Why does the pledge to Grover Norquist seem to take precedence over a congressperson's oath of office? Don't our Senators and Representatives swear to uphold the constitution not some private document.

While the words in common parlance are used interchangeably it seems to me that an oath (particularly when taken on a bible) is like what a lawyer would call a 'contract with consideration,(that is both sides give something) while a pledge is more like a promise without consideration (that is, one sided).

Thusly, I think it could be argued that an oath trumps a pledge.

I think law suits should be brought against every one of them who says they can't change their vote because they signed an unenforceable document. They will probably loose, but these folks will have to explain themselves

Anyone like to pledge to research this?

From Wikipedia
An oath (from Anglo-Saxon āð, also called plight) is either a statement of fact or a promise calling upon something or someone that the oath maker considers sacred, usually God, as a witness to the binding nature of the promise or the truth of the statement of fact. To swear is to take an oath, to make a solemn vow. Those who conscientiously object to making an oath will often make an affirmation instead.

A promise is a commitment by someone to do or not do something.