"Look, whether or not you agree with the solution or not, we have a problem in America when you've got 8 million undocumented workers here," he told The Washington Times in an interview for Wednesday's editions.
"I believe the president has got to set big agenda items and solve big problems," the president said. "Obviously, we're going to have to work on it, just like Social Security (search). This will require the expenditure of capital."
He said the "solution is not instantaneous citizenship" but "something more rational than that."
The president expressed confidence that he can persuade Congress to move on his immigration bill, despite opposition from some in his own party.
Bush's plan, outlined a year ago, would grant temporary work visas to foreign workers as long as U.S. workers cannot or do not want to fill the job.
Critics say this would encourage more people to cross the Mexican border illegally to obtain work.
Insisting his plan is not an amnesty, Bush described the current situation as a "bureaucratic nightmare."
"We've got people living in the shadows of our society, and we've got a border patrol that's overstressed because we've got people streaming across," Bush said. "The system has broken down. ... And I think by legalizing work, we take a lot of pressure off our borders."
He said he strongly opposes instant citizenship. "I think all that would do is cause the problem to occur again," he said. "I believe that if they want to be a citizen, they need to get in line like the other people have done. And if Congress is worried about logjams for certain countries becoming citizens, they need to change that part of the law."