Bloomberg's numbers will dwindle (as Nader's did). He will then face a stark choice: accept that he's been made a monkey of—or up the ante. Nobody gets to be as rich as Bloomberg if he is not a fierce competitor. So—assuming he has followed the path thus far—he will double down. He will go negative, filling the airwaves with harsh attack ads.
Against whom will those ads be aimed? A lot will ride on that question. Attack ads are dangerous things, because they damage both the attacker and the attackee. Their main effect is not to change votes from D to R or R to D, but to depress turnout among potential supporters of the targeted candidate. Candidates refrain from excess negativity for fear of damaging their own image. But a Bloomberg in the polling basement will feel no such constraint.
The ads will be a free gift to the candidate Bloomberg dislikes less at the expense of the candidate he dislikes more.
And the candidate he dislikes more will almost certainly be the Republican.
That's certainly one plausible scenario, but I don't know that it's the likeliest. Bloomberg is no dummy. I think he could run a competitive race against two major party competitors in a harsh anti-Washington year. He'll present himself as a successful, non-partisan, no-nonsense businessman who knows what it takes to maintain economic growth, solve difficult problems, and work with Republicans and Democrats. With a billion dollars to spend, I'm not sure he reaches the point where 'his numbers dwindle.'
And if you presume that he's willing to unleash hundreds of millions of dollars of negative ads to destroy a rival, why wouldn't he do that early in the race? It's true that negative advertising typically hurts both the target and the candidate who 'goes negative.' But isn't it possible that Bloomberg sees an opening in the spring, decides he has the best chance of co-opting the support of the Democratic candidate, and then decides to spend his millions trashing that person?