Abdul El-Sayed has won Michigan’s Democratic U.S. Senate nomination, defeating Rep. Haley Stevens in a major victory for progressives in the key battleground state.

The real test of El-Sayed’s and progressives’ strength will now come in a November matchup against Republican Mike Rogers, a former congressman who ran unopposed for the GOP nomination.

The seat is pivotal to Democrats’ chances of flipping the Senate and El-Sayed will face the challenge of uniting a divided Democratic base after a bruising primary.

The race remained tight throughout Tuesday night, with a winner not declared until early Wednesday. In his final remarks at a primary night party before the race was called, El-Sayed said "tomorrow we begin to mend fences."

El-Sayed’s win represents a significant shift for the Democratic Party. In 2018, he lost the gubernatorial primary to now-Gov. Gretchen Whitmer by 21 percentage points.

A former health official, El-Sayed campaigned on "Medicare for All," halting military aid to Israel and campaign finance reform. He had support from progressive leaders, including Sen. Bernie Sanders and U.S. Rep. Alexandria Ocasio-Cortez, who framed the race as a grassroots-focused movement and a broader fight against the establishment and big-money spending.

Stevens, a more moderate candidate, was backed by Senate Minority Leader Chuck Schumer and tens of millions in outside spending, including the American Israel Public Affairs Committee’s largest outlay ever in a single race.

Now comes the more difficult challenge for Democrats: uniting the party to defeat Rogers, for whom Republicans have already reserved large sums of money ahead of the general election. Democrats consider the seat a must-win if they are to flip control of the Senate this year.