Millions of air travelers may find going through airport security much more complicated this spring, as the Bush administration heads toward a showdown with state governments over post-Sept. 11 rules for new driver's licenses.

By May, the dispute could leave millions of people unable to use their licenses to board planes, but privacy advocates called that a hollow threat by federal officials.

The plan's chief critic, the American Civil Liberties Union, called Chertoff's deadline a bluff -- and urged state governments to call him on it.