Steep reductions can be carried out by START II or by less formal guidelines. Either way, the ABM Treaty is the guarantor and facilitator of drastic cuts. It serves as a safety net against a free fall in U.S.-Russian relations. Remove it, and many equations change for the worse.
To kill the ABM Treaty in order to facilitate missile defenses will make effective defenses immensely harder to achieve while short-circuiting deep cuts in nuclear forces. If we're smart and if Moscow is sensible, we can have the ABM Treaty, drastic cuts and effective theater missile defenses--as long as we defer plans for continental defenses.
Why push technology and money away from where the real threat lies, toward a far less serious but more intractable problem? The first priority now is learning how to intercept short-range missiles. Next comes the problem of defending against intermediate-range missiles, which are possessed by a few states, most notably North Korea. Then comes the problem of defending against intercontinental missiles that might be launched accidentally or without authorization.
Despite the best efforts of defense enthusiasts, strategic vulnerability would exist even in the absence of the ABM Treaty, because Moscow will continue to have the means to destroy the United States. Rather than fighting this problem, a far wiser approach calls for drastic reductions in Russian and U.S. nuclear arsenals while phasing in theater defenses. This can be accomplished--but only as a cooperative enterprise with Moscow.*