Why not nominate Ron Paul, couldn't he get a good chunk of democratic voters?

I'm a Liberal and I'd vote for Ron Paul over Obama but probably none of the other candidates. The only other one who looks like he could be a decent leader is Mit Romney but I can't support presidents like Bush and Obama who think they are above the constitution. Patriot acts, war without congressional approval, etc unconstitutional. How about someone with a shred of integrity.