So, if a congressman pledges that he will not raise taxes, he is violating the constitution. Utter nonsense.
By your logic, if a congress didn't establish any new post offices or post roads, they would be in violation of the constitution. There are already post offices/roads in place, what if we didn't need any new ones? Must congress establish one just because it's their "constitutional duty" as laid out in Article 1, Section 8? Heaven forbid a congressman sign a pledge that states he won't build any new ones. That would just be treasonous.