US President Donald Trump said Friday that he had instructed his administration to file a lawsuit to block the certification of the US Senate race in Alaska if Republican Senator Dan Sullivan loses, arguing that voters would be confused because two Republicans on the ballot have similar names. The legal battle is ramping up questions over whether Trump will accept a potential midterms defeat.

The Trump administration has sued Alaska to preemptively block the state from certifying the results of its US Senate race if the Republican incumbent candidate loses narrowly, saying voters could be confused because two Republicans on the ballot have almost identical names.

The Department of Justice's lawsuit, filed in federal court in Anchorage on Thursday, 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, where his fellow Republicans narrowly control both chambers.

The administration has taken an increasingly aggressive approach to election oversight, pressing states for voter-registration data and arguing that stricter measures are needed to ensure public confidence in the vote. Critics, including Democratic and some Republican election officials, view the moves as federal overreach.

Control of the Senate could come down to Alaska, where incumbent Republican Dan S. Sullivan is running a close race against Democrat Mary Peltola.

The suit, which names the state and local election officials as defendants, takes issue with the presence of another 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 DOJ asks a 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 on Friday that he supported the lawsuit, saying "I'm the one that said do it." A DOJ spokesperson said in a statement that 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 country have told Reuters they are preparing for potential federal interference in the 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 unfavorable.

Alaska's other US senator, Senator Lisa Murkowski from Trump's Republican Party, said on Friday the Justice Department "should never have filed this lawsuit".

Election law experts said the 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".

Trump's administration "has absolutely no authority to tell Alaska when or how to certify the results of its Senate election", said David Becker of the Center for Election Innovation and Research, a nonpartisan group that advises election officials, adding that "this issue has already been fully litigated by Alaska's courts".

Read more'Thanks a lot, Trump': Wyoming conservatives divided ahead of US midterms The unelected Sullivan, a former US Forest Service worker and elementary school teacher, filed to run as a candidate on May 29, seeking to unseat 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.

The Alaska Supreme Court in June permitted the challenger Sullivan to appear on the ballot. He will appear without a party designation next to his name, while the senator will be listed as the Republican incumbent.

Peltola, the Democratic candidate, told MS NOW on Friday that because of how the candidates are displayed on the ballot "there just isn't much room for confusion". The lawsuit is "sending a message that Dan Sullivan does not believe that Alaskans are sophisticated enough to know exactly who they're voting for", she added.

Trump's approval rating has fallen to a record low, hit by rising diesel and gasoline prices during the Iran war, which has proven deeply unpopular with voters.

Public opinion polls suggest Republicans may lose control of one or both chambers of Congress, which would likely limit Trump's ability to enact some of his policy agenda in the final two years of his presidency.