Correction: An earlier version of this article had an incorrect dateline. This version has been corrected.

Heritage Foundation President Jim DeMint participates in a question-and-answer session Aug. 22 after speaking to attendees during Heritage Action for America’s “defund Obamacare” meeting at the Marriott in Franklin, Tenn. (Luke Sharrett/For The Washington Post)

It was a sleepy night in this historic town south of Nashville, but inside the ballroom of the local Marriott hotel, Michael Needham had more than 800 conservative activists on their feet.

“Can we defund Obamacare?” he called out.

“Yes, we can!” the crowd shouted back.

The rally was not the work of a tea party group or the local Republican congressman. Instead, it was produced by Heritage Action for America, the new advocacy arm of the venerable Heritage Foundation that is emerging as one of the most pugilistic forces on the right.

Such red-meat, campaign-style events have boosted Heritage’s standing with tea party activists, but they have also alienated many of the group’s longtime Republican allies on Capitol Hill. On Wednesday, Heritage Action came out against President Obama’s proposal to launch military action against Syria, putting the group at odds with House Speaker John A. Boehner (Ohio) and other GOP leaders who back the strikes.

As of Sept. 4, lawmakers appear to be tentatively dividing into four camps over military action in Syria.

The Republican Study Committee, a group of the most conservative House members, recently barred Heritage analysts from its weekly strategy meetings, where they had played a central role for years, according to multiple people familiar with the episode. The move came after noisy disputes over a farm bill and other legislation, which left many lawmakers feeling blindsided by Heritage’s positions.

“I pay no attention to Heritage Action,” said Sen. Tom Coburn (R-Okla.), who has accused the group of “destroying the Republican Party” with its push to strip funding for Obamacare. “They’ve become a purely partisan group that never asks anybody’s opinion.”

Since its founding in the 1970s, Heritage made its mark primarily as a purveyor of conservative ideas, exercising influence through dense policy papers and expert testimony. But through its new Heritage Action arm, the group is transforming itself into an overtly political force — scoring legislators on key votes, training “sentinels” to track lawmakers’ activities and marshaling loud rallies aimed at forcing Republicans to act.

Last month, the group roused the base with a series of packed “Defund Obamacare” meetings around the country featuring Needham, chief executive of Heritage Action, and former U.S. senator Jim DeMint, the new president of the Heritage Foundation.

“Folks, we can’t allow this plague to come onto the American people,” DeMint told the audience in Franklin, which murmured its assent.

Heritage veterans’ exit

The shift toward political activism has dismayed some longtime Heritage scholars; more than half a dozen have left in recent months. But Heritage officials say the mission of the think tank has not changed and note that it has a long track record of disagreement with Republican leaders.

“We’re doing more research and policy work than we’ve ever done,” DeMint told reporters before the Franklin rally. “The only difference is now that we have a sister organization that is taking those ideas to the people and Congress in a more aggressive way.”

Still, the campaign to threaten a government shutdown Oct. 1, unless Obama agrees to strip funding from his signature health law, marks a dramatic departure from what former employees call “the old Heritage.” Until recently, those staffers said, the think tank was reluctant to even place scolding editorials in local newspapers.

Now, representatives of the advocacy arm are traveling the country in an attempt to pressure lawmakers to vote against funding the government in the new fiscal year unless the bill also halts spending for Obamacare. Top GOP aides believe the proposal has little chance of success and is likely to backfire.

Boehner has promoted a different strategy aimed at challenging House Democrats to take a stand on controversial parts of the health-care law in a series of votes. Meanwhile, liberal groups have played up the possibility that GOP hostility to Obamacare could shut down the government: Americans United for Change followed Heritage Action on its tour last month, staging counter-events highlighting the law’s benefits.

GOP strategist Brian J. Walsh, a former communications director for the National Republican Senatorial Committee, said the Heritage approach is encouraging “fratricide” within the party. “There are a number of people who are frustrated, because what may be good for Heritage Action’s fundraising or growing its mailing list is not necessarily good for the Republican Party heading into 2014,” he said.

Among the conservative lawmakers in the group’s crosshairs is Rep. Renee L. Ellmers of North Carolina, who voted 40 times to defund or dismantle the health-care law. But Ellmers believes the Heritage plan is unworkable, and was among 100 House Republicans targeted by Heritage Action last month with online ads.

