It's often thought inconceivable that a party's nominee for vice president could be from the other party. The reasons are: that slot is used to reward party factions not delighted by the presidential nominee; the slot is thought to lead to a nomination for president in the future, so it must be given to one of the party's own members; it would be difficult for a vice president from the opposing party to operate successfully in the White House; the person might be an enemy within, politically speaking.
And so conventional thinking goes. But in fact if Republican Colin Powell had been willing to run as Al Gore's vice presidential nominee, not only would President Gore have been sworn in -- since Powell would have added a million votes nationally and certainly more than 500 in Florida to the D ticket -- but also the Gore Administration would have handled the country's challenges in a vastly different and superior manner, focussing on building a centrist approach to the Arab world, global warming, income inequality, and the rising business challenge from China.
Powell now would be able to change parties and run as a centrist Democrat and win the party's nomination for President. Of course, he could not run as a Republican, since they do not welcome apostates, but of course history shows he did not feel he could do that in any case.
In 2008, any Democratic presidential nominee would be lucky to have Republican Senator Chuck Hagel agree to run as the vice president on his or her ticket. On many social issues he is far more conservative than most Democrats but his fiscal and foreign policy views are intelligent, moral, and courageous. He not only showed immense personal courage fighting in Vietnam, but also has developed a deep understanding of security matters since then.
Of course the Republicans should want him to be their Presidential nominee, but Americans should not expect to see such wisdom on the part of the party formerly known as the "majority."
In any event, the likely Democratic nominees for President may well want a military man as vice president.