Former Nevada Republican state lawmaker Jim Marchant announced Tuesday he is entering the race for U.S. Senate, looking to unseat incumbent Democratic Sen. Jacky Rosen in 2024.

Marchant, who has led a group of Donald Trump supporters who claim the 2020 election was stolen from the former president, was endorsed by Trump in his failed secretary of state bid in Nevada last year. He also lost his bid for a House seat in 2020 to Democrat Steven Horsford and sued unsuccessfully to overturn that result.

He is the founder of the America First Secretary of State Coalition, a group that advocates for more restrictive ballot access laws in their states. Marchant himself has been a proponent of counting ballots by hand, a process that some election officials have said is less accurate and more costly than a machine count.

His announcement Tuesday makes him Rosen’s first significant Republican challenger since she announced her reelection bid in April.

On Tuesday, Marchant’s Twitter bio read: “Candidate For U.S. Senate NV. I promise, I will aggressively fulfill all of the duties of the office and I will specifically focus on election integrity.”