SANFORD, Maine — Former state Senate President Troy Jackson is racking up support to become Maine’s next Democratic Senate nominee, with his top opponents ending their campaigns on Sunday.

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Maine Secretary of State Shenna Bellows and physician Nirav Shah, a former top public health official in the state, both announced they would be leaving the race.

Bellows made her announcement Sunday morning, while Shah exited the race Sunday evening after Democrats across the state selected delegates to next week’s state nominating convention. Jackson quickly established a seemingly insurmountable advantage in delegates as his supporters swept counties throughout the state.

Delegates at next week’s convention will select a new nominee to take on GOP Sen. Susan Collins, after primary winner Graham Platner abruptly ended his campaign last week after a sexual assault allegation, which he denied.

In a nod to Jackson’s strong showing this weekend, Shah threw his support behind the former legislator.

“To Maine Democrats: Susan Collins will not be defeated without a united Democratic Party. And we cannot unite without treating one another with respect,” Shah said in a statement. “No matter whom you supported in this race, I urge us all to remember that in the weeks ahead.”

Bellows did not mention Jackson in her announcement, saying in a statement, “Donald Trump’s attacks on our democracy are only accelerating, and the best way that I can serve our state and country at this moment is by focusing exclusively on my role as Secretary of State.”

Maine Beer Company founder Dan Kleban also ended his Senate bid Sunday night, saying in a statement that it has “become clear from the county nominating meetings that we lack a path forward.”

Kleban signaled support for Jackson as well, saying, “From the very beginning, my focus has been on defeating Susan Collins. I will do whatever I can between now and November to help Troy Jackson finally achieve that goal.”

The abrupt ends to their campaigns capped a whirlwind week in which the candidates scrambled for an early edge in the race.

Democrats in York County had just started voting for delegates to next week’s state convention when county party Chairwoman Kathie Purdy announced the news that Bellows was leaving, prompting gasps throughout the crowd.

Jackson’s slate of delegates captured nearly all of the slots up for grabs in Saturday’s county meetings and continued that streak on Sunday. Those delegates are not required to support Jackson at the convention, even if they were elected as part of his slate.

At a rally Sunday morning with supporters ahead of the York County meeting, Jackson already sounded like a likely nominee.

“Every candidate and every supporter deserves our respect, because this moment, I know for a fact, is way bigger than any one candidate, certainly, is way bigger than I have ever been,” Jackson said. “And a lot of people poured their hearts in this movement, before I even entered this race.”

Jackson later hinted at a winnowing field, telling reporters at the county meeting, “If we have a day like [Saturday], the rest of the candidates will drop out and we’ll just be getting the nomination next week.”

Jackson said he had heard from a couple candidates who expressed their “congratulations.” He said they “pretty much” suggested they would end their campaigns before the convention, but declined to say whom he spoke with.

Shah had been making the case that he is the strongest contender to take on Collins, saying in a recent debate that the race came down to “electability.” And Shah told NBC News’ “Meet the Press Now” on Friday that he thought Jackson would struggle to win statewide.

“I don’t think he would have as good of a chance to beat Sen. Collins as I would,” Shah said. “Sen. Jackson came in third place in the primary, did not have the same number of first-choice [votes], and thus may not have the same ability to unify the party in all parts of the state — north, south, east, west — as I do.”

But Shah said he was backing Jackson with the goal of beating Collins in mind.

“Susan Collins, Donald Trump and Senate Republicans are banking on disunity and frustration from Maine Democrats to try and win this seat, and we are not going to give them that,” Shah said. “Maine Democrats are united, angry and energized.”

Shah, Bellows and Jackson all competed in the governor’s race but did not win the nomination. All three candidates support similarly progressive policies, like “Medicare for All” and abolishing the U.S. Immigration and Customs Enforcement agency.

They were able to quickly spin up their old campaigns after Platner’s exit, and Jackson has stressed that his team focused on organizing his slate of delegates ahead of county meetings this weekend.

“As soon as we got the delegate list, I mean, I was calling, people were calling to find out where people stood and if they needed any more information about me,” Jackson told reporters Sunday, the former logger pointing to his union roots as a strength.

“In the union, that’s what we do. We organize and go after it because it’s so important,” Jackson said.

But Jackson could still face competition at the convention.

Jordan Wood, a former congressional staffer who is running for the Senate, told NBC News at the York County meeting that he plans to stay in the race. He raised concerns about Jackson’s vocal support for Platner, suggesting Republicans could use those ties against Jackson if he is the nominee.

“I think that Troy can beat Susan Collins. I think it’s going to be difficult, though, because he helped introduce us to Graham and got a lot of — conveyed a lot of trust and authority onto him, as did Bernie Sanders,” Wood said. Sanders endorsed both Platner and Jackson in their primaries earlier this year, but has stayed neutral after Platner ended his Senate bid.

Jackson “can overcome it, but it’s going to be tough,” Wood said.

But some Mainers who supported Jackson’s opponents are beginning to view Jackson as their likely nominee.

“At the end of the day, this entire process is about whose campaign can organize the best effort,” said Shaun Donnelly, a 44-year-old computer graphics artist who had been running to be a Bellows delegate from York County. He had been supporting Bellows because after Platner, he “was not excited about getting behind another white dude.”

“If the numbers are true and Troy has the majority of the activists in the party behind him, those are the people who we need energized to win in the fall,” Donnelly said.

“So I think overall it’s a good thing, and I look forward to voting for him if he wins,” Donnelly said.