Conservatives initially saw the selection of Mr. Ryan as a hopeful sign that Mr. Romney had fully embraced their small-government agenda and eagerness to turn the election into a head-on clash of ideologies. But now, with Mr. Romney encountering a host of problems and Republicans openly fretting about the outcome in November, Mr. Ryan’s slow fade back into the secondary and sometimes afterthought role traditionally played by running mates has given conservatives a new outlet for frustration over the state of the race.

“I was enthused when Mitt Romney picked Paul Ryan because I thought it was a signal that this guy was getting serious, he was getting bold,” Gov. Scott Walker of Wisconsin, Mr. Ryan’s home state, told a Milwaukee radio show host on Friday. “It’s not necessarily even a frustration over the way Paul Ryan’s been used but rather in the larger context: I just haven’t seen that kind of passion I know that Paul has transferred over to our nominee, and I think it’s a little bit of some pushback from some of the folks in the national campaign.”

Mr. Ryan still has high-profile moments of combativeness and takes on fights that Mr. Romney does not. On Friday, he appeared at the annual AARP convention and drew boos as he called for repeal of Mr. Obama’s health care law and laid out the approach that he and Mr. Romney would take to address Medicare’s financial troubles, which would encourage more private-sector competition in the government-run program.

It was a classic example of what Chris Chocola, president of the conservative political action committee Club for Growth, admiringly called Mr. Ryan’s “ ‘You’re damned right’ answer” to critics.

“If someone says you’re going to change Medicare as we know it, you say, ‘You’re damned right.’ Paul Ryan can give that answer,” Mr. Chocola said, adding, “The Romney ticket would be well served to let Paul Ryan be Paul Ryan.”