Washington — Democrat Graham Platner and GOP Sen. Susan Collins will face off in a November contest that could determine which party controls the Senate next year, a race that was shaken up in recent weeks by a spate of allegations against Platner.

Platner won the Democratic primary on Tuesday night after facing minimal opposition, CBS News projects. Collins was unopposed in the Republican primary.

The primaries set up a race that could be key to Democrats' hopes of winning control of the Senate. Democrats have long been trying to oust Collins, a five-term GOP senator who's survived a handful of tough reelection fights since coming to the Senate in 1997. The 73-year-old, who wields significant power as chair of the Appropriations Committee, is known for regularly breaking with her party and is among the most independent-minded in the GOP conference. But her Democratic opponent has campaigned against that record, saying for Maine, "symbolic opposition" isn't enough.

Platner, a 41-year-old veteran and oyster farmer, had seen surging support among Democrats in Maine when he jumped in the race last year, billing himself as a working class, anti-establishment candidate with a focus on opposing the "billionaire class." He quickly gained support from leading progressives like Sens. Bernie Sanders of Vermont and later, Elizabeth Warren of Massachusetts. 

But Platner's introduction on the political scene also unearthed a series of allegations and controversies. And in the lead up to Tuesday's primary, the allegations ramped up. 

In the days ahead of the primary, The Wall Street Journal reported that Platner's wife told his campaign that he sent sexually explicit text messages to other women soon after the couple were married in 2023. Then, The New York Times reported on allegations of Platner's "unsettling" behavior toward women he dated, including one claim that he was physically abusive. Platner has denied that he was physically abusive and suggested that the allegations are politically motivated. 

The allegations came after Platner effectively locked up the nomination when Maine Gov. Janet Mills dropped out of the race in April. The 78-year-old governor was recruited by Senate Minority Leader Chuck Schumer and had the backing of national Democrats. But Mills was unable to catch up to Platner in the polls or fundraising efforts, leaving skeptical moderates no choice but to fall behind Platner.

Although she suspended her campaign, Mills' name was still on the ballot on Tuesday. 

Schumer, asked repeatedly about his support for Platner amid the allegations last week, told reporters that "we're going to beat Susan Collins and take back the Senate."

CBS News on Wednesday obtained a memo from Senate Republicans' campaign arm that slammed Democrats for "propping up" Platner after the allegations came forward.

"The lesson is simple: Graham Platner is wounded, but Democrats are not treating him as disposable," the memo said. "They are treating him as a vehicle for Senate control. National donors and supporters should not mistake Democrat discomfort for Democrat retreat. The party is consolidating, the money is moving, and the grassroots are already rationalizing the choice. Republicans need to match that urgency immediately. Define Platner. Defend Collins. Resource Maine."

Platner's supporters in Congress have largely come to his defense in light of the allegations. Sanders told reporters that instead of worrying about Platner's personal life, people should "worry about what's happening to the working families in this country." Democratic Rep. Ro Khanna of California said on CBS News' "Face the Nation" that there should be acknowledgement that Platner's actions were "misogynistic" and "wrong." Then, Khanna said the attention should be on "moving on with redemption" and a focus on the issues on which Platner has campaigned.

The most recent allegations followed a handful of other personal controversies that have punctuated Platner's campaign. He's apologized for a number of problematic internet comments he made years ago and covered up a tattoo he got during his time in the Marines that is widely recognized as a Nazi symbol.

On the eve of the primary, Platner's former political director, Genevieve McDonald, wrote an op-ed arguing that he shouldn't be a U.S. senator, telling Maine Democrats that they "don't have to accept" him as the sole choice to take on Collins. McDonald, who exited the campaign amid upheaval over Platner's posts last fall, suggested that Platner could withdraw after the primary and allow for Maine Democrats to select a different nominee.

Maine election law dictates that a nominee can be replaced if they withdraw before 5 p.m. on the second Monday in July, at which point a political party would have two weeks to select a new nominee.

In an acceptance speech Tuesday evening, Platner said: "I've made mistakes in my life — mistakes that I regret, that I live with and that I continue to learn from. I'm still far from perfect, but every day, I wake up and I try to be a little bit better and a little bit kinder than I was the day before."

Five months out, the race is expected to be one of the most competitive this cycle. Collins is the only Republican seeking reelection in the Senate in a state that Vice President Kamala Harris won in 2024. The Cook Political Report rates the race a toss up.

With 53 seats controlled by Republicans in the upper chamber, Democrats would need to flip four seats and defend a handful of incumbents in tight races to win back the Senate, making the task an uphill battle — and one they likely can't do without Maine.