Boebert had switched districts after Buck announced his impending retirement to avoid a stiff challenge from Democrat Adam Frisch in the 3rd district, but Buck’s immediate departure will set up a special election in which the state Republican and Democratic parties will choose a nominee – and there’s no guarantee they’ll pick the controversial Boebert.
Further complicating matters, Boebert would have to resign her seat in the 3rd district to run in that special election because Colorado law prohibits a candidate from running for more than one office at a time, and her resignation would set up yet another U.S. House special election before the end of the year.
This is incorrect. If the Republicans choose to run someone other than her in the 4th district, she has no office to exist in, and she will no longer be a US politician, and this is not her intentional choice. If she instead chooses to participate in the special election in the 3rd district, she will probably lose, and that’s also not her intentional choice.
It remains to be seen whether she would stop seeking office, but I think that’s a fair reason to say it “could end [her] career”.