The lawsuit signals the willingness of President Donald Trump's administration to contest the results of the November 3 midterm elections that will decide control of the US Congress.
Control of the US Senate could come down to Alaska, where incumbent Republican Senator Dan S. Sullivan is running a closely contested race against Democrat Mary Peltola.
The Justice Department's lawsuit, filed in federal court in Anchorage, takes issue with the presence of a third contender in the race, another Republican named Dan J. Sullivan Jr, alleging he is running "with the intention of misleading supporters of Senator Sullivan into voting for him."
The lawsuit asks a federal judge to block the certification of an outcome "that may be impacted by voter confusion," so that there can be "an investigation to determine whether votes cast for Daniel J. Sullivan were meant to be cast for Senator Sullivan."
Trump told reporters he supported the lawsuit, saying "I'm the one that said do it."
A Justice Department spokesperson said in a statement the agency is taking "necessary steps to ensure that this frivolous candidacy does not undermine the integrity of Alaska's election or the will of its voters."
Election officials around the US have told Reuters they are preparing for potential federal interference in November's elections, including legal challenges and disputes over voting that could disrupt certification of the results.
Trump has a long history of challenging election outcomes and encouraging supporters to contest results he views as unfavourable.
Election law experts said the Justice Department's lawsuit has no merit and seeks relief not allowed under federal law.
"In short, like many Department letters over the last few years, it is designed to win the press release rather than the litigation," wrote Notre Dame law professor Derek Muller in a blog post, adding that "the legal argument is one of the worst ones I've ever seen."
The unelected Sullivan, a former US Forest Service worker and school teacher, filed to run as a candidate on May 29 seeking to unseat Senator Dan S. Sullivan and "put Alaska first," according to his campaign website, which advertised "Dan Sullivan Challenges Dan Sullivan for US Senate Seat."
Dan J. Sullivan has denied in interviews with US news outlets that his aim is to cause confusion or divert votes from Dan S. Sullivan.
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