If Congress enacted a bill that was in violation of the ADA, this is what would likely happen:
A citizen or a citizen's group would likely challenge the bill in Court. They would sue Congress, in essence.
A Federal District Court would hear the case. And then decide one way or another.
There is a chance that the Court would make it the entire way up to the Supreme Court.
Either way, though, the judiciary would review it (only after a case was brought) to determine whether the law was constitutional.
