Progressives scored their biggest win of the year Tuesday in Michigan's Senate primary, narrowly defeating the Democratic establishment in the clearest sign yet of how a populist surge is remaking the party.Why it matters: Left-wing outsider Abdul El-Sayed underperformed predictions, but his victory over moderate Rep. Haley Stevens was still a rebuke to party leaders in D.C. and Michigan, and to big donors who flooded Stevens' campaign with cash.El-Sayed was outspent 9 to 1 by Stevens and her allies — including AIPAC's political arm. He ran on an unapologetically progressive platform to abolish ICE, support Medicare for All and staunchly oppose aid to Israel.In a bitter campaign, El-Sayed was painted as a sexist, an antisemite and critic of former First Lady Michelle Obama — much of it on the airwaves. Michigan Gov. Gretchen Whitmer cut a slick video backing Stevens.It wasn't enough.Key takeaways from Michigan:1. Progressives lay the groundwork for 2028.Progressives and democratic socialists have been eager to show that one of their own can carry a battleground state that will be crucial in the next presidential election. They'll get a chance to prove that in November, now that El-Sayed has been nominated in Michigan.If El-Sayed succeeds in November, it'll be seen as a boon for the prospects of a left-wing 2028 presidential contender such as New York Rep. Alexandria Ocasio-Cortez.On the other hand, if El-Sayed were to stumble, AOC — who's mulling a run for Senate or the White House in 2028 — likely would share some of the backlash. She's been among El-Sayed's top surrogates.2. Bernie hands Chuck an "L."With the progressive movement he'd fostered for so long building unprecedented momentum, progressive icon and Vermont Sen. Bernie Sanders went on a mission to elect progressives up and down the ticket this year — often opposing Senate Minority Leader Chuck Schumer's favorites, including Stevens.Sanders backed El-Sayed on the first day of his campaign. Stevens' loss marks the second time this year that Schumer failed to get his preferred candidate over the finish line in a Senate primary. The last time the Democratic Senatorial Campaign Committee whiffed in a major primary before this year was in 2010.3. Money isn't everything.El-Sayed, a former a former Wayne County health director, compared himself to David going up against Goliath.The Biblical allusion made many moderate Democrats roll their eyes, but the spending disparity in the race was truly enormous: Stevens and her allies spent $65 million on ads compared to $7.5 million by El-Sayed and his supporters, according to the ad-tracking firm AdImpact.Stevens' loss is a large face-plant for AIPAC, the pro-Israel lobby whose super PAC poured more than $30 million into the race against El-Sayed. The results could lead more Democrats to distance themselves from AIPAC money at a time when Israel's image has nosedived among the party's voters.4. Republicans like the result, too. El-Sayed has excited the left, but many Republicans are happy with his win as well. Recent polls have suggested that former Rep. Mike Rogers, the GOP's Senate nominee, would fare better against El-Sayed than Stevens. Trump already has branded El-Sayed a "communist," a theme voters can expect to hear from the GOP throughout the fall. (El-Sayed has said he's a capitalist, not a socialist.)El-Sayed, meanwhile, faces the challenge of reuniting a deeply divided Democratic Party, wooing independent voters and trying to attract some Republicans. Many centrist Democrats are skeptical he can pull it off — and point to Tuesday's closer-than-expected results as evidence he could lack enough big-tent appeal in November.5. The left also shows its muscle in down-ballot races.For progressives, it wasn't just El-Sayed: Will Lawrence won a battleground House nomination, and Donavan McKinney was threatening to oust Rep. Shri Thanedar.Yousef Rabhi declared victory over incumbent Christopher Taylor in the Democratic primary for mayor in solidly blue Ann Arbor. And Abbas Alawieh of Dearborn — cofounder of Uncommitted, a group formed to protest U.S. support of Israel — was ahead of state Rep. Erin Byrnes in the race for the Democratic nomination of a state Senate seat.