An influential Harrisburg conservative group that controls one of the biggest political action committees in Pennsylvania endorsed former U.S. Rep. Lou Barletta for governor on Sunday, calling on other contenders to do the same to stop State Sen. Doug Mastriano from winning the Republican nomination.

In doing so, Commonwealth Partners Chamber of Entrepreneurs, a membership association, publicly joined a last-ditch effort to rally behind Barletta as some Republicans worry Mastriano’s views are too extreme to win in a purple state.

Commonwealth Partners had previously endorsed former US attorney Bill McSwain, and its political group has spent nearly $13 million boosting his candidacy. On Sunday, the group called on McSwain and another GOP gubernatorial hopeful, former Delaware County Councilman Dave White, to withdraw from the race and endorse Barletta in Tuesday’s primary election.

“It’s become abundantly clear in recent days that nominating Lou Barletta for governor is Republicans’ best chance to defeat Josh Shapiro in November,” Commonwealth Partners president and CEO Matt Brouillette said in a statement. Shapiro, the state attorney general, is running unopposed in the Democratic primary.

McSwain’s campaign manager, James Fitzpatrick, pushed back Sunday night, saying in a statement that McSwain “is the only candidate who can beat Josh Shapiro.”

“Anyone who wants to ensure a Republican victory in November should vote Bill McSwain for governor,” Fitzpatrick said.

Commonwealth Partners is worried the primary election voters will go for Mastriano.

“Months ago, Commonwealth Partners began cautioning of the dangers of nominating Doug Mastriano, as he would not be able to win the swing voters necessary to win in November,” Brouillette said.

“Unfortunately, although Mastriano is garnering support from less than 30% of likely primary voters, given the large field of candidates, this could be enough to secure him the nomination,” Brouillette said.

A representative for White did not immediately return a message seeking comment.

Scores of current and former Republican elected officials have rallied behind Barletta over the last week, as some in the party argued that Mastriano would march to the nomination absent a major campaign shakeup. The eleventh-hour effort to stop Mastriano already faced steep odds – which grew even worse when former President Donald Trump announced his support for Mastriano on Saturday.

Mastriano has said the effort shows he’s running against the establishment.