If he picks a pro-choice running mate:

Several prominent conservatives, one of whom serves on the board of the Arlington Group, a coalition of social-conservative organizations, and is a delegate to the Republican convention, say that should the Republican platform currently being shaped in Minneapolis and Sen. John McCain’s vice presidential selection not reflect the views of the conservative movement, they will attempt to mount a challenge to McCain’s nomination on the floor of the Republican convention next week.

Maybe he should have kept his mouth shut since there are ways for them to stop this kind of protest:

A floor challenge would require the majority of delegates from a total of six states, one to make a motion to open the floor to a new nomination, and five others to essentially second the motion. But it would also require good timing and some luck. The convention schedule — even when not televised — is tightly filled and heavily scripted, and it isn’t clear that the secretary of the convention would make it easy for such a motion to be placed.

Don’t get too excited, it probably won’t happen. I bet McCain will be picking a pro-life running mate since his trial balloon of Lieberman and Ridge wasn’t well received.