Of course it’s a viable challenge! The nature of the Constitution is that it can be amended and those amendments can be challenged in the courts for Constitutionality. There is no such thing as settled law. Otherwise, lawyers would not have a job!
Some cases have been on the books for long enough without any substantial challenge rising up against them as to be settled, however controversial they were at the time. Obvious examples would be Brown v. Board of Education (school desegregation) and Marbury v. Madison (establishing the right of the courts to strike down laws as unconstitutional).
There has never been a supreme court case involving the definition of natural born citizen as it applies to eligibility to be President under the Constitution. It is far from being settled law.
Who would have to say Obama was qualified, no question about it?
Pelosi? Reid?
Who would have to say Obama was qualified, no question about him being legal for the office?
Pelosi? Reid?