An Iowa group is calling on the Federal Election Commission (FEC) to investigate Bob Vander Plaats' endorsement of Rick Santorum.

Progress Iowa on Friday began circulating an online petition calling on the FEC to look into potentially illegal campaign coordination by The Family Leader CEO.

Santorum won Vander Plaats' personal endorsement on Tuesday.

Vander Plaats is best known for leading last year's successful campaign to oust three of the seven Iowa Supreme Court judges who overturned the state's ban on gay marriage in 2009.

Santorum, along with Minnesota Rep. Michele Bachmann and Texas Governor Rick Perry, signed the group's controversial 14-point anti-gay marriage pledge, which asks each presidential candidate to “vigorously” oppose marriage equality, be faithful to his or her own spouse, vow to protect women and children from pornography and reject Sharia law because it is a “form of totalitarian control.”

On Wednesday, Santorum told CNN that Vander Plaats told him that “he needed money to promote” an eventual endorsement, but didn't make “a direct ask.”

“What he talked about was he needed money to promote the endorsement and that that would be important to do that,” Santorum said of a conversation which took place earlier this fall. “There was never a direct ask for me to go out and raise money for it.”

Santorum said during a town hall, also reported by CNN, that Vander Plaats suggested that he drop out of the race before he endorsed his campaign.

Robocalls promoting Vander Plaats' endorsement of Santorum for president began in Iowa on Thursday. The calls say they were paid for by a super PAC called Leaders for Families, the Des Moines Register reported.

“Any coordination between a campaign and an outside group on campaign expenditures such as advertising is illegal,” said Progress Iowa in a statement. “Progress Iowa's petition calls on the FEC to investigate the Family Leader and Bob Vander Plaats for any potential illegal coordination between the group and the Santorum campaign.”

“In light of news that Vander Plaats' PAC is launching robocalls in Iowa on behalf of Santorum after he solicited fundraising help, the FEC must look into any coordination between the Family Leader and the campaign.”

Speaking to the Des Moines Register, Vander Plaats said there was “absolutely no 'quid pro quo'” for the endorsement and no “coordination” with the campaign.

He said promoting his endorsement was an “ethical responsibility.”

“You can't say, 'We endorse you. Now see you later,'” Vander Plaats told the paper. “That's not going to do a lot in the long run.”

ABC News quoted multiple sources as saying that Vander Plaats was soliciting as much as $1 million for his endorsement.

Santorum has moved up in some recent polls and his campaign released a new pop-up ad in Iowa this week which quotes 2008 Vander Plaats endorsee Mike Huckabee as saying, “Loves Rick Santorum's convictions.” (The video is embedded in the right panel of this page. Visit our video library for more videos.)