And They're Both Named David!

Peter Beinart jumps aboard the Petraeus-for-president meme:

Pundits have mused about the Eisenhower-Petraeus comparison before, but the Afghanistan slugfest gives it new relevance. In the late Truman years, MacArthur, Joseph McCarthy, and the rest of the Republican right wing were a bit like Sarah Palin and Glenn Beck today. They succeeded in bloodying the Democrats and scaring the country about overseas threats. But their overseas warmongering and domestic radicalism made them too extreme to ever win national office themselves. Ike was different. He exploited the right’s hysteria, and yet sailed above it at the same time. [...] Parties that have grown narrow and extreme tend to spiral downward until they nominate someone who is not beholden to their narrow, extreme base. That person has to be so popular that he or she can defy the normal rules about how candidates get nominated. Right now, David Petraeus is the only Republican who fits the bill.

I sure can see the potential here. But, ahem, how do we know that Petraeus is, or will be, a Republican? Does anyone actually have evidence of this? Or is all this speculation based on the stereotype that military = Republican?