Republican presidential candidate Mitt Romney called on voters to help him defeat President Barack Obama in order to repeal a landmark healthcare law that was upheld on Thursday by the U.S. Supreme Court.

"This is a time of choice for the American people. If we're going to get rid of Obamacare, we're going to have to replace President Obama," Romney said. "My mission is to make sure we do exactly that."

The decision is a setback for Romney and his fellow Republicans, who had hoped that the Supreme Court would strike down Obama's central policy achievement. They may have better luck at the ballot box.

Most Americans oppose the law even though they strongly support much of what it does, according to a Reuters/Ipsos poll released on Sunday. Obama's Democrats acknowledge that they have done a poor job of selling the Affordable Care Act, as it is known.

omney repeated his vow to repeal the law if elected. But even if his fellow Republicans win both chambers of Congress in the Nov. 6 elections, Democrats will likely still retain enough seats in the Senate to keep the law in place.

"What the court did today was say that Obamacare does not violate the Constitution," Romney said on the roof of a building overlooking the U.S. Capitol. "What they did not do is say that Obamcare is good law, or good policy."

(Reporting by Andy Sullivan, Deborah Charles and Steve Holland; Editing by Jackie Frank)