It looks as if Kerry's thinking of a bipartisan ticket here, but McCain won't budge.
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Democratic presidential candidate John F. Kerry has discussed the vice presidency with Sen. John McCain (R-Ariz.) on several occasions, the most recent in the past two weeks, informed sources said yesterday. But the conversations have gone nowhere because McCain believes such a bipartisan ticket would not work and could weaken the presidency, they said
Washington Post articleI'm not certain how McCain believes this would create a constitutional crisis, it isn't as if we've never had a split-ticket executive branch before.