Mallory McMorrow, the Democratic Michigan state senator who tried to cut a middle path between moderate and left-wing rivals, suspended her campaign for the U.S. Senate on Sunday amid poor polling numbers and fund-raising that did not keep pace with her two main rivals.

“Today, I’m announcing that I am suspending my campaign for United States Senate,” Ms. McMorrow wrote on social media. She thanked her volunteers and donors.

Ms. McMorrow’s departure leaves Michigan’s Democratic Senate primary a two-way contest between Representative Haley Stevens, a moderate, and Dr. Abdul El-Sayed, a progressive former public health official. Ms. Stevens is backed by Senator Chuck Schumer of New York and pro-Israel groups, while Dr. El-Sayed has endorsements from Senator Bernie Sanders of Vermont and Representative Alexandria Ocasio-Cortez of New York.

Ms. McMorrow did not make an endorsement in her announcement Sunday but said she would support whoever wins the primary.

Ms. McMorrow’s argument to Michigan voters was that they did not have to choose between Ms. Stevens, a four-term congresswoman with deep ties to the party establishment, and Dr. El-Sayed, whose left-wing views have made some Democrats nervous about his ability to win a general election. He lost the 2018 Democratic primary for governor to Gretchen Whitmer, who subsequently won two general elections by wide margins.

“We are being presented right now with what I believe is a false binary choice” between “the status quo in Haley Stevens” and “a candidate who has never won a campaign before,” Ms. McMorrow said in a CNN interview last month.

Michigan Republicans are set to nominate former Representative Mike Rogers, who narrowly lost the state’s Senate race in 2024. The Senate seat is being vacated by Senator Gary Peters, a Democrat who is not seeking a third term. The primary is Aug. 4.

Holding Michigan’s Senate seat is critical to Democratic hopes of winning control of the chamber in November’s midterm elections. The party must retain the Senate battlegrounds it holds now and flip at least four Republican-held seats to seize a majority.

Long adept at cable news interviews and creating viral moments online, Ms. McMorrow began to bleed support in public polling after she condemned Dr. El-Sayed for campaigning in March with Hasan Piker, the left-wing streamer who has defended Hamas and called Israel “an apartheid state.”

The move backfired quickly. Her remarks helped burnish Dr. El-Sayed’s claim that he was the lone progressive candidate in the race and the one most willing to criticize American funding of the Israeli military. And she won little support from moderate voters as millions of dollars of pro-Stevens advertising poured into the state from the American Israel Public Affairs Committee and other groups.