All United States senators and congress people are required to swear or affirm the same oath of office.
I do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter: So help me God.
Signing other pledges, such as the Taxpayer Protection Pledge, prior to entering office sets up a potential conflict of interest. The only pledge elected officials should be following is their formal oath of office. They are here to serve our country, not special interests.