Fred Karger, president of Rights Equal Rights, plans to file a complaint on Thursday against Rick Santorum and the National Organization for Marriage (NOM).

The Federal Election Commission (FEC) filing centers on Santorum's unsuccessful bid for the 2012 GOP presidential nomination.

Karger alleges that an endorsement from social conservative Bob Vander Plaats, CEO of the Family Leader, included cash to pay for ads promoting Vander Plaats' endorsement of Santorum for president.

Santorum told CNN at the time that Vander Plaats told him earlier 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 in the fall of 2011. “There was never a direct ask for me to go out and raise money for it.”

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 claiming that Vander Plaats was soliciting as much as $1 million for his endorsement.

Karger's complaint will argue that NOM and its consultants “played a major role in paying for and securing the Vander Plaats endorsement of Santorum for president.”