WASHINGTON (Reuters) - Republican presidential candidate Mitt Romney said on Thursday that the American people must defeat President Barack Obama in order to overturn his landmark healthcare overhaul.

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

(Reporting by Andy Sullivan, Deborah Charles and Steve Holland)