And here we thought we were getting to know Mitt Romney. His selection of Rep. Paul Ryan of Wisconsin as his running mate, it seemed, would bring a much sharper definition to his presidential candidacy: The challenge to President Obama in an election dominated by uneasiness about the economy would come straight from the political right.

Or so it seemed, ever so briefly.

Instead, there are signs of potential fissures already between Mr. Romney and a running mate who is clearly determined to radically reshape fiscal policy. Most notably, Mr. Ryan would turn Medicare into a voucher system and cut federal spending — in almost every area except defense by $6 trillion over 10 years.

That has Mr. Romney sufficiently so ill at ease that he felt compelled to declare Sunday, with Mr. Ryan at his side, “I have my budget plan. And that’s the budget plan we’re going to run on.”

A fair notion, indeed, but only if Mr. Romney’s positions on such a critical issue are better known and more clearly articulated.

Why, for instance, does Mr. Romney feel he must promise to “make sure we can save Medicare?” Could it be that he needs to draw a contrast with the budget plan that Mr. Ryan has succeeded in making the favored position of Republicans in Congress?

For Mr. Romney to declare suddenly that he has determined the budget priorities of the GOP ticket raises all sorts of new questions. How do they differ with Mr. Obama’s positions (vague as they, too, can sometimes be)? How, as well, are they consistent with Mr. Ryan’s zealous advocacy for such sweeping cuts in essential social programs — ranging from Medicaid to food stamps to college loans to veterans programs to job training?

Here we were waiting for Mr. Ryan and the GOP’s congressional leaders to say exactly what they’d cut or which of those infamous tax loopholes they’d close; now we’re waiting for Mr. Romney to get specific. Where would he cut the budget if he’s to carry out his vow to reduce non-defense spending by 5 percent?

And how does he reconcile his plan to keep the Bush-era tax cuts and even extend them for the rich with Mr. Ryan’s more expansive call for some $4 trillion in additional tax cuts, including all capital gains taxes?

The answers can’t come soon enough and forthrightly enough. Will Mr. Romney go along with the idea of “saving” Medicare by leaving future senior citizens without health care if the government subsidies Mr. Ryan envisions don’t cover the bills? Would he, too, privatize Social Security and turn it into another Wall Street gamble?

Say this for Mr. Romney’s most critical political decision to date: It puts a bright and intensely serious student of government by his side, a substantial and substantive contrast to John McCain’s choice of Sarah Palin four years ago. But it’s Mr. Ryan’s fiscal philosophy that has made him the preferred candidate of influential conservative forces who were wary of Mr. Romney. What if they sense that Mr. Ryan was picked for his conservative credentials, but not the actual ideas behind them?

More elaboration from Mr. Romney is required to determine if he’ll be running with Mr. Ryan or, at times, from Mr. Ryan.

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