They said it couldn't be done.

Vermont Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE's rocket rise from the brunt of jokes to an official menace should be deeply concerning to Democrats who didn't see it coming. With more boffo numbers — of record-setting crowds, strengthening polling and an impressive campaign warchest – the scrappy socialist could soon reach the level of threat.

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It isn't likely Sanders will topple Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE and win his party's presidential nomination, but there is evidence of a more potent liberal energy he successfully engages that she does not. Clinton thought she caught a break when Sen. Elizabeth Warren (D-Mass.) decided against a run but it seems her supporters are running to Sanders. Should Clinton fail to stave off his surge, Sanders could not only embarrass her with strong showings or wins in early primary contests but may ultimately weaken her for the general election next year. She should plan accordingly.

Sanders's gathering of 10,000 supporters gathered in Wisconsin Wednesday night was larger than any crowd any presidential candidate has had all year. In the latest Quinnipiac poll of Iowa voters Sanders is at 33 percent to Clinton's 52, still behind but making solid gains. In May her lead was 60-15, which means in just two months he has more than doubled his support there. What's more is that this is the first time her support has been lower than 60 percent in that poll. Sanders's $15 million in donations in two months also represents only one-third of Clinton's $45 million in three months but far exceeds expectations and illustrates a level of grassroots enthusiasm that no one is laughing at any more. As Clinton scrambled to collect as many $2,700 checks as she could, worried that Jeb Bush was collecting larger ones as a still undeclared candidate, Sanders was counting dollar bills. His haul includes 250,000 donors making 400,000 donations of $250 or less, the average totalling $33.51.

The Sanders boomlet is rippling throughout the party and could potentially alter the primary by more quickly choking off the candidacies of former governors Martin O'Malley and Lincoln Chafee and now former Sen. Jim Webb who strangely chose to jump in Thursday at the height of Bernie buzz. But the most dangerous consequence could be that Sanders's popularity renders Clinton vulnerable enough to invite a challenge from Vice President Biden.

At least for now the Clinton camp, while perhaps genuinely stunned, is smart enough to be eating some humble pie, seeming to take Sanders seriously.

"We're never going to have a coronation in my party," Clinton surrogate Paul Begala said on CNN Thursday. "Never Never. Sen Sanders has tapped into something real."

Of course this was supposed to be a coronation, the turn Clinton waited for for eight years. It was one thing when Barack Obama, who Clinton insiders and many party operatives dismissed as a threat, cut down Clinton's shot at her dream in 2008 - he was the walk-on-water candidate from central casting. Sanders, with his often too-liberated hair and a clunky accent, is perhaps the last person anyone would cast as a potential giant killer.

Should he maintain his current trajectory eight months will be plenty of time for an old-fashioned ass kicking. But Sanders doesn't have to win Iowa on Feb. 1, 2016 - just coming close to Clinton there would be a terrible start for her default-nominee standing and could invite additional dissent in New Hampshire the following week and other contests shortly thereafter.

So far Sanders isn't attempting to reach as the broad coalition Clinton is targeting. He doesn't highlight issues that attract attention from Hispanic or African-American voters, but his rants against the billionaire class with white progressives are surely doing the trick. The real question, of course, is whether Sanders ever wants to tap into liberal hunger for government transparency and exploit Clinton's willingness to hide and destroy her email record of her tenure as secretary of state. So far he has refused to attack her integrity - heaven help her if he changes his mind.

Stoddard is an associate editor of The Hill.