In defending Mr. Romney against an attack ad highlighting a cancer patient who had no health insurance, the campaign touted the Massachusetts health care plan, coming perilously close in the minds of some conservatives to sounding like the president.

“If people had been in Massachusetts, under Gov. Romney’s health care plan, they would have had health care,” Andrea Saul, a spokeswoman for Mr. Romney, said Tuesday. That prompted Erick Erickson, of the conservative blog, Redstate, to write on Twitter: “OMG. This might just be the moment Mitt Romney lost the election. Wow.”