Representative Liz Cheney of Wyoming, the highest-ranking Republican woman in the House, said Thursday that she would not seek the Senate seat being vacated by Michael B. Enzi, choosing instead to focus on her re-election campaign.

The announcement, which was first reported by The Casper Star Tribune, ended speculation that Ms. Cheney — the chairwoman of the House Republican Conference and a daughter of former Vice President Dick Cheney — would give up her place in House leadership to join a competitive primary for the Senate seat. She had run for the seat in 2014 but withdrew from the race.

Ms. Cheney confirmed the news in a statement posted on Twitter, in which she also attacked House Democrats and Speaker Nancy Pelosi, saying that they “are threatening our freedom and our Wyoming values every day” and “must be stopped.”

“Our nation is facing grave security challenges overseas and the House Democrats are working to weaken our president and embolden our enemies,” she said. “I believe I can have the biggest impact for the people of Wyoming by remaining in leadership in the House of Representatives.”