ANN ARBOR, Mich. — Former Gov. Mitt Romney of Massachusetts came here on Thursday to address the biggest threat to his nascent presidential campaign, defending core elements of the health care law enacted in his home state five years ago even as he tried to reassure conservatives that he would work to roll back the similar national overhaul President Obama signed into law last year.

It was the start of a treacherous balancing act for Mr. Romney, one that forced him to confront not just the complexities and passions surrounding health policy but also questions about his willingness to stick to his principles under political pressure.

Speaking before a group of doctors, health policy experts and local officials at the University of Michigan medical complex, he embraced the aspect of the Massachusetts law most criticized among Republicans, the mandate that individuals buy insurance. He said it was necessary, given the needs of his state at the time.

But Mr. Romney said that if he were president he would seek to repeal the new national health care law with its similar mandate, arguing that it was inappropriate for the federal government to prescribe such a sweeping measure for states.