I don't believe that we do have a choice when it comes to the people who are already here illegally. The vast majority of them are working at jobs where they are needed, and they can't be replaced overnight.
The only way we can find out who they are and where they are is to make the employers report them. Employers won't do that if they risk their businesses by doing so.
I don't think there's any way even a giant bureaucracy can find many of them unless the employers cooperate.
I can see options like giving currently illegal workers a limit of something like 3 years, then making them go back to their home country to reapply. That would give both the workers and the employers time to plan for a change.
I think the goal should be to make sure that everyone who is here has some legal status, so we know who they are and where they are. Driving the illegals farther underground is a terrible option.
And like I said, I'm not arguing for replacing them overnight. If we have a guest-worker program available only to aliens applying in their countries, it won't make the situation any worse than it is now. Illegals will probably continue to work at their jobs, as before. And if we're able to stem the tide of illegal entry, then we can work on slowly deporting the ones already here. Once the illegals here see the handwriting on the wall, they'd most likely consider it advantageous to go back and apply the right way, rather than risk getting caught sooner or later and being deported permanently.
Above all, there's no reason for insisting, as Bush and many Congressmen do, on making sure this type of program is included as part of a general border-security bill. We can pass a bill beefing up the Border Patrol, or fencing off areas of the border, without a guest-worker plan attached to the same bill, and that would still at least improve the situation. My accusation of the politicians there is that they're trying instead to use the unsecured border as blackmail to get this program enacted over the objections of the people. That's why, left to their own devices, they would never accept any plan to split this into two separate pieces of legislation - which is how they properly belong, because they're really two separate approaches that can work independently of each other.