Instead, Noem said, she'll run for reelection to the House. Noem pledged to work to help a Republican win back the seat to be vacated by retiring Sen. Tim JohnsonTimothy (Tim) Peter JohnsonSenate GOP rejects Trump’s call to go big on gun legislation Court ruling could be game changer for Dems in Nevada Bank lobbyists counting down to Shelby’s exit MORE (D-S.D.).

"It is critical that we elect a Republican to the U.S. Senate in 2014 who will hold this Administration accountable, rein in wasteful spending and reduce our debt, and protect our liberties. I will work hard to help elect a South Dakota Republican to the U.S. Senate next year," she wrote.

In late May, Noem told The Hill that she was discussing a potential Senate run with groups interested in her candidacy, though she wouldn't elaborate. 

She was considered a more conservative alternative to the only announced Republican in the race, former Gov. Mike Rounds. 

But some Republicans were worried that if Noem launched a primary challenge against Rounds, that would ultimately weaken the eventual nominee and jeopardize Republican chances at the seat.

Noem's decision not to run eliminates a hurdle to Rounds's nomination, but outside conservative groups are still actively looking for a potential challenger.