“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.”
What do you think the duties of the office entail?
Turns out, selling out your constituency and betraying people who voted for you only has short term benefits.
If she hadn't left the party, she could count on the party's overwhelming support in the primaries, ostensibly because she's the incumbent, but actually because any challenger would be more progressive than her.
The Republican party seems to have an ethos of “any win at any cost”, which has led us to Trump. It might sting in the short term to lose a senate seat, but benefit longer term to oust someone who does you no good anyway.