Impeachment should not be for disagreements over policy or what is Constitutional or the proper rationale of a President when determining if he should sign legislation.
If he opined that he thought it was Constitutional then it wouldn't have been an impeachable offense? How about if he had just kept his mouth shut on the Constitutionality?
That being said I would hesitate to vote for any candidate who thought he should just go ahead and sign legislation that in HIS OPINION exceeds the powers given under the Constitution - because it is the role of the SCOTUS to make that determination.
WRONG - (but not an impeachable offense) - the Constitutionality of a law should be considered FIRST, LAST, and ALWAYS by EVERYONE involved.
I see your point. Yeah, impeachment is probably too strong in that case, but I was more upset with him over that than anything else he did during his time in office, as I considered that action a violation of his oath of office to defend the Constitution(of course the current occupant violates it continually).