I can't imagine anyone would support that scarecrow.
The point of the ABM treaty was for no one to have ABMs. No one having them meant that people would be less likely to use nukes because effective retaliation was assured. If somebody has a missile defense, they are free to start doing numbers games on how safe they might be if they decided to nuke someone who was pissing them off.
Alternatively, how seriously would China take our friendship with Taiwan, if they thought that our nuclear arsenal was not to be feared?
There are cans of worms to consider, when you unravel treaties.