Six weeks ago, Biden jumped in and bounced out to a big lead, seemingly taking some “soft” Bernie supporters with him. He’s since cooled off and Sanders has regained some of that lost support — but not all of it. Where’d the rest go? Today’s new polls from Fox News and South Carolina’s Post & Courier solve the mystery, which wasn’t much of a mystery.

Whatever bounce Biden received after joining the race in late April is gone, returning him to the status quo ante. Not so for Bernie, who’s lost 10 points to a combination of two other progressives, Elizabeth Warren and Pete Buttigieg. The secret to the Warren/Buttigieg surge, Fox suggests, is perceptions that each wants a “bold agenda” as president. Among voters who list that as a priority, Bernie scores 17 percent but Warren and Buttigieg are close behind at 12. The left loves Sanders foremost because he’s the boldest advocate for socialism in the race, the only candidate who openly embraces the label. To the Warren/Buttigieg progressives, for whatever reason, that’s not enough. Maybe they’re skittish about the electability of an avowed socialist or maybe they think winning the purity contest isn’t enough. (Or maybe they prefer a Democratic nominee who … actually identifies as a Democrat?) Warren’s the one who’s distinguished herself so far with policy specifics. Buttigieg is the young, gay, highly educated veteran from middle America, arguably the member of the top-tier with the best “story.” Each feels less stale than a rerun of Bernie 2016.

Meanwhile, in South Carolina:

Biden fell to 37 percent in the latest poll, a number closer to his previous S.C. results. Warren sits second at 17 percent followed by Buttigieg at 11 percent with Harris and Sanders tied at 9 percent…

Sanders had been the solid second choice of S.C. voters in previous polls. But Warren reversed deficits with Sanders among men, women, voters under age 50 and, most importantly, African Americans who make up a majority of Democrats going to the polls.

And she now leads the race among voters ages 18 to 34, a spot previously held by Sanders.

He got crushed in South Carolina in 2016 because of weak support among black voters but nine percent is a brutally bad showing, down six points in a month. The only other poll I’ve seen this year in which he failed to reach double digits was one of Massachusetts, Warren’s home state, conducted a few days ago, but he’s been flirting with single digits in a bunch of recent surveys — 13 percent in today’s Fox poll, 13 percent in a Monmouth poll of Nevada published a few days ago, 12 percent in a YouGov national poll around the same time. A few more weeks of momentum for Warren and Buttigieg and America’s First Socialist President really might be a single-digit proposition consistently.

I wonder why the voters Bernie momentarily lost after Biden entered the race didn’t all return to him as Biden’s big lead eased. Best guess: Some faction of Sanders’s early support was tepid about him, possibly backing him just because he was the best known “Not Biden” option available, possibly sticking with him through sheer force of habit after supporting him against Hillary three years ago. Biden’s announcement and the hype that greeted it might have forced those voters to pay closer attention to the race, and as they did, they saw something they liked in Warren or Buttigieg. It may be, in other words, that only 15 percent or so of the Democratic electorate are true Berniebros. The rest of the Bernie 2016 base may be people who disdained Hillary personally or chose him because they wanted a more left-wing nominee than Clinton and he was the only item on the menu.

His best shot at the nomination, I think, is simply the calendar. A third new poll today, this one from CBS and YouGov, has him slipping to third place nationally at 16 percent, a point behind Warren. He slides to fourth place when Democrats are asked whom they’re considering voting for, behind Biden, Warren, and Harris. But in specific early states like Iowa and New Hampshire, he does better. He’s second in Iowa with 22 percent, 10 points ahead of Warren, and second in New Hampshire with 20 percent, three points ahead of her. If Bernie out-organizes everyone to win the caucuses and then parlays that into winning New Hampshire, which he won easily three years ago, his national numbers will skyrocket. He’d likely end up an also-ran in a true national primary, in which everyone votes on the same day, but that’s not the system we have. Until he starts heading towards single digits in IA and NH he’s a threat.

Here he is today defending public ownership of private initiatives. Bold agenda. By the way, Fox’s new poll today also ran hypothetical match-ups of Trump against all five top Democratic candidates and found POTUS trailing in every one (by 10 points in Biden’s case) and never reaching higher than 41 percent against any of them. Another “suppression poll”!