Officially, Senator Chuck Schumer of New York, the Democratic minority leader, has no preferred candidate in his party’s unruly primary race for Senate in Michigan.

But in conversations with major donors behind closed doors, Mr. Schumer, who for two decades has tried to elevate his favored primary candidates in battleground states, has been clear about his desire.

He is asking party donors to support Representative Haley Stevens, a moderate candidate running against two more progressive Democrats, according to four people briefed on the communications over the last few weeks, who spoke on the condition of anonymity to disclose private conversations.

They said that Mr. Schumer viewed Ms. Stevens, a four-term congresswoman, as the strongest general-election candidate in what is expected to be a hard-fought contest in a state that President Trump won in 2024. Mr. Schumer, who has faced growing frustration from the Democratic base and some liberal senators, often helps his preferred candidates behind the scenes without making an endorsement, rallying support from donors and the party infrastructure.

Mr. Schumer’s effort comes as Ms. Stevens is locked in a tight three-way race ahead of Michigan’s primary on Aug. 4. She is up against Dr. Abdul El-Sayed, a progressive former public health official backed by Senator Bernie Sanders of Vermont, and Mallory McMorrow, a liberal state senator who is supported by Senator Elizabeth Warren of Massachusetts and who has sought an ideological middle lane in the contest.

Dr. El-Sayed has made an apparent rise in the limited polls of the race, and the influential United Automobile Workers union endorsed him on Friday. Last week, Ms. Stevens delivered a shaky debate performance in which she dodged questions about her support from donors to the American Israel Public Affairs Committee, the pro-Israel lobbying group, and struggled to explain the mechanics of how the filibuster works in the Senate.

The quiet message from Mr. Schumer appears to have been heard loud and clear by Democratic donors. In recent weeks, supporters of Ms. Stevens have begun at least four outside efforts to give her a sizable advantage in advertising spending.

Mr. Schumer’s preference for Ms. Stevens has been largely assumed in Washington. Technically, Senate Democrats’ campaign arm and its allied super PAC, Senate Majority PAC, are neutral.

“Leader Schumer is laser-focused on winning in Michigan and taking back the Senate,” said Allison Biasotti, a spokeswoman for Mr. Schumer. A spokeswoman for Ms. Stevens declined to comment about Mr. Schumer’s actions.

Ms. Stevens is preferred by pro-Israel donors. AIPAC has supported her campaigns in the past, and its fund-raising website has a page through which donors can give to her. She is the only Democrat in the Michigan race who supports continued sales of American weapons to Israel’s military. Dr. El-Sayed and Ms. McMorrow have said that if elected, they would oppose such sales.

Mr. Schumer has broadly succeeded in getting most of the nominees he wants in battleground Senate races this year. Josh Turek, a Paralympic gold medalist, won the primary for Senate in Iowa with $10 million in help from a Schumer-aligned group — although as in Michigan, Mr. Schumer did not formally endorse him. The minority leader cleared the field for his recruits in Alaska, North Carolina and Ohio.

Only in Maine did a Schumer-backed candidate struggle to advance past the primary stage. There, Mr. Schumer supported Gov. Janet Mills, who suspended her campaign in April after struggling against Graham Platner, a progressive oyster farmer. Since then, Mr. Platner has continued to face scrutiny over his past behavior.

In Michigan, the three Democratic candidates are competing to take on Representative Mike Rogers, who is running unopposed in the Republican primary, for the seat of the retiring Senator Gary Peters, a Democrat.

The race is crucial to Democrats’ hopes of winning back the Senate. To do so, they must flip four Republican-held seats and defend all of their current seats that are up for election, including in Michigan.

Ms. McMorrow is the only candidate in the Michigan race who has said explicitly that she would not support Mr. Schumer to continue as the party’s Senate leader.

Mr. Schumer and the Senate Democratic campaign arm quietly indicated that Ms. Stevens was their preferred choice in October, when she was the only Michigan Senate hopeful invited to attend a planned donor event at a hotel and vineyard in Napa County, Calif. (The event was later canceled because of the government shutdown.)

Several pro-Stevens outside groups have taken off recently. Allies of Mr. Schumer have been helping the efforts, approaching several super PACs and nonprofit groups and encouraging them to spend money for Ms. Stevens, according to four people briefed on the discussions.

Ms. Stevens is slated to be the beneficiary of over $10 million worth of ads that have been bought in recent weeks. About $6.5 million of that came from a shadowy nonprofit group called the Center for Democratic Priorities, which was started last year, has said nothing about its donors and did not respond to requests for comment.