WASHINGTON — Conservatives are calling it their August Revolt — a surprising upsurge of activism against President Barack Obama’s proposed health care overhaul.

Spurred on by the success of their efforts to dominate the news at Democratic town hall meetings, conservative groups are reporting increases in membership lists and are joining forces to plan at least one mass demonstration in Washington next month. But the conservative mobilization has also created an unusual dilemma for Republican leaders, who want to turn the enthusiasm into election victories next year but find themselves the target of ire from many of the same activists.

Sen. John Cornyn of Texas, chairman of the GOP’s Senate campaign committee, was booed at a “tea party” rally in July for his support of bailing out the financial services industry. And one of the party’s most reliable conservatives, Rep. Bob Inglis of South Carolina, was shouted down at a recent town hall meeting when he criticized a conservative broadcaster and tried to counter claims that children would soon be forced to receive swine flu vaccinations.

“You cannot build a movement on something that is not credible,” said a frustrated Inglis, referring to the vaccine issue and other false rumors being spread by some more aggressive critics of the health bill. “Going door to door, I found opposition tending toward hostility,” added Inglis. “At town meetings, the hostility went straight through to hysteria.”

Some GOP leaders, such as former vice presidential nominee Sarah Palin, have tapped into the unrest — with Palin stoking fears on her blog of “Obama death panels” that would result from the health care legislation.

Others are still trying to figure out how to balance the desires of the base with the need to appeal to moderate swing voters who might be turned off by high-volume rhetoric. Whether they find that balance could determine whether the GOP can win back independents who voted overwhelmingly for Obama last year but now, according to several polls, are questioning their commitment to him.

The GOP might take comfort in a new Gallup survey which shows that more than a third of independents who have followed the protests have gained sympathy for the protesters’ views, while just 16 percent have lost sympathy for them. About 35 percent of independents approve of Obama’s handling of health care policy.

But Republican leaders eager to win the middle on other issues such as immigration have failed in recent years to appease the conservative base. Complicating matters now is that some activists have mounted their anti-health care overhaul effort largely outside the party machinery. Instead, they are relying on Internet social networking sites to recruit volunteers for town hall meetings and spread YouTube videos. One new group, Smart Girl Politics, has drawn about 10,000 participants using the networking site Ning.

“I don’t know that anybody would want to be associated with either party at this point,” said Michelle Moore, a suburban St. Louis business owner and mother of two who joined Smart Girl Politics and has helped drive activists to four town hall meetings hosted by Missouri’s Democratic senators.

Anti-tax activist Grover Norquist, who for years has served as a moderator of sorts between factions of the conservative movement, said the new insurgency is all “fed up” with Obama but not unified around a leader.

There is some organization to the conservative agitation.

About a dozen groups, including the large and well-financed FreedomWorks, led by former GOP Rep. Dick Armey, are sponsoring a march on Washington on Sept. 12.

Another longtime conservative group, the 60 Plus Association, purchased a nearly $2 million cable TV ad buy alleging that the Obama plan would put seniors’ well beings in jeopardy.

Republican officials hope those efforts will dovetail with signs of a GOP resurgence. Leaders are trying to rebrand the GOP as the party of fiscal discipline, fighting Obama on his economic stimulus plan, attacking his proposed global warming legislation as a massive tax increase and, now, portraying his health care agenda as a socialist takeover of the private sector — messages that GOP strategists had hoped might appeal nicely to the base and the middle.

But the newly energized grass roots don’t appear ready to let Republicans off the hook for supporting recent government bailouts of automakers, banks and Wall Street investment giants, even if Obama and the Democrats were the primary targets for their anger.

When conservative Rep. Gresham Barrett, R-S.C., addressed a mid-April anti-tax rally in Greenville, S.C., members of the audience booed and turned their backs to the speaker. His crime against conservative orthodoxy: Barrett had voted in late 2008 for the Wall Street bailout bill.

“I know you are mad,” Barrett told the crowd. “I know you are frustrated and I hear it. You may boo, you may turn your back. But I have devoted my life for the conservative cause. We are fighting for you and I will never turn my back on you.”

Cornyn’s status as a party campaign leader was no shield against catcalls at an Austin rally sponsored by Americans for Prosperity, one of the conservative groups involved in the town hall protests. Although he joined them in railing against “more spending and more government” in Washington, Cornyn was called a “traitor” and heckled throughout his speech to the Fourth of July crowd, apparently because of his support for the Wall Street bailout.

Another strand of the conservative movement burst onto the scene at a town meeting held this summer by moderate GOP Rep. Mike Castle of Delaware. He was confronted by a woman asking why Congress was not more aggressively pursuing claims that Obama was not born in the U.S. He drew a hailstorm of boos and roars when he insisted that Obama was a U.S. citizen.

In many places, tensions between conservative activists and the party establishment have gone beyond catcalls to frontal political assaults. In Florida, conservatives have backed a primary challenge to Republican Gov. Charlie Crist, who is running for the U.S. Senate. He is viewed with suspicion from the right because of his support of global warming legislation and Obama’s stimulus plan. In Indiana, even a Republican as conservative as GOP Rep. Dan Burton — a leader of the effort to impeach President Bill Clinton — is being opposed by several Republicans who say he has lost touch with the cause.

Inglis, who first came to Congress as part of the 1994 conservative class, now faces a primary challenge because of recent departures from party dogma on the Iraq war and some environmental issues. After he was besieged by activists at this month’s angry, unruly town meeting, Inglis sent the YouTube clip of the confrontation to his supporters and donors with the message: “This is what the opposition looks like.”