Dan Kleban, a co-founder of the Maine Beer Company, announced on Wednesday that he was running for Senate in Maine, joining an expanding Democratic primary field that may not be settled for months.

National party leaders are anxiously waiting to hear whether Gov. Janet Mills, a two-term Democrat, will join the list of those vying to unseat Senator Susan Collins, the lone remaining Senate Republican in New England.

Ms. Mills, 77, said recently that while she was “seriously considering” a run against Ms. Collins, she was unlikely to make any announcement until mid-November. “I’m not in any rush to make a decision,” she told reporters.

But Mr. Kleban, 48, is not waiting for Ms. Mills to decide, hoping his personal story — a college dropout from a middle-class family who went on to found a successful brewery — will appeal to voters. He’s drawing on his company’s motto, “Do what’s right,” to ground his campaign.

“I just don’t think Susan Collins is up to the job anymore,” Mr. Kleban said in an interview.

But before taking on Ms. Collins, 72, Mr. Kleban must navigate a primary contest that already includes Graham Platner, a 40-year-old oyster farmer and former Marine, and Jordan Wood, a former congressional aide.

Notably, Mr. Kleban, a political newcomer, did not pledge to continue to his run if Ms. Mills ultimately enters the race.