Graham Platner’s bid for Maine’s U.S. Senate seat was dealt another significant blow when his national finance director became the third senior staffer to abandon the embattled campaign in a matter of weeks.

Ronald Holmes announced his departure Friday in a LinkedIn post, citing a misalignment between his professional standards and the campaign’s direction. His exit follows a tumultuous period that has seen Mr. Platner’s organizing team crumble amid controversies over old social media posts and a tattoo with alleged Nazi symbolism.

“I joined this campaign because I believed in building something different — a campaign of fresh energy, integrity, and reform-minded thinking in a political system that often resists exactly those things,” Mr. Holmes wrote in his post. “Somewhere along the way, I began to feel that my professional standards as a campaign professional no longer fully aligned with those of the campaign.”

The staffing crisis began earlier this month when political director Genevieve McDonald resigned, delivering a scathing rebuke of her former boss. Ms. McDonald said she could not overlook Mr. Platner’s previous Reddit posts, in which he had self-identified as a communist and made dismissive comments about sexual assault in the military.

Days later, campaign manager Kevin Brown stepped down after less than a week in the role, citing the impending birth of a child as his reason for leaving.

The rapid succession of departures leaves serious questions about the viability of Mr. Platner’s campaign, which had shown early promise with a strong fundraising performance. The first-time candidate raised more than $3 million in his first six weeks, largely from small-dollar donors who were attracted to his progressive platform and an endorsement from Senator Bernie Sanders, a Vermont independent.

Maine’s primary election is not until June 2026, so candidates have seven more months to make their case to voters.

The personnel shake-up comes as Mr. Platner, an oyster farmer who is running as a progressive for the Democratic nomination, grapples with damaging revelations from his past.

Before launching his campaign, he maintained a Reddit account where he made several comments years ago that were dismissive of sexual assault and questioned why “Black people don’t tip.” While Mr. Platner has disavowed these posts, saying he doesn’t think “any of that is indicative” of who he is today, the comments have dogged his campaign.

Adding fuel to the fire, Mr. Platner was forced to address a tattoo on his chest that bears resemblance to a Nazi symbol used by SS police divisions. The Maine Democrat said he got the skull and crossbones tattoo in 2007 during a night of drinking while on leave in Croatia as a Marine Corps member. He covered the tattoo last week, claiming he was unaware of its Nazi connotations until others informed him.

The controversies surrounding his campaign intensified following Governor Janet Mills’s entry into the Senate race in mid-October. Ms. Mills, 77, who has the backing of the Senate minority leader, Chuck Schumer of New York, is now seen as the establishment favorite to challenge the Republican incumbent, Susan Collins in what Democrats view as a must-win race to flip the Senate majority.