David McIntosh — like Chocola, a former Republican congressman from Indiana — will take charge on Jan. 1. A chairman of the Republican Study Committee in the 1990s, McIntosh is said to have close ties to Indiana Gov. Mike Pence, who is considering a run for the White House in 2016.

Moving into 2016 — when Republicans look to maintain control of Congress and pick a presidential nominee from a wide-open, ideologically varied field of candidates — the Club’s challenge now is whether it remains the chief antagonizer for Republicans on the right, or if establishment victories and upstart competitors weaken its influence.

Though 2014 was a great year for establishment Republicans, it was not as great a cycle at the Club. It helped elect several senators who will shape the Senate long term, such as Nebraska’s Ben Sasse, Arkansas’ Tom Cotton and Alaska’s Dan Sullivan. And it knocked off two relatively moderate House Republicans. But it was also the most significant outside backer of Mississippi state Sen. Chris McDaniel, who lost in a Mississippi runoff to Republican Sen. Thad Cochran.

At the start of the cycle, Chocola attacked Karl Rove and American Crossroads for planning to get involved in primaries. He also announced that South Carolina Sen. Lindsey Graham would be a top target. Graham wound up coasting to another term, and no Senate incumbent lost a primary for the first time since 2008.

And now, as Republicans prepare to take control of the Senate for the first time in eight years, the GOP cannot merely resist Barack Obama’s agenda. The necessity of governing has already begun to marginalize groups like the Club that promote ideological purity.

Meanwhile, the Club faces growing pressure on its right from younger, more zealous groups, such as the Senate Conservatives Fund, which constantly try to poach its donors.

Chocola can correctly argue that the Senate Republican Conference is much more conservative than when he took the reins. The group offered key early support to big-name senators like Florida’s Marco Rubio, Kentucky’s Rand Paul, Utah’s Mike Lee and Texas’ Ted Cruz. But it also helped topple Sen. Richard Lugar during a 2012 GOP primary in Chocola’s home state; the GOP subsequently lost that seat. And the group badly damaged former Wisconsin Gov. Tommy Thompson in a Senate primary the same year, arguably costing him the general election.

Just one week ago, a spokesman for the Club denied rumblings around town that Chocola was on his way out. “Chris Chocola remains safely ensconced at the head of the Club for Growth,” said spokesman Barney Keller. “The rumors of his demise are as exaggerated as the rumors of the Obamacare website being fixed.”

But the group announced Thursday that Chocola is “retiring” and will stay on the board of directors. “The Club for Growth is the most effective advocate for pro-growth policies in Washington, and I know it will only get stronger under David’s leadership,” Chocola said in a statement.

The Club was the forerunner to many of the outside groups that now play in primaries on the right. Its former leader, Pat Toomey, chased then-Sen. Arlen Specter out of the Republican Party in 2009 by launching a primary challenge. Specter became a Democrat but lost that party’s primary in 2010, clearing the way for Toomey to pick up the seat in the general election.

The 2012 cycle was more problematic for the Club, as Republicans lost ground in the Senate after some of their nominees flamed out in the general election. Chocola was unapologetic, saying it was not his problem to worry about the party’s win-loss record.

Party leaders — frustrated by the Club’s efforts — fumed that Chocola’s voting record as a member was not nearly as conservative as the positions he espoused as Club president. Chocola, 52, served in the House from 2003 to 2007.

McIntosh, 56, worked in Ronald Reagan’s administration as special assistant to the president for domestic affairs. He worked under Vice President Dan Quayle in George H.W. Bush’s administration. In the GOP wave of 1994, he won an Indiana House seat and became chairman of the Republican Study Committee. He’s been working most recently as a partner in the government relations practice at Mayer Brown LLP.

While the Club has mostly eschewed social and national security issues, McIntosh has pushed fiscal conservatives — this group’s constituency — to work more closely with the religious right.

Such collaboration was one of the goals of an initiative McIntosh co-chaired with former Reagan Attorney General Ed Meese called the Conservative Action Project. The project, which is funded by the secretive Council on National Policy, sought to bring together conservative leaders and donors to try to shape the movement. Almost exactly a year ago, it hosted a private summit of top conservatives at the Ritz-Carlton in Tysons Corner, Va., where McIntosh moderated a panel called “Why conservative leaders and organizations must unite and work together to achieve shared goals,” according to an agenda obtained by POLITICO.

McIntosh is defiant as he takes on the new job.

“The Club for Growth is known as America’s premier organization for fighting Big Government liberals in both parties, and I’m looking forward to building on that reputation,” he said in a statement. “The Club is the tip of the spear when it comes to holding Members of Congress accountable for their actions, and we’ll continue to do just that under my leadership.”

Kenneth P. Vogel and Maggie Haberman contributed to this report.