A rift has opened up between presidential challenger Mitt Romney and the rest of the Republican party over the question of whether a key aspect of Barack Obama's healthcare legislation is classified as a tax.

Chief justice John Roberts ruled the individual mandate to be constitutional under the federal government's taxation powers, a fact seized on by Republicans in Congress.

But on Monday, one of Romney's senior advisers, Eric Fehrnstrom, undercut the Republican party line in an interview with MSNBC. Fehrnstrom positioned Romney alongside the White House, which describes the levy on anyone who refuses to take out insurance as a "penalty".

Fehrnstrom said of Romney's position: "He disagreed with the ruling. He disagreed with the findings of the ruling. He disagreed with the logic that supported those findings. He said that he agreed with the dissent, which was written by Justice Scalia, and the dissent clearly stated that the mandate was not a tax."

The comments contrasted with those made by the Republican senate leader Mitch McConnell on Sunday, when he said he would turn the November Congressional elections into a referendum on what he described as the tax-raising healthcare legislation. McConnell, speaking after the supreme court ruling, was unequivocal. "The supreme court has spoken. This law is a tax," he said.

Fehrnstrom made a campaign gaffe in March when he spoke about an "Etch a Sketch" moment in the Republican presidential campaign. Romney's opponents seized on this at the time as evidence that the candidate's commitment to conservative policies was only superficial and he would move to the centre during the White House campaign.

Fehrnstrom's "tax" comment does not appear to be a gaffe, but instead reflects the dilemma that healthcare reform poses for Romney. As governor of Massachusetts, Romney introduced a similar policy to Obama's.

Romney has repeatedly described it as a penalty, not a tax. Fehrnstrom reiterated this in his MSNBC interview. "The governor believes what we put in place in Massachusetts was a penalty, and he disagrees with the court's ruling that the mandate was a tax."

Fehrnstrom's comments provoked a backlash from conservatives. Conservative commentator Stephen Hayes, who writes for the Weekly Standard, said on Twitter: "So the official, considered position of the Romney campaign is that the Obamacare tax isn't a tax? That makes no sense."

The Republican and Democratic parties launched ads Monday targeting members of Congress seen as vulnerable on the health issue.

The Democratic Congressional Campaign Committee began a series of robocalls aimed at Republican members of Congress it claims received funding from insurance companies.

"Democrats are on offense as we expose these House Republicans forstanding up for insurance companies and Congressional perks instead ofprotecting consumers," said DCCC chairman Steve Israel.

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