Though Trump would have to commit fairly early, the deadline for putting a party on the ballot in some states is very early, like November.
I don't think this is necessarily true. There are a plethora of minor parties which are already qualified in many states. If Trump could be nominated by as few as 2 or 3 of these he could be on the ballot in almost all states. For instance, the Libertarian party is already qualified in 26 states.
How does he do this? Easy. Money and political payoff. If he promised almost any party enough money (a war chest for future elections, or even this one) and a cabinet position (a national platform to promote their views) even the most dedicated minor party could see this as a good deal. And, it would not be unprincipled. Especially since these parties wold see the potential for a rise in their popularity in the major restructuring which would inevitably follow a Trump victory as a third party candidate.
There would be an interesting side effect. If Trump were nominated by two minor parties, both on the ballot in some states, would his name appear twice in those states?
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To me, the way the Republicans prevent Trump from becoming a disaster for them is to embrace him. He clearly has appeal and a base. Let him into the debates. And into the primaries. If he wins, he is their best candidate. If he loses, the process was "fair" and by then it is too late to run on a third party ticket.
Give him the VP slot, or promise a cabinet position and he will open his wallet...