President Obama is casting Mitt Romney as beholden to conservative activists such as Rush Limbaugh who are pushing the former governor to shift his views to the political right.

“The fact that a whole bunch of Republicans in Washington suddenly said this is a tax — for six years, he said it wasn’t, and now he suddenly reversed himself. And so, the question becomes ‘Are you doing that because of politics? Are you abandoning a principle that you fought for for six years simply because you’re getting pressure for two days from Rush Limbaugh,” Obama told NBC affiliate WLWT in Cincinnati in an interview.

The president was referring to Romney’s recent comments that the fine for people who don’t purchase insurance under “Obamacare” is effectively a tax. Romney backed a similar mandate in Massachusetts and did not describe it as a tax then. His campaign, like Obama’s, had also described it as a penalty, not a tax, but shifted from the position after sharp criticism from conservatives. (In fact, Limbaugh has attacked the entire health law and said the tax/penalty discussion is one of “semantics”).

Obama’s decision to invoke Limbaugh is not surprising, as he is a popular figure among conservatives but not the moderate voters both campaigns are courting.