PORTLAND, Maine (AP) — Fighting for political survival, Republican U.S. Rep. Bruce Poliquin joined a lawsuit Tuesday over Maine’s new voting system, used for the first time in U.S. House and Senate elections.

A lawyer for Poliquin’s campaign asked the secretary of state to stop the tabulations taking place in Augusta to allow a judge to rule, but the secretary declined to stop the process.

Poliquin received the most first-place votes on Election Day and believes he should be declared the winner. But the ranked-choice system requires additional voting rounds because neither he nor Democrat Jared Golden won an outright majority.

Poliquin’s Democratic challenger decried the 11th-hour attempt to derail the process following the most expensive political race in Maine history.

“Any attempt by Bruce Poliquin to change the rules after votes have already been cast is an affront to the law and to the people of Maine,” Jon Breed, Golden’s campaign manager, said in a statement Tuesday.

The lawsuit, filed Tuesday in federal court, seeks a preliminary injunction. A separate letter was sent to the secretary of state seeking a suspension of tabulations.

“We are continuing to process ballots to complete the tabulation of votes and will continue to do so,” said Kristen Schulze Muszynski, spokeswoman for Secretary of State Matthew Dunlap. “If we receive a court order to halt the process we will review it with our legal advisers.”

The ranked-choice voting system approved by referendum in 2016 lets voters rank candidates from first to last on the ballot. It provides for eliminations of last-place candidates and reallocations of votes to ensure that the winner gets a majority.

The lawsuit, brought by Poliquin and three other voters, says the “foundation of our ‘democratic process’ is the right of all qualified voters to cast their votes effectively.” It suggests that the ranked voting system “denies Plaintiffs the opportunity to cast their votes effectively.”

The lawsuit makes several arguments, including that the voting system violates the “one-person, one-vote” principle, violating the Equal Protection Clause.

Supporters say the system already has cleared legal challenges. Several courts, including the 9th U.S. Circuit Court of Appeals in San Francisco, have upheld the constitutionality of ranked-choice voting, supporters said.

“We’re seeing these kinds of post-election sore loser lawsuits across the country. That’s what this is,” said Rob Richie, of FairVote, an electoral reform group that pressed for Maine’s adoption of the voting system.

The first round of voting on Election Day ended with Poliquin and Golden both collecting 46 percent of the vote, with Poliquin maintaining a slim edge of about 2,000 votes in unofficial returns.

Under the system, two independents in the four-way race who together collected about 8 percent of the vote will be eliminated, and their supporters’ second-choice votes will be reallocated to Poliquin and Golden.

For now, the voting system is used only in federal races and in statewide primary elections in Maine. It cannot be used in the governor’s race or legislative races because of concerns it runs afoul of the Maine Constitution.

Democratic Gov.-elect Janet Mills has vowed to seek to amend the constitution so the system can be used in all elections.