Republican Liz Cheney lost her seat in Congress in Wyoming to a Trump-backed candidate from her own party.
He admitted defeat to Harriet Hageman, whose candidacy was backed by the former president in the Republican primary.
Ms. Cheney, 56, was one of the most outspoken critics of Trump in the Republican Party and was one of 10 Republicans who voted to impeach Trump in 2021 following the January 6 Capitol riots.
Meanwhile, Ms. Hageman backed up her claim that the 2020 election was stolen.
Observers say the loss of Ms. Cheney indicates this Mr. Trump still has a grip on the Republican Party.
Speaking after the polls closed, Ms. Cheney said she could “easily” win the primary again if she chose to support the former president.
“It is a path that I could not and would not want to take,” he added.
Ms. Cheney, daughter of former Vice President Dick Cheney, spoke at length about the Capitol riots calling it a “tragic chapter” and added that no candidate should support the “election deniers”.
He then targeted Mr. Trump for his reaction to the FBI search of his Florida estate in Mar-a-Lago.
Ms. Cheney, a three-term Conservative MP, used her campaign to keep attention on Trump’s actions around the uprising and his continuing false claims about fraud in the 2020 election, in an attempt to persuade fellow Republicans that the l former president is a threat to democracy.
While his record of voting in Congress supports the former president’s agenda, Ms. Cheney’s criticisms of Mr. Trump have dismissed many Wyoming Republicans.
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Trump congratulated Ms. Hageman on her victory, calling it a “great and very decisive victory,” on his Truth Social platform. He added that it is a “wonderful achievement for America”.
Most of the candidates Trump has supported this election season have triumphed in a sign, say his supporters, of his continued influence on the party. During the 2020 campaign, Trump won the widest margin in Wyoming.
It comes amid speculation that the former president will run for office again in 2024.
Four of the 10 Republicans who voted to impeach Trump have decided to pull out, three have already lost the primary and two have survived the primary.
Ms. Cheney will now be forced to leave Congress at the end of her third and final term in January.