The wage differential between the jobs illegals will take and the wages in Mexico must be low enough to truncate the economic incentive to illegally migrate in the first instance. The unspoken deus ex machina of the Bush plan is to do that with workers that are truly temporary, just like the Palestinians used to be that worked in Israel.
If the Dems decide to highlight the deus ex machina, the plan will probably die, and the status quo ante will be left in place. Many Dems will be evaluating if the temporary bit is a fig leaf, and if it is, might support it. The only way for the Bush plan to get traction is to persuade two different voting blocks that for the one block, the temporary bit is a fig leaf, and for the other that it is not. Maybe Bush needs to speed dial Clinton or something, to figure out how to do that.
Meanwhile the Dem block, will be pushing for details to increase the fig leaf odds, and the GOP block will be pushing for details to make the fig leaf more impermeable.
Meanwhile, many lower wage legal workers will hate it either way.
It should be interesting. The odds favor the status quo ante. But Bush does like to take risks to move the ball, and I cannot really fault him for that. At least it opens the matter up for further discussion.