It really depends on the democrat. Are they happy with Obama? I wouldn't even bother. Are they disillusioned with Obama? Talk to them about how in a general election, Ron Paul is the only candidate that can hold Obama accountable for his warmongering, for his trampling of civil rights by signing the NDAA and other things, for his cracking down on medical marijuana at far higher rates than Bush ever did, for his bailing out corporations, for his collaborating with lobbyists, etc. If they're worried about these things, those issues will only be drowned out during a general election and, assuming Obama wins reelection, over the next four years.