Ridge and Lieberman campaigned for McCain in Pennsylvania last week.

(CNN) – Republican sources tell CNN that John McCain campaign manager Rick Davis and senior advisor Charlie Black have been calling activists to sound out the pros and cons of several possible running mates – including former Pennsylvania Governor Tom Ridge, a choice that could spark major discontent from the conservative base.

Several campaign officials say they do not believe a final decision has been made – though two sources say McCain may have already decided, and told only Davis. One party insider tells CNN that Davis has called several state party chairs indicating Ridge will be the Republican vice presidential pick this cycle – although it is possible that Christian conservatives may be playing up the former governor’s chances in an effort to raise alarm and prematurely kill off his candidacy.

Several conservative activists say they know McCain’s campaign manager has been warned of possible dire consequences for the Arizona senator’s presidential bid at the convention and beyond if he picks a candidate who supports abortion rights.

"If he picks Ridge, the convention blows up, you will have a base completely demoralized,” said one activist. “And it will be viewed that he is ceding the future of the GOP to a liberal Republican. It would be a disaster."

But the same activist also says McCain is unlikely to make that move, pointing to comments by the presumptive Republican nominee that he would most likely have an anti-abortion running mate, and remarks this weekend that he would have a "pro-life" presidency.