Anxiety is rising among some Republicans over the party’s embrace of a plan to overhaul Medicare, with GOP lawmakers already starting to face tough questions on the issue at town hall meetings back in their districts.

House leaders have scheduled a Tuesday conference call in which members are expected in part to discuss strategies for defending the vote they took this month on a budget that would transform the popular entitlement program as part of a plan to cut trillions in federal spending.

Democrats, eager to win back the seniors and independents who abandoned the party in last year’s midterm elections, have declared the vote a “moment of truth” and this week launched a media campaign accusing GOP House members of dismantling Medicare and endangering retirees.

The assault has taken some Republicans by surprise, prompting concerns that the party is ceding ground in a policy debate that GOP strategists already viewed as perilous.

Some Republicans fear a repeat of 2005, when President George W. Bush tried to turn the political capital of his reelection into a push to privatize Social Security. Republicans abandoned the effort after Democrats vigorously attacked them, accusing the GOP of trying to cut benefits.

A Washington Post/ABC News poll published this week found that two-thirds of Americans want Medicare to remain as is. That includes 62 percent of independents and nearly eight in 10 people 65 and older — making for an uphill climb for House Republicans trying to reassure constituents.

“People have to believe there’s a fundamental problem with Medicare that has to be fixed,” said Carl Forti, a veteran GOP strategist. “If they don’t believe there’s a problem, then why would they support a change?”

Republican officials said they understood the political risks of tackling entitlements, but they point to other polling numbers showing that the public supports spending cuts and tends to trust Republicans more than Democrats on fiscal issues.

“Republicans don’t want to be talking about Medicare every day for the next year and a half,” said a Republican Party official, speaking on the condition of anonymity to address internal strategy deliberations. “But if they keep the conversation on broader issues of spending and taxes, they can win.”

The GOP official added that the party “can fight the Medicare issue to a tie” by “muddying the waters” and painting Democrats as choosing status-quo options that would have Medicare “die a slow death.”

The tensions underscore the central role Medicare is likely to play in the upcoming policy battles over spending cuts — and then in election campaigns next year.

President Obama and House Republicans said they are open to tweaking Social Security, but neither side has revealed detailed plans — in effect removing that program, at the moment, from the partisan cauldron.

The GOP budget plan drew overwhelming support this month from the House Republican majority, with just four members voting against it.

The Medicare provision, one piece of the larger document drafted by House Budget Committee Chairman Paul Ryan (R-Wis.), would fundamentally change Medicare from the federal entitlement it has been since the 1960s. Seniors who join Medicare in 2022 and beyond would receive subsidies, or “premium supports,” to help pay for private insurance they would buy on their own.

Republicans argue that Medicare is on a path to bankruptcy and will not be available for future generations without a dramatic overhaul. Democrats, whose support among seniors has been slipping in recent elections, say the plan would mean a sharp reduction in benefits, and they see the debate as a political bonanza.

A newly formed pro-Democratic group, the House Majority PAC, is spending as much as $150,000 on radio ads in 10 GOP districts, accusing lawmakers of trying to “end Medicare as we know it,” according to a person familiar with the effort.

Rep. Steve Israel (D-N.Y.), chairman of the Democrats’ House campaign committee, said Friday that the GOP’s vote on Medicare would be the “defining vote that secures the majority for the Democrats.” He said the issue “is the most intense driver in the polls for independents and seniors.”

The GOP’s challenge was evident Friday to Rep. Charles F. Bass (R-N.H.), who fielded questions at a senior center in his district and said later that Democrats “have beaten the world record for getting misinformation out fast.”

“The first thing [the seniors] asked me is whether or not I’m planning to vote to end Medicare completely,” said Bass, elected last year in a swing district that he had previously represented for 12 years.

Bass said the encounter has convinced him he needs to compile a “fact sheet” to distribute to the senior centers in his district that would include the assurance that nobody 55 or older would be affected by the changes.

Earlier in the week, Bass faced similar questions at a town hall. He defended the GOP plan on Medicare, according to a video of the event, but at one point sought to reassure the room that it was not a done deal and still would be debated.

“Remember that budgets are not law,” he said.

At a town hall this week captured on video by a critic, Rep. Patrick Meehan (R-Pa.) argued with a woman who asked why he had voted to abolish Medicare, calling the charge “factually wrong.” But then he, too, offered a reassurance, calling the House GOP budget a “blueprint, a sense of what we would like to do, a direction that we’d like to go in.”

Conservative activists, hoping to tilt the debate back in the GOP’s favor, also mobilized this week.

The conservative group Americans for Prosperity is targeting seniors in about two dozen House districts with radio and Internet ads and recorded phone calls lauding Republicans who backed the budget plan, said Tim Phillips, the group’s president. The group is spending about $400,000 on the campaign.

One mail piece sent to people 55 and older said the House budget would “improve and protect” Medicare.

“There’s absolutely anxiety” among Republican lawmakers, “and there’s a reason for it,” Phillips said.

Another pro-GOP group, the 60 Plus Association, is placing calls and sending mail thanking Republicans for backing a plan that “strengthens and preserves Medicare,” according to the group’s Web site.

The tea party group FreedomWorks has introduced a new social networking tool to connect activists for a budget debate that the organization’s president, Matt Kibbe, describes as a pivotal moment for the movement. And Heritage Action for America, a newly formed advocacy group affiliated with the Heritage Foundation, is distributing budget talking points for volunteers to use at lawmakers’ town hall meetings.

“Our role is to make it possible [for GOP lawmakers] to vote for stuff that might now be seen as politically impossible,” said Michael Needham, chief executive of Heritage Action, which hopes to raise millions this year for issue advocacy campaigns.