Rick Santorum has given his former rival some tepid endorsements since officially backing him last month, but today he said he wouldn't even accept a cabinet post from Mitt Romney if asked to serve in a hypothetical Romney administration.

"I want to help Mitt Romney get elected president," Santorum told CNN's Candy Crowley. "I'll be happy to help him and advise him if … (he) wants my advice as president. But, no, my … objective right now … is to serve my family and provide for them."

Santorum mentioned that the two eldest of his seven children are in college, but when Crowley pointed out it wasn't a "flat no," Santorum said it "pretty much" was.

"It's pretty much a flat no," Santorum said. "And it's not because I don't want to help Gov. Romney. … It's just, for me … it's a matter of my priorities and my time of being a husband and father and … I have to take care of them."

Santorum backed Romney in a middle of the night email to supporters, which first raised questions whether the endorsement was heartfelt. Since then, the two have not campaigned together and as of Friday no official invitation has been made to Santorum for the two to hit the trail together, not even when Romney visited his home state of Pennsylvania Saturday.

Last week, also on CNN, when asked if he trusts Romney, Santorum answered, "Well, I trust him more than I do Barack Obama."

But, Saturday in an address to the Faith and Freedom Coalition, Romney did mention Santorum and a study the former Pennsylvania senator frequently mentioned on the campaign trail. The former rival then praised Romney in his own speech, saying Romney's remarks were "right on and not because he quoted me."

In the interview today, Crowley also asked the former presidential candidate about comparing Romney to President Barack Obama during the primary campaign and Santorum answered he had "no problem with questioning the authenticity," because those were "all things" he "thought were legitimate differences between us as candidates."

"I mean there are differences between us," Santorum said. "I don't back away from any of those things. But at the same time, clearly, the difference between President Obama and Mitt Romney is a chasm. … I have a great degree of comfort in supporting Gov. Romney as the choice between the two."

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