Jim Marchant, a Trump loyalist, is G.O.P. voters’ choice to be Nevada’s top election official.

Jim Marchant, one of the organizers of a Trump-inspired “America First” slate of candidates who continue to question the legitimacy of the 2020 election, easily won the Republican nomination for secretary of state in Nevada, a key political battleground.

His victory was called by The Associated Press.

Mr. Marchant, who was also a member of Nevada’s alternate slate of pro-Trump electors seeking to overturn Joseph R. Biden Jr.’s victory in the state in 2020, has said he would have refused to certify that year’s election had he been in office.

In his push to become Nevada’s top election official, he has proposed decertifying Dominion voting machines and pushing for the hand counting of paper ballots, which experts say would bring lengthy delays and chaos to the voting process.

In the general election, he will face Cisco Aguilar, a Democratic lawyer who once worked for former Senator Harry Reid, in what will be a closely watched race that could hinge on the outcome of Senator Catherine Cortez Masto’s re-election bid. Down-ballot races for offices like secretary of state have often — though not always — closely mirrored results for top-of-the-ticket elections.

A former state assemblyman, Mr. Marchant unsuccessfully ran for Congress in 2020. But rather than concede a race he lost by more than 16,000 votes, or about 5 percentage points, he blamed fraud for his loss, filing a lawsuit that echoed many debunked claims about the 2020 election that Republicans put forward elsewhere.

The lawsuit, and an effort to hold a new election in Clark County, which includes Las Vegas, eventually failed, but Mr. Marchant continued to be a vocal challenger of the 2020 results.

His efforts soon became fodder for a run for secretary of state. In an interview with The New York Times this year, Mr. Marchant said he had been approached by allies of former President Donald J. Trump who encouraged him to run for secretary of state.

Soon after he announced his candidacy, Mr. Marchant played a crucial role in establishing the “America First” slate of candidates for secretary of state and other offices.

The roster initially included Mark Finchem, a candidate for secretary of state in Arizona who has led efforts to decertify the 2020 election, and Kristina Karamo, the likely Republican nominee for secretary of state in Michigan, who has pushed the fiction that voting machines were “flipping” votes for Mr. Biden, among other falsehoods.

The group, which has since grown to encompass more than a dozen candidates, was buoyed by the financial support of prominent Republican donors who have aligned themselves with the movement to discredit the 2020 election, like Mike Lindell, the founder of MyPillow, and Patrick Byrne, the founder of Overstock.com.

Recent meetings and events held by the “America First” slate have featured prominent adherents of the QAnon conspiracy theory, and the candidates have often traveled to other states to speak at rallies focused on the 2020 election.

Mr. Marchant is the fourth member of the group to succeed in securing a Republican nomination:

• Audrey Trujillo, a candidate for secretary of state in New Mexico, ran unopposed in her primary.

• Doug Mastriano, a Republican candidate for governor of Pennsylvania, won his primary last month. If elected, he will be able to appoint the top election official in the critical swing state.

• And Ms. Karamo has won the backing of Republican delegates in Michigan, all but assuring her nomination in the state’s August primary.