Troy Jackson, who is expected to become the Democratic nominee for Senate in Maine, said he would not support Senator Chuck Schumer of New York to continue as the party’s leader in the chamber. Mr. Jackson also said that he was not sure he would want Mr. Schumer’s help in the general election.

Mr. Jackson made his most definitive rejection of Mr. Schumer to date in an interview on Monday with The New York Times.

“The Democratic Party needs to have a different approach — getting people back in the party, getting working-class, everyday people back in the party — and I don’t believe it’s going to happen with Senator Schumer,” he said.

Mr. Jackson, a progressive, was never likely to express strong support for Mr. Schumer, who has grown deeply unpopular with the Democratic base. But his comments are notable because he has little time or money to build a campaign after Graham Platner’s unexpected exit following a rape allegation.

The Republican incumbent, Senator Susan Collins, is backed by an array of well-funded conservative groups. Any Democratic path to victory in Maine, widely seen as a key state for Senate control, is likely to require powerful party committees and super PACs aligned with Mr. Schumer to throw their weight behind Mr. Jackson.

Still, Mr. Jackson appeared wary of support from Mr. Schumer and national Democrats.

“I’d have to find out what that entailed,” Mr. Jackson said. “I mean, if his help comes, absolutely no strings, I would say, you know, possibly. There’d have to be no strings.”

Mr. Jackson, a former president of the Maine Senate, has few ties to big donors and raised just $1.2 million in his unsuccessful campaign for governor this year. A Senate campaign in a battleground state would cost many times that amount.

“You know, Susan’s got $12 million. I got $12,” he said. “I’m hopeful that, you know, we’re going to tell the story about what we want, what we’re going to be fighting for, and people are going to come on board.”

As Democratic voters have pushed party leaders to show stronger opposition to President Trump, Mr. Schumer has suffered a precipitous drop in popularity in the party. Accustomed to choosing many Senate nominees, he has seen some of his choices this year lose primaries or face stiff challenges. Several other Democratic candidates for Senate have said they would not support Mr. Schumer for leader.

In Maine, Mr. Schumer heavily recruited Gov. Janet Mills, a Democrat who is barred by term limits from seeking re-election, to challenge Ms. Collins. Mr. Schumer discouraged other Democrats from entering the race, but in the end Ms. Mills was no match for Mr. Platner, an oyster farmer who galvanized progressives with support from Senator Bernie Sanders of Vermont. Out of money, she suspended her campaign in late April. Mr. Platner won the primary but ended his campaign two weeks ago after the rape allegation, which he has denied.

Mr. Jackson shares a political ideology with Mr. Platner, who also rejected the prospect of supporting Mr. Schumer as leader. Mr. Schumer and the Senate Democratic campaign arm offered only tepid support for Mr. Platner, even after Ms. Mills dropped out and he won the primary.

An aide to Mr. Schumer referred to a refrain the senator has often fallen back on when asked about the Maine Senate race, which he repeated again on Monday to reporters at the Capitol. “We’re going to beat Susan Collins and take back the Senate,” Mr. Schumer said.

When Mr. Jackson was asked in the interview whom he would prefer as Senate leader, he did not hesitate: “Obviously Bernie, but that ain’t going to happen,” he said. “If I am lucky enough, I guess I’ll have to survey the field. But one thing I’ve been pretty good at is judging people’s character.”

One person Mr. Jackson said he did not wish to have support from in the general election was Mr. Platner. Mr. Jackson had supported the former nominee but rescinded his endorsement after the rape allegation, which he called “credible.”

“This is about the movement, this is about issues — ‘Medicare for all,’ prescription drugs, workers’ rights, child care, having a government that works for people,” Mr. Jackson said. “It’s not about Graham; it’s not about me. It is about people having a government that is for them, and I don’t see his endorsement as being helpful at all.”