For the better part of a year, Bernie Sanders enjoyed a polite if slightly bemused welcome from the non-radical quarters of the Democratic firmament. The wing of the party represented by Sanders and Elizabeth Warren had been ascendant for most of Barack Obama’s presidency, enlarging the potential constituency for a populist presidential primary challenge to Hillary Clinton. As a grumpy-yet-affable elderly Jewish socialist who wasn’t actually a Democrat, Sanders struck members of the liberal establishment as the least-viable tribune of the party’s insurgent wing.

One week from the Iowa caucuses, we now know their assessment was wildly inaccurate. Sanders is within striking distance of Clinton in Iowa, and leads her in most New Hampshire polls. He still trails badly in more ethnically diverse Southern and Western states, but the Clinton campaign and its allies are suddenly contending with the possibility that Sanders will convert victories in both of the first two contests into polling surges elsewhere in the country, imperiling Clinton’s nomination, or at least making her path to it much longer, costlier, and more divisive.

Where center-left liberals were once sanguine about the state of the Democratic primary campaign, or even grateful to Sanders for premising the debate on progressive assumptions, they are now alarmed that Sanders might pull off an upset and become the party’s nominee.

New York Times columnist Paul Krugman, for one, laments that “while idealism is fine and essential—you have to dream of a better world—it’s not a virtue unless it goes along with hardheaded realism about the means that might achieve your ends. ... [T]here’s nothing noble about seeing your values defeated because you preferred happy dreams to hard thinking about means and ends.” New York writer Jonathan Chait concluded his article “The Case Against Bernie Sanders” on a note of astonishment: “[I]t seems bizarre for Democrats to risk losing the presidency by embracing a politically radical doctrine that stands zero chance of enactment even if they win.”

Center-left liberals are jittery about Sanders for obvious and understandable reasons. But neither Krugman’s nor Chait’s case constitutes an airtight argument that Sanders should be defeated. To its chagrin, the Democratic establishment hasn’t made that argument either. And until that case is laid out fully, his supporters can make a persuasive counterargument that nominating Sanders is a worthwhile gamble.