Clinton isn’t the only high-profile Democrat to air concerns about Sanders — according to reports from Barack Obama’s inner circle, the former president said that he would try to stop Sanders if he were poised to clinch the nomination.

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High-level Democrats aren’t crazy to oppose Sanders, an Independent and a Democratic Socialist whose views differ significantly from those of many mainstream members of the party. But it’s not clear how they could stop him. Obama, Clinton and other Democrats might be able to sway voters with a well-timed endorsement, but beyond that, it’s hard to say what else they could do to take Sanders down.

The optimal move for Obama, Clinton and other Democrats might be to wait for results in Iowa and New Hampshire, assess whether Sanders has momentum and only endorse a rival if he looks like he’s taking off. If they endorse someone else earlier, it could backfire: Imagine the momentum Sanders would get if he beat Joe Biden or another establishment candidate after Democratic luminaries intervened in the race.

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And if the party’s most important figures were to endorse a Sanders opponent too late, they might not be able to stop him. Say Sanders racks up some wins in the early states and takes a real lead in the delegate count following Super Tuesday. Party leaders might not have the strength to push anyone else into a position to compete with him in the delegate race. And even if they did, they’d be opposing a candidate with small-d democratic legitimacy and risking party unity heading into the general election.

Sanders’s opponents have few other options available to them. Sanders doesn’t depend on top donors for money: He spent years building a network of small-dollar donors who have sustained him during long fights with mainstream Democrats like Clinton. Polls show that rank-and-file Democrats generally like Sanders and wouldn’t be disappointed if he were the nominee. And superdelegates, who don’t vote until the second ballot at the convention, might have trouble throwing the nomination to someone else if Sanders clearly had the most votes and delegates.

History doesn’t offer many helpful suggestions, and it offers a great deal of caution. In the winter and spring of 2015, top Republicans were still dead-set against Donald Trump’s candidacy. They wanted to oppose him, but they couldn’t fully get behind one candidate in time. If a fractured, multicandidate field comes out of Iowa, Sanders-skeptical politicians might struggle to get on the same page.

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Obviously this is only one of many scenarios: Biden, Elizabeth Warren or Pete Buttigieg could win Iowa and keep Sanders from the nomination without Obama’s help. But Sanders’s candidacy underlines the weakness of the Democratic Party. Twenty years ago, the idea of a socialist taking over the party and running a general election campaign would be pure nonsense. But the guardrails of our party norms are weak. Trump broke through them. For better or worse, Sanders has a chance to do the same.