Jon Ossoff, a documentary filmmaker and political neophyte, has already accomplished far more than anyone expected in his bid for the U.S. House of Representatives. On Tuesday, in an 18-candidate “jungle primary” in Georgia for the seat vacated by Health and Human Services Secretary Tom Price, Ossoff fell just shy of the 50 percent needed to win outright. If he defeats former Secretary of State Karen Handel in the June runoff election—indeed, even if he comes close—it will be the first time a Democrat has run competitively in the state’s Sixth Congressional District in nearly 40 years.

There are no downsides to Ossoff’s over-performance for any faction of the party. Just as in last week’s special House election in Kansas, where we also witnessed a major swing in the Democrats’ direction, his unexpected support hints at a Trump-driven combination of Democratic mobilization and Republican drop-off that’s large enough to portend major gains for Democrats in the 2018 midterms—if the dynamic persists for the next year and a half.

Developments like these not only bolster voter mobilization efforts—they suggest the party can compete credibly in all kinds of districts without rummaging through talent-scout books looking for Blue Dog–types who will force the party’s agenda to the right.

This backdrop explains why Democrats were taken aback this week when Senator Bernie Sanders—in the midst of a nationwide speaking tour with Tom Perez, chair of the Democratic National Committee—questioned Ossoff’s political merits. Asked by The Wall Street Journal whether Ossoff was a progressive, Sanders said, “I don’t know.” The Washington Post got a more categorical quote: “He’s not a progressive.”

As a result, Ossoff, is now vying for victory—which will require him to drive overwhelming turnout—under a cloud of skepticism conjured by the most popular progressive in the country.