PORTLAND, Maine (AP) - Two challengers from opposite ends of the political spectrum are looking at an uphill battle to knock off a popular incumbent in the first U.S. Senate race to use a ranked-choice style of voting.

Independent Sen. Angus King, first elected in 2012, has enjoyed high approval ratings in his state and is favored to win another term. But Maine’s adoption of ranked choice, approved in a state referendum in 2016, adds a wrinkle to his bid against Republican State Sen. Eric Brakey and Democratic activist Zak Ringelstein.

King, 74, is a former Maine governor who caucuses with the Democrats and has a track record as a political moderate. Brakey, 30, is a Libertarian-leaning Republican who favors limited government, protecting gun rights and keeping the U.S. out of foreign wars. Ringelstein, 32, is a member of leftist group Democratic Socialists of America who supports expanding Medicare to everyone.

The ranked-choice ballot will allow voters to select their top two candidates in the race, creating an “instant runoff” in case no candidate breaks 50 percent. King coasted to victory with 53 percent of the vote against a Democrat and a Republican in 2012, and expects a similar result this time around.

“I’ve got one opponent on the Republican side who says I’m way too liberal and one opponent on the Democratic side who says I’m way too conservative,” King said. “I try not to be ideological so much as pragmatic about what will work for Maine people.”

King has spent his time in the Senate prioritizing issues such as protecting veterans, tackling the opioid crisis, expanding rural broadband access and supporting Maine’s traditional industries and food producers. More recently, he has cast key votes in favor of preserving the Affordable Care Act and against Supreme Court nominee Brett Kavanaugh as well as come out in support of Planned Parenthood.

He has vastly outraised his opponents, collecting more than $5 million as of Sept. 30. By that same date, Brakey had raised a little more than $700,000 and Ringelstein had raised about $324,000.

But both challengers think they have a chance to topple the incumbent, and Brakey believes he could attract support from progressive voters despite his status as a Republican. Brakey, a state senator since 2014, opposes military interventionism and wants to expand legal cannabis, which are two stances he believes will appeal to liberal voters.

Brakey is also a staunch supporter of the free market who wants to cut what he called “corporate welfare” to private industry.

“There are certainly issues I have in common with some of those progressive voters that Angus King does not,” Brakey said. “They’ll have common ground with me.”

Ringelstein is a former public school teacher who is campaigning on issues such as investing in education, tackling income inequality and creating a single-payer health care model. He has received little support from the Democratic Party, but like similar candidates in other parts of the country, he said he expects his message to resonate with disaffected voters who feel overlooked by machine politics.

Ringelstein’s campaign platforms are reminiscent of those of independent Sen. Bernie Sanders in the run-up to the 2016 presidential election, and while he did not have a role in Sanders’ campaign, he said “a lot of people associate my policies with those of his.” Similar to Sanders, he said he wants America to have an economy that works for everyone, not just the wealthy.

He has called on voters to use the ranked-choice ballot to select him first and King second.

“There is this awesome opportunity in this race to vote your hopes and your conscious and not your fear,” Ringelstein said. “Angus is a good senator, not a great senator.”

The ranked-choice voting format was used for the first time in Maine in the June primaries. It is allowed in federal races, such as for Congress, and in partisan primaries, but not for general elections for state office, such as governor.

Residents of Maine’s 1st Congressional District will also vote in a November ranked-choice election between Democratic incumbent Rep. Chellie Pingree, Republican Mark Holbrook and independent State Rep. Martin Grohman. Voters in the race for the 2nd Congressional District will use ranked-choice as the election includes incumbent Republican Rep. Bruce Poliquin, Democratic State Rep. Jared Golden and a pair of independents.