STORY HIGHLIGHTS

Washington (CNN) -- As Massachusetts governor, Mitt Romney imposed a penalty on people who could afford health insurance but chose to go without it. As Republican presidential candidate, Romney opposes the federal health care law that does the same thing.

Last week, Romney's top campaign adviser said the federal penalty for refusing to get health coverage was exactly that -- a fine, not a tax. This week, Romney said it is a tax because the Supreme Court opinion he opposes declared it a tax.

Confused? So is the Romney campaign, apparently.

The back-and-forth shows the tightrope Romney must walk on the health care reform issue now that he is the certain Republican presidential nominee.

Rebounding from last week's Supreme Court decision that upheld the constitutionality of President Barack Obama's signature legislative achievement, Republicans seek to galvanize conservative contempt for the measure into a wave of electoral support in November.

They know that the only way to repeal the 2010 health care law is a GOP landslide that wins the White House and both chambers of Congress, so they want to make the act a major campaign issue.

Specifically, Republicans have seized on the high court's 5-4 ruling that the government has the power to enforce the health insurance mandate through its taxing authority. They contend the ruling shows Obama lied to the American public when he repeatedly denied in 2009 that the individual mandate amounted to a tax increase.

However, Romney's support for the same kind of individual mandate -- the requirement that eligible people have health insurance -- during his term as governor in Massachusetts leaves him vulnerable to questions about where he really stands.

Former GOP candidate Rick Santorum focused on that vulnerability during the Republican primary campaign, constantly pointing out that Romney lacked credibility in opposing the 2010 federal law known as Obamacare because of his support for similar reforms in Massachusetts.

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Romney walks health care high wire