PPP finds that Trump "gets 31% of the Republican vote running as an independent candidate, holding Romney to just 56% within the party":
It's highly unlikely that Trump would really end up doing this well as an independent, but even if he just pulled 5-10% it would probably be enough to ensure Obama's reelection. And these numbers show that if Romney's the GOP nominee there may be enough consternation on the far right that another third party candidate and not necessarily just Trump could earn enough support from those voters to have a spoiler effect much as Ralph Nader did for Al Gore in 2000.
Is he that serious? One fears he may be. Or if one is a Democrat, perhaps a licking of the lips is in order. But you never know. This is America. Right-wing populism has its moments. They rarely last; but they often shine brilliantly in the meantime.