Division of Elections Director Carol Beecher in a letter to challenger Dan Sullivan said her office had received two complaints regarding his eligibility and determined “that the preponderance of evidence does not support your eligibility for the office of United States Senator.”

She gave him a Thursday deadline to submit “any additional information and evidence” in response.

Sullivan, the challenger, did not immediately respond to messages seeking comment about the letter from Beecher, a registered Republican who in the past has donated to Republican groups and campaigns. Her letter, dated Wednesday and published by the Anchorage Daily News, did not specify the evidence it found to potentially remove him from the primary ballot, and her office did not respond to requests for comment.

Sullivan’s candidacy has caused a stir in one of the most prominent U.S. Senate races in the country. It’s a seat Democrats have targeted as they try to regain the majority in the chamber in this year’s midterm elections.

Sen. Sullivan has accused his namesake challenger of working with Democrats to try to trick voters and boost the chances of his top opponent, former Democratic Rep. Mary Peltola, a claim both deny. The challenger, who lives in the small fishing community of Petersburg south of Juneau, told The Associated Press earlier this week that the decision to run was “my choice.” He said he had no contact with the Peltola campaign — “zero, none, zilch.”

This week, the challenger also pushed back in response to Republican Lt. Gov. Nancy Dahlstrom’s announcement that she was opening an investigation into his candidacy.

“The law forbids your office from denying me access to the ballot just because Senator Sullivan and the NRSC would prefer I not be allowed to run,” he wrote, referring to the National Republican Senatorial Committee.

He called the investigation “an unprecedented affront to my rights as a candidate and the rights of Alaska voters to select their own representation in the U.S. Senate.”

It was not immediately clear whether he had retained an attorney to help him remain on the ballot.

Some attorneys also have raised questions about Dahlstrom’s investigation, which among other things demanded that Sullivan explain his party affiliation, how long he had been going by the name Dan Sullivan, his affiliation with a consultant and any interactions he might have had with other candidates in the race or the Democratic Party.

Dahlstrom, who oversees elections, said in her letter to the challenger that the investigation pertained to “credible allegations” that he did not declare his candidacy “with a good faith purpose to seek office but rather with a purpose to confuse voters and have them mistakenly vote for you rather than the incumbent with the same name and same political party affiliation.”

The questions are in line with claims outlined in a letter to her and Beecher earlier this month from an attorney with the National Republican Senatorial Committee.

The ACLU of Alaska, in a statement, said it is “unaware of any other instance where the Lieutenant Governor has investigated a specific candidate for reasons other than determining whether a candidate meets federal, state and local eligibility requirements.” The group said it was monitoring the situation.

Jahna Lindemuth, who was an Alaska attorney general under an independent governor, said investigating why someone would run for office “starts infringing on free speech concerns and other protections under the Constitution.” She said Dahlstrom could label the senator as the incumbent on the ballot if she were concerned about voter confusion.

The Constitution requires senators to be at least 30 years old, a U.S. citizen for at least nine years and live in the state they’ve chosen to represent at the time of election. Sullivan, who will turn 69 this weekend, told the AP he moved to Alaska in 1980 and worked for the U.S. Forest Service before switching careers and becoming a teacher. He’s now retired.

The declaration that the elections division requires candidates to fill out asks for their name, the party affiliation they want on the ballot, their address and how they want their name to appear. In signing the declaration, candidates are asked to affirm they meet citizenship, age and residency requirements.

The division previously certified challenger Sullivan’s candidacy, noting him on the candidate list as Dan J. Sullivan. The senator is listed as Dan S. Sullivan and as the incumbent.

At least one group running ads in support of the senator, One Nation, has begun referring to him as Sen. Dan S. Sullivan.