Rick Santorum isn't exactly brimming with enthusiasm about the man he is supposed to be supporting for the presidency.

When asked on CNN if he now "trusts" Mitt Romney after railing against him during the primary, especially when he consistently compared his health care plan in Massachusetts to the president's health care plan, he gave quite a tepid endorsement.

"Well, I trust him more than I do Barack Obama," Santorum told John King. "This election is about a choice."

Santorum went on to say he believes Romney when he says he will "repeal Obamacare," and that, through his new organization Patriot Voices, he will keep a watch on the presumptive GOP nominee as well as other conservative candidates to hold "their feet to the fire."

King asked the former presidential candidate if he worries that his supporters will see him as just choosing the "lesser of two evils."

Santorum answered that he's "passionate and enthusiastic about a lot of things that Governor Romney supports," but he's "made no bones about the fact that I have concerns about Governor Romney's health care plan in Massachusetts."

He was also asked if he is just "tolerated" by the Romney campaign or more welcome into the fold and whether he will be out campaigning for his former rival.

Santorum answered he feels an "obligation to go out and work hard" in the campaign and will be "helping the Romney campaign wherever they believe we can be helpful in working with them."

Santorum and Romney have made no public appearances together since their meeting last month. The former Pennsylvania senator did formally endorse him last month, but it was in a middle-of-the-night e-mail to supporters, which also seemed to push the message he was less than enthused. Just last week, a Santorum senior adviser told ABC News they have had no formal requests from the Romney campaign to help out on the trail. On Monday night in Atlanta, Romney was joined by another former competitor, Newt Gingrich, for a fundraiser.

Santorum also wouldn't weigh in on who he thinks should be Romney's running mate, saying that since Romney is the person who "vanquished" him in the primary, "I'm not going to be out there as a former foe making recommendations to who he should pick."