“They’ve used bully tactics, and they’re going way beyond the scope of promoting conservative ideology,” she said. “They are now trying to influence members of Congress through what I consider very threatening actions.”

Ellmers said she relied on Heritage Foundation research when she was first elected in 2010 but no longer: “To me, it is tainted.”

Heritage Action officials say the politicians who are complaining simply do not want to be held accountable for their conservative pledges.

“We understand that people are upset,” said Tim Chapman, the advocacy group’s chief operating officer. “This does hit them where it hurts.”

It’s not the first time Heritage has faced blowback from Republicans. The foundation was founded by Paul Weyrich and Edwin Feulner in 1973 to have a more aggressive presence on Capitol Hill than the conservative think tanks of the time. Since then, Heritage has weathered numerous clashes with GOP leaders unhappy with its policy pronouncements, including officials in the Reagan White House and both Bush administrations.

“This is not new: We get into battles with our friends,” said Phillip Truluck, the foundation’s longtime executive vice president.

The creation of Heritage Action in 2010 meant “some changes for us, no doubt about that,” he added. “But primarily, we are still and always going to be a research institution. The tail is not wagging the dog here.”

Heritage officials said the passage of the Affordable Care Act in 2010 crystallized the need to engage explicitly in political advocacy, which the foundation, as a 501(c)(3) charity, cannot do. (Some key components of the law, including an individual mandate to buy insurance, were once embraced by Heritage research; the group has since disavowed the idea.)

Heritage Action, a 501(c)(4) “social welfare” group, was seen as a way to harness the rising tea party movement and provide a vehicle for Heritage to press its arguments without having to register its analysts as lobbyists, according to current and former staffers.

The advocacy group quickly began issuing “key vote” pronouncements, scoring lawmakers and contacting voters in Republican districts. Hometown activists, dubbed sentinels, are tasked with raising the alarm when incumbents stray from what Heritage considers conservative orthodoxy.

Heritage Action jumped into the foreign policy arena Wednesday when it declared there was “not a vital U.S. interest at stake” in attacking Syria for its alleged use of chemical weapons. That stands in sharp contrast with recent statements from Heritage analysts, including several who called for urgent action on Syria last November and said the United States should “work closely with allies to accelerate the fall of the Assad regime.”

DeMint makes his mark

In its latest anti-Obamacare campaign, Heritage Action has coordinated closely with the Senate Conservatives Fund, a political action committee started by DeMint and run by one of his former advisers. The group is airing radio ads attacking Senate Republicans who do not support the defunding plan.

The arrival this year of DeMint as the foundation’s president — succeeding Feulner, who held the post for 36 years — accelerated a cultural shift within the institution, according to several former employees, who spoke on the condition of anonymity to discuss the group’s inner workings.

The former South Carolina senator was a tea party leader who frequently clashed with his GOP colleagues. At Heritage, DeMint brought in a team of young acolytes who “were absolutely convinced that you were a sellout, and they wanted to do serious work to make sure we were ideologically centered,” said one former policy analyst.

Another former staffer who left for Capitol Hill said: “Heritage Action was sold to us as a small group that would give fire to the organization. Now, it’s taken over the entire foundation.”

The group has experienced a wave of departures to Capitol Hill and rival think tanks in recent months, losing its top statistician, its chief lobbyist, a senior tax analyst, a key retirement expert and a Latin America expert. Heritage Vice President Matthew Spalding announced last month that he was decamping for conservative Hillsdale College, where he will become vice president and dean of educational programs in Washington.

Truluck said that the think tank’s rate of turnover is consistent with past years and that DeMint fully supports independent analysis. “There has been no change in our policy direction,” he said. “Zero.”

Still, DeMint has clearly brought a new feistiness to the group. In Tennessee, he drew the largest cheers of the night when he declared that “the president just has a different set of beliefs about our country than we do.”

“I’m sure in his heart he believes he’s helping,” DeMint added, prompting boos and shouts of “No!”

“I’m trying to be nice, okay?” he responded.

“We don’t want nice!” a man shouted from the audience.

“Okay,” DeMint said, chuckling. “I know you don’t want nice.”

Montgomery reported from Washington. Alice Crites contributed to this report.