JUNEAU, Alaska (AP) — Republican U.S. Sen. Dan Sullivan is running for reelection in Alaska and faces a field of 15 competitors. One of them is Dan Sullivan.

The senator told reporters in Washington, D.C., on Tuesday that he thinks the appearance on the state’s primary ballot of another Republican with the same name is a dirty political trick coordinated by Democrats and the campaign of his chief rival for the seat, former Democratic U.S. Rep. Mary Peltola. He threatened a lawsuit to get to the bottom of it.

“Everybody in Alaska knows I’m Dan Sullivan-R. So he’s trying to do that. Why?” the senator said of the other Dan Sullivan. “He’s not an R. He’s purposely trying to trick my constituents to rig the election for Peltola.”

A spokesman for the Peltola campaign, Harry Child, said it “has no involvement with either Sullivan campaign.” Jenny-Marie Stryker, executive director of the Alaska Democratic Party, said her organization “is in no way affiliated with either Dan Sullivan.”

The kerfuffle over the dueling Dan Sullivans on Alaska’s August primary ballot has drawn the attention of state and national Republicans. They claim that adding a second Dan Sullivan to the ballot will sow confusion among voters who support the incumbent and help Peltola.

In Alaska’s primary, the top four vote-getters, regardless of party affiliation, advance to the ranked-choice general election.

Blake Murphy, an attorney for the National Republican Senatorial Committee, sent a letter dated Monday to Alaska election officials outlining concerns about the potential for voter confusion. Murphy also raised questions about the party affiliation of the challenger, calling the other Dan Sullivan a “sham” candidate.

Murphy wrote the NRSC could consider legal action “to ensure that the Alaska electoral process remains fair.”

Carmela Warfield, the state Republican Party chair, said in a statement accompanying Murphy’s letter that until recently, the challenger was registered as undeclared. In previous years, ballots have not identified which candidates were incumbents.

A spokesperson for the Alaska Division of Elections, Steve Kirch, did not immediately respond to a request for comment on the letter and said a records request would be required to get details of the challenger Sullivan’s voter registration history.

Sullivan, the incumbent senator, is an ally of President Donald Trump and is seeking a third term. He and Peltola are the highest-profile candidates in a race that’s being closely watched nationally as Democrats try to retake the Senate majority in this year’s midterm elections.

State election officials certified a Republican Dan J. Sullivan, from the southeast Alaska fishing community of Petersburg, as one of the candidates in the U.S. Senate race. Efforts to reach him were not immediately successful on Tuesday, and he did not appear to be registered with the Federal Election Commission.

His campaign website says he is a Midwesterner who moved to Alaska to work for the U.S. Forest Service and has been in Petersburg for nearly 50 years. After becoming disillusioned with what he saw as government inefficiency and “lack of long-term thinking,” he switched careers and became an elementary school teacher, the site says.

The challenger Sullivan said on his campaign website that the state deserves a senator who “puts Alaska first every single day. That’s the commitment I’m making to the people of this state, and together, we’re going to elect a Sullivan that actually stands up for Alaska.”

Sen. Sullivan, emphasizing his words with an expletive, told reporters in Washington that having the second Sullivan on the ballot was a scandalous attempt to trick Alaskans: “That’s an insult.”

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Freking reported from Washington.

Becky Bohrer And Kevin Freking, The Associated Press