WASHINGTON – Mitt Romney’s assertion that President Obama’s mandate to buy health insurance amounts to a tax increase may boomerang by opening the likely Republican nominee to the same charge.

As Massachusetts governor, Romney championed a state health care plan with a similar mandate to buy insurance – one he says today should be described as a penalty, not a tax. He also raised hundreds of millions of dollars for the state budget by ending some tax benefits for businesses and approving higher fees on professional licenses and identification cards for the blind.

The fees and corporate tax changes allowed Romney to close a projected $3 billion state budget gap, while preserving his pledge not to raise taxes. Not everyone saw it that way.

“Any time the government mandates that you buy something or imposes a penalty, it’s a tax by any other name,” said Matt Kibbe, president of FreedomWorks, a group aligned with the anti-tax tea party movement. Kibbe said he still intends to support Romney.

The shifting political rhetoric means Romney is “playing defense,” said Dan Schnur, director of the Jesse M. Unruh Institute of Politics at the University of Southern California and former communications adviser to Republican Arizona Sen. John McCain’s 2008 presidential campaign. “Romney doesn’t want to talk about Massachusetts. He wants to talk about what’s wrong with the American economy.”

The line of attack on Romney, the presumptive Republican presidential nominee, isn’t new: In primary debates, opponent Rick Santorum highlighted the fee increases. An Obama campaign ad charges him with enacting “over 1,000 fee hikes” on such services as school bus rides, hospitals and health care.

In an interview with a Cincinnati television station, Obama framed Romney’s comments as a flip-flop done for political convenience. Romney’s characterization of the mandate as a tax reversed six years of calling it a penalty, Obama said.

“The question becomes, are you doing that because of politics?” the president said in the interview that aired Friday. “Are you abandoning a principle that you fought for, for six years simply because you’re getting pressure for two days from Rush Limbaugh or some critics in Washington?”

Still, it’s a turn in the political conversation that troubles some Democrats, too.

“Why does the Obama campaign want to have a fight over who raised the most taxes?” said Tad Devine, a Democratic political strategist,

Speaking to reporters Thursday, Obama spokeswoman Jen Psaki dismissed the discussion over whether the mandate is a penalty or a tax as a “silly debate.”