“Wow, it’s almost like 97% of incumbents win re-election to Congress or something.” FiveThirtyEight’s Nate Silver was clearly reaching for snark, but his reaction to the news of Ed Markey’s victory in the Massachusetts Senate primary last night captured what could well become the retroactive pundit consensus on the race.

Markey, in this telling, simply followed the trajectory enjoyed by many sitting lawmakers and swatted down a challenge that was never likely to succeed in the first place. The truth is, this was not a typical race and Markey’s victory over Joe Kennedy III — which looks set to be a rout of around ten or eleven points — was anything but inevitable.

For one thing, polling since last summer frequently suggested Kennedy would win. A survey conducted just after Labor Day 2019, for example, found him ahead by fourteen points — the incumbent trailing in most polls until just a few months ago. As recently as May, Emerson College found Kennedy leading by a margin of sixteen. And while we don’t yet have a complete picture of fundraising throughout the race, we do know that Kennedy brought in more money in its early stage, eclipsing Markey’s total in the final quarter of 2019 by a million dollars.

Kennedy’s edge, quite atypical of a Senate primary challenge, owed itself in large part to big endorsements from within the Democratic Party. Though House Leader Nancy Pelosi’s late-in-the-game nod deservedly received the most attention, the bid to take down Ed Markey also got a blessing from the late John Lewis, House Majority Leader Steny Hoyer, Joaquin Castro, Beto O’Rourke, and more than two dozen House Democrats. The challenge once appeared so certain to succeed there was briefly speculation that Markey might commit political seppuku and retire by his own hand to avert humiliation.

So, how did Markey win? There’s no denying that he did enjoy certain advantages of incumbency, counting plenty of centrist Democrats (notably Chuck Schumer) and local allies among his endorsements. But these alone would not have been enough to defeat Kennedy. Markey’s remarkable comeback was instead made possible by his eager embrace of the progressive left and willingness to champion its values and agenda — a strategy that expanded his coalition and brought in voters under thirty by a stunning margin.

Having helped introduce the most potentially transformative piece of environmental legislation in American history, Markey earned the backing of the Sunrise Movement and its House sponsor Alexandria Ocasio-Cortez. Producing campaign ads like this one — which includes references to revolution, his union background, and images from the Black Lives Matter protests — he staked out clear ground as a progressive figure. Thus, a traditionally conventional Democratic incumbent became an unlikely hero of young voters who desire sweeping change — a fact made all too clear by the tenor of his victory speech:

In this race, justice was on the ballot. Health care justice, that’s Medicare For All and universal health care … Economic justice, so that three billionaires don’t hold more wealth than the bottom 50 percent of our population combined. That’s an economic system that’s gone off the rails. Racial justice, so that we confront our history, make reparations, and root out systemic racism … Today and every day we say “black lives matter,” “black voices matter.” And environmental justice, that’s the Green New Deal … There will be no peace, no justice, and no prosperity unless we stop the march to climate destruction … The very future of our civilization depends upon it. There is no time for simply “doing what we can.” There is no time for compromise on the existential threat to our time. We must pass a Green New Deal … The time to be timid is past. The age of incrementalism is over. Now is our moment to think big, to build big, to be big.

Though his career until now had been very different, Markey sounded a lot like Bernie Sanders, and he accordingly attracted many of the same voters. As Claire Sandberg, who served as Sanders’s national organizing director, quite rightly put it: “Corporate Democrats want to make the left out to be purity-obsessed and unwilling to compromise, but the left rallied around a longtime politician with a mixed record because he actively courted their support and became a champion of one of their major legislative priorities.”

Far from being a familiar tale of incumbent victory, Ed Markey’s triumph over Joe Kennedy III was the unlikely outcome of a race that was always Kennedy’s to lose.