Mr. Romney’s aides are exasperated by the skepticism, because he has formally checked the right conservative boxes, pledging opposition to same-sex marriage, strong support for Israel and the military budget, and embracing the stringent fiscal proposals of Representative Paul D. Ryan, Republican of Wisconsin, a Tea Party favorite. But his inconsistent history on social issues, in particular, leads some to question whether he would carry through as president.

Gary L. Bauer, the president of American Values, a Christian advocacy group, had been one of Mr. Santorum’s most visible supporters because, he said in an interview, Mr. Santorum best personified the Reagan legacy of fiscal and social conservatism and a strong defense. Still, he is not reluctant to shift his allegiance now, though with a warning.

“Going to the general election, I will do everything I can for Governor Romney,” he said.

“But his campaign has got to make it easy for me to help them,” he added, “and not make it hard by being tempted to pull back on conservative issues.”