Adopting a new tone, Mitt Romney on Sunday said he would retain elements of President Obama’s health care overhaul, blamed Republicans as much as Democrats for the “mistake” of agreeing to automatic cuts in military spending and said Mr. Obama’s national security strategy had made America in “some ways safer.”

The remarks were a marked departure from Mr. Romney’s frequently harsh and openly partisan critiques of the president on the campaign trail over the last year, and seemed to amount to a different tenor now that he has officially become the Republican presidential nominee: bipartisanship, of sorts.

They came in a rare interview on the NBC News program “Meet the Press,” and not the friendlier terrain of Fox News, on which Mr. Romney prefers to appear.

The approach, however fleeting it may be, appeared to be a direct and deliberate appeal to middle-of-the-road voters who have not made up their minds yet and are likely to decide the race. At one point, Mr. Romney said the speech last week by the country’s previous Democratic president, Bill Clinton, had “elevated” the party’s convention in Charlotte, N.C.