It boils down to money - if it starts drying up now, it’s a very long shot.
Just got my first email from Jeb's campaign. Clicking "unsubscribe" didn't cost me a dime.
They could learn faster from those than anything else; but you're right, it will probably be money. The good news is that if Walker starts out-polling Jeb in Floriduh this early and without having spent a dime, the money people will know he's a bad bet.