He will pick a vice president he thinks can help him govern, and whom he thinks will be prepared to take over as president if necessary. That persons view on abortion is irrelevant to anything a McCain Administration would do to actually impact life in this country.
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One heartbeat away could impact life or death for millions of tiny beating hearts of the unborn...
In addition, it is a HUGE signal about the moral compass of the GOP (there is none) and a signal about what sort of judges McCain would pick.