Troy Jackson has further tightened his grip on Maine’s Democratic nomination for Senate.

Mr. Jackson, a logger and former state lawmaker, is set to appear on the ballot at Democrats’ nominating convention on Saturday in Bangor alongside just two other candidates who have won almost no public support from delegates: David Costello and Saundra Pelletier.

Hundreds of delegates aligned with Mr. Jackson were elected last weekend, giving him a seemingly insurmountable advantage that forced his leading primary rivals to suspend their campaigns.

The lack of well-known opponents appears to set up the convention as a celebration of Mr. Jackson’s nomination. His campaign announced on Monday that it was turning its effort toward defeating Senator Susan Collins, a five-term Republican, in a race widely seen as crucial to control of the Senate.

While the other top contenders had announced that they were withdrawing from the race, there had been indications that at least one of them hoped to appear on the ballot at the convention.

But when the state party announced on Tuesday which candidates had made the ballot, the other leading candidates were not included — meaning that they had not turned in the 500 verified signatures required for a spot.

They included Dr. Nirav Shah, a former public health official who came in second in the primary for governor in June; Shenna Bellows, the Maine secretary of state, who came in fourth in that race; and Jordan Wood, a progressive activist who placed third in the Democratic primary for Maine’s Second Congressional District. Dr. Shah had previously announced that he had gathered more than 800 signatures.

Dan Kleban, a brewery owner, qualified for the ballot but filed paperwork to withdraw, a representative of the Maine Democratic Party confirmed to The New York Times.

“I’m proud to throw my support behind @TroyJackson207 to beat Susan Collins and fight for working Mainers,” Mr. Kleban said in an X post on Tuesday announcing his withdrawal. He ran for the same Senate seat last fall, but dropped his bid.

Each candidate needed to collect 50 signatures per county in at least eight counties, a requirement that had some contenders bouncing among county meetings last weekend to find signatories. All of the signatures had to come from registered Democratic primary voters.

The wave of campaign suspensions prompted CNN to call off a debate on Thursday that was set to feature Mr. Jackson, Dr. Shah, Ms. Bellows and Mr. Wood.

While delegates still have until Saturday to change their minds, the likelihood that large numbers would abandon Mr. Jackson continues to diminish as the week goes on.

Mr. Costello, an environmental policy consultant, placed third in the Democratic primary for Senate in June. Ms. Pelletier, the chief executive of a pharmaceutical company focused on women’s health, is a first-time candidate.

The party remains on edge after Graham Platner, who won 72 percent of the vote in the Democratic primary for Senate in June, relinquished the nomination following an allegation of rape and a string of other scandals. The upheaval left Democrats rushing to find a new nominee to face Ms. Collins.

Just over 600 delegates will vote at the convention in Bangor on Saturday, including 500 who were elected over the weekend and 101 from the state Democratic Party committee. More than 17,000 Maine Democrats voted or otherwise participated in county meetings to elect delegates over the weekend, the party said, including nearly 3,700 delegate hopefuls. Voters overwhelmingly favored delegates who were committed to Mr. Jackson.

“Today’s certification marks another important milestone in this open, transparent, and inclusive nominating process,” Devon Murphy-Anderson, the executive director of the Maine Democratic Party, said in a statement on Tuesday afternoon. “We look forward to hearing directly from these candidates and bringing Democrats from every corner of Maine together on Saturday to choose the strongest nominee to defeat Susan Collins.”

Any candidate has the option to withdraw from the race before Saturday, according to the party.