Covering All Your Bases

Why is it that the Bush Administration says both that they are perfectly within the law to wiretap American Citizens without a warrant AND that they urgently need to pass a law legalizing that practice (retroactively) before January when the Republicans lose majority in Congress?

If it isn't illegal why do they need a law legalizing it? And especially why do they need one before January.

The answer of course is that it isn't really legal and the chance of a Democratic Congress after January legalizing it is extremely unlikely. The only chance is to force it though now hoping that the Democrats won't go to extreme measures to block it. I can't think of a good reason for the Democrats to go along with that since the argument that its urgent is a bit of a stretch since the wiretapping is already ongoing and the Administration is adamant that it is a legal program.