Susan Collins and Sara Gideon face off in key Senate race that could determine whether Mitch McConnell remains majority leader

Four candidates running for the U.S. Senate in Maine will appear on the debate stage together Friday for the first time in one of the most closely-watched races in the country.

The running battle between Republican Sen. Susan Collins and Democratic rival Sara Gideon, the speaker of the state House of Representatives, could determine control of the U.S. Senate. It has drawn national attention and a steady flood of out-of-state contributions — the campaigns and outside donors have so far poured more than $60 million into appealing to Maine's one million registered voters.

The two will be joined onstage by Max Linn and Lisa Savage, independents who, while not polling double-digits, could play spoiler roles in the ranked-choice election.

"We're glad we can offer voters an up-close look as the candidates make their case under questioning," Cliff Schechtman, the executive editor of the Portland Press Herald/Maine Sunday Telegram, said. "It's so vital now for the public to have enough information to make an informed decision."

Gideon has for months held a slim but steady lead over Collins, who has struggled to square her moderate, bipartisan reputation with the vagaries of hardline Trumpism.

A Bangor Daily News/AARP poll released Thursday shows Gideon with a one-point lead — significantly smaller than her margins to date. However, while that poll included Savage, it did not include the conservative independent Linn, who could pull from Collins in November.

The nominally pro-choice Collins has taken hard hits for her support for Supreme Court Justice Brett Kavanaugh, especially among critical women voters in the state, and has recently come under fire over allegations of corruption.

Salon exclusively reported Wednesday that the ethics watchdog American Democracy Legal Fund filed a complaint with the U.S. Senate Ethics Committee requesting an investigation into accusations that the three-term incumbent used her office to financially benefit her husband, and subsequently, herself.

The Democratic PAC American Bridge will attempt to capitalize on the recent allegations, launching a multi-pronged campaign — through digital ads, as well a website and text messages — targeting registered independent voters in the Pine State.

"Susan Collins is a corrupt politician who used her office to benefit herself, which is why ethics watchdogs are calling for an investigation into her unethical behavior," American Bridge 21st Century spokesperson Zach Hudson told Salon. "Susan Collins' net worth rose by millions as she was using her Senate office to help her husband's lobbying business. She's working for herself — not Mainers."

Gideon, an experienced and popular politician who won the Democratic primary easily, has attacked Collins over her vote to confirm Kavanaugh and her support for President Donald Trump's 2017 tax bill. She has also tried to elevate her own efforts on climate change, education, the opioid crisis and women's health issues, such as abortion access and reproductive care.

Collins, for her part, has leaned on the Paycheck Protection Program, the provision of the CARES Act she co-authored, which doled out more than $2 billion in forgivable small business loans.

The Collins campaign also sees Gideon as vulnerable to financial woes of her own, in particular her tax delinquency following a failed real estate venture.

Collins has refused to say whether she supports Trump's re-election. And where Collins stands on Trump – or her continued refusal to weigh in – could be a key litmus test for Maine voters who despise the president, as well as for die-hard Trump supporters whose support Collins needs to be re-elected.

Collins and Gideon have stolen the headlines, but the debate will give voters an opportunity to learn more about the independents, Savage and Linn.

Savage, a Green Independent, is running to Gideon's left on a platform reminiscent of Bernie Sanders' failed presidential bid — including the Green New Deal, Medicare for All and tuition-free college education.

"I'm amazed at the interest in this debate not just in Maine but across the nation," Savage told The Portland Press Herald in a statement. "People are curious about how ranked-choice voting changes the game, allowing people to vote their hopes rather than their fears without being told they are 'spoiling' the election. I'm interested to see other candidates' RCV strategies."

Linn ran as a Trump Republican against Sen. Angus King, I-Maine, in 2018, but he failed to qualify for the primary. He claims that state Republican leaders bullied him out of challenging Collins this year.

"When I first started my campaign for United States Senate, I was told by establishment operatives that I would never be in a debate and my voice would never be heard," Linn told The Press Herald. "Well they were wrong. What I can promise is that I am bringing high energy and the best damn platform in the race, and the $100 million dollar dark money Goliath better be ready for a fight."