Rick Santorum slammed Mitt Romney as being farther to the left on same-sex marriage than President Obama.

"I would argue that Mitt Romney as governor of Massachusetts was to the left of Barack Obama on gay marriage," Santorum said Monday on Laura Ingraham's radio show.

Santorum's comments are likely in the context of Romney's tenure as Massachusetts governor, during which the blue state legalized same-sex marriage.

Santorum, a former Pennsylvania senator, touted his own conservative bona fides, saying he fought for the Defense of Marriage Act when he served in Congress.

Obama has been hailed by same-sex marriage advocates for successfully repealing the military's "Don't ask, don't tell" policy last year against gays serving openly. He has also had federal lawyers stop defending the Defense of Marriage Act.