Hello Southern Chicken,
“The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.”
— U.S. Constitution, Article VI, clause 3
Here is the actual Oath, as determined by the House, that they all took:
“I, AB, 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.”
US House of Representatives
I draw your attention to the phrase:
"without any mental reservation or purpose of evasion,"
and the phrase:
"will bear true faith and allegiance to" [the Constitution]
So their job is to openly consider the evidence without any partisan consideration, not to defend the President from the get-go.
What they did is showing allegiance to the Republican party, to President Trump, not the Constitution.
Therefore, they have violated their oaths of office.