Such refusal would be tantamount to what has taken place since Marshall. The refusal would be a precedent, and if followed faithfully, would be tantamount to a pragmatic invalidation of the "law."
But note, here: The Congress and President are bound by the same oath. Consequently, you have multiple checks on the application of Government. Even as the Supreme Court--or for that matter, any local Court--is honor bound upon their oaths, not to enforce or apply legislation believed to be unConstitutional; so too the President is honor bound not to apply any legislation he deems to be unconstitutional--and not to apply a judicial decision that has the effect of making new law, if he feels it to be in error. (This is not an issue with the Rehnquist Court, which has not been legislating. But it was very much an issue with the Warren Court, which clearly did.)
Of course, Congress has no business passing legislation that is unConstitutional in the first place.
The bottom line is that if Washington were full of men of honor, we would not have most of the laws that are in the United States Code, today.
As for Gephardt or Kucinich ruling by decree? The day after either of them is elected, will be the day that every forewarned patriot takes such action as may be appropriate--with "Committees of Correspondence" to coordinate what is needed. We should be very glad for that "forewarning," because it will also give us all plenty of time to discuss these matters with family members in the armed services, to make sure that everyone understands what the newly elected intends, etc..
William Flax Return Of The Gods Web Site