Sen. Jeff Sessions, R-Alabama, dons a "Make America Great Again" hat as he endorses Republican frontrunner Donald Trump. | AP Photo Club for Growth warns against Trump endorsements

The Club for Growth is issuing a warning to its stable of congressional candidates this fall: Endorse Donald Trump and you might just lose your backing.

The conservative group, which specializes in supporting outsider candidates that challenge centrist-leaning Republicans, is actively considering denying endorsements to any candidate that endorses Trump while the primary is still being contested, according to a source familiar with the Club's endorsement process. The reason: The Club hates Trump's "liberal views on economic policy."


"A Trump endorsement, before anyone clinches the nomination, would be viewed as a serious error in judgment, and would be more difficult to reconcile with the kind of fiscal conservative who would ordinarily meet the Club PAC's rigorous standards for endorsement," the source said.

The Club's PAC is backing four conservative incumbent senators, two candidates in open Senate races and five House candidates thus far this cycle. None of the Club's candidates have endorsed Trump thus far, and some, like Sen. Pat Toomey (R-Pa.) are seeking distance from what he called Trump's "appalling" remarks on the KKK and David Duke.

The threat from the Club also applies to prospective candidates that are trying to win the group's endorsement through the Club's already rigorous endorsement process. Losing a Club endorsement could make the difference for a candidate that needs a fundraising edge: The Club has already spent $3.5 million against Trump.

David McIntosh, the president of the Club, said in an interview on Monday that the group has put about $1 million in Arkansas and Oklahoma in the hopes of hurting Trump in a couple of states where his rivals Marco Rubio and Ted Cruz are within striking distance. More spending is expected over the next two weeks, which could be decisive in determining whether Trump is the nominee, sources at the Club said.

"Our polling shows that it’s close enough in several of these winner take-all-states that if there’s a real advertising campaign, he’ll come in no. 2," McIntosh said. "Now’s the time and a lot is at stake, or we’re going to lose the Senate and the Supreme Court."