The six primary contests taking place on Tuesday are the last best hope for Bernie Sanders' promised "revolution."

Promising to activate large numbers of young, infrequent, and disaffected voters has not only been at the core of Sanders' campaign to win the Democratic nomination, it has been the central element of his electability argument in the general election, and it is how he plans to pass his agenda once elected. So far, though, it hasn't happened, something even Sanders publicly admitted after Super Tuesday. If he can't make the increased voter turnout that has been the central promise of his campaign happen now, it is very unlikely to ever materialize.

Turnout in the primaries so far has been higher than it was in 2016 but, for the most part, has not exceeded turnout for 2008 when adjusted for population growth. Even worse, the places where turnout seems to have grown the most have often been in states won overwhelmingly by Joe Biden on Super Tuesday.

If one is being generous, it is possible to come up with decent excuses for why Sanders' revolution hasn't yet arrived but still could. Iowa and Nevada are caucuses, which creates a serious timing and time commitment problem for many voters. Sanders did surprisingly well in the satellite caucuses set up for those who couldn't attend regularly timed caucuses. In New Hampshire, turnout was the highest ever (although not after adjusting for population growth), but voters there faced a choice overload. There were multiple candidates who infrequent voters might have been considering, and they may have split Sanders' potential gains. South Carolina was always inherently unfavorable to Sanders and where Biden concentrated his campaign. Finally, Sanders supporters might have been overly confident on Super Tuesday with headlines just days before claiming Sanders' leads could be nearly insurmountable. The shocking swing to Joe Biden was so late and so big it didn't even really get picked up in the polling.

None of these excuses really apply for the primaries on Tuesday — Idaho, Michigan, Mississippi, Missouri, North Dakota, and Washington. Sanders is currently behind in most of these states, but not so far behind that his voters should think it is hopeless. His potential supporters in these primaries should know he can still win and how much their votes matter. The large black population in Mississippi is a real challenge for Sanders, but the other states are theoretically winnable. In Idaho and Washington state, Hispanics, a group Sanders has done very well with, are the largest minority group, making up just over 12 percent of the population. Michigan has a large union population, which the Sanders campaign has aggressively courted. In 2016, Sanders won the North Dakota caucus overwhelmingly although this year the party switched to a more open "firehouse caucus."

Equally important, the choice between candidates has never been so simple. With all the other candidates out of the race, the stakes are clear and the choice truly binary. There are no questions about strategic voting and reaching delegate thresholds or tactical arguments about the best way to achieve progressive goals. Biden has always been one of the most conservative candidates in the race, and Sanders the most progressive. Sanders and Biden offer very different visions of the country and support very different policies. On issues ranging from health care to trade to foreign policy, they are the two Democrats who have been furthest apart. Biden's message is a return to normalcy while Sanders' message is a big change. For the first time, Sanders is not just trying to get voters to turn out for something — he can also pitch them on turning out against something.

If there is truly a large, untapped pool of young voters and infrequent voters who can be turned out with the promise of big change and big new policy ideas, then there has never been a better moment to prove it. The choice is simple, and it will never be more clear that their vote could make a difference. Either we see the revolution Tuesday, or we need to conclude it just isn't there.

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