Suddenly, Bernie Sanders’ presidential campaign is being taken seriously.

For months, the Vermont senator was written off by Democratic Party insiders as a candidate with a committed but narrow base who was too far left to win the primary. Elizabeth Warren had skyrocketed in the polls and seemed to be leaving him behind in the race to be progressive voters’ standard-bearer in 2020.

But in the past few weeks, something has changed. In private conversations and on social media, Democratic officials, political operatives and pundits are reconsidering Sanders’ chances.

“It may have been inevitable that eventually you would have two candidates representing each side of the ideological divide in the party. A lot of smart people I’ve talked to lately think there’s a very good chance those two end up being Biden and Sanders,” said David Brock, a longtime Hillary Clinton ally who founded a pro-Clinton super PAC in the 2016 campaign. “They’ve both proven to be very resilient.”