The defining characteristic of Bernie Sanders's early surge in the 2016 presidential race was his crowds. Big swarms of people waving blue signs and cheering on the unexpected Democratic contender, a physical manifestation of the narrowing gap between himself and Hillary Clinton. Revolutions are always more exciting than baby steps, and Sanders is 2016's Che.

But one of the weirder things about the Iowa caucuses was that Sanders's supporters, those big crowds of people shouting and finishing his sentences, didn't turn out as heavily as you might expect. Young people turned out much less heavily than they did for the 2008 caucuses, according to entrance polling, as did less wealthy voters (acknowledging that there's overlap between those groups). Very liberal voters turned out more, but that may have been in part thanks to the party itself moving to the left.

Caucus turnout on the Democratic side was down from 2008 substantially. It was up since 2004 -- but so was the number of voters. Turnout in 2004 was 5.6 percent, according to Edison Research. In 2016, it was 7 percent -- compared to 10.7 percent in 2008. On the Republican side, turnout was up both in terms of the number of voters and as a percentage of the voter base. In 2008, 5.4 percent of Republicans in Iowa caucused. In 2016, 8 percent did.

When New Hampshire rolled around, same deal. Sanders won big, but the number of people voting this year was lower. The Republicans had more voters in both Iowa and New Hampshire than did the Democrats.

This doesn't exist in isolation. Other factors come in to play, including the number of candidates. (More viable candidates, one might assume, would prompt more people to come out to vote in a tightly contested race.) But this is not the only indicator that there may be an enthusiasm gap between the parties (as they say).

In January, a CBS News/New York Times poll looked at the enthusiasm of each party, finding that 73 percent of Republicans felt very or somewhat enthusiastic about voting this year, versus 65 percent of Democrats.

It's still too early to try to predict what will happen in November, especially based on data like this. But if the Democrats want to sweep a new Democrat into the White House this November, they're going to need to catch up to the Republicans in terms of getting people to the polls. And if Sanders wants that revolution to kick in, he's going to need to get the enthusiasm of his big crowds to trickle out into the party a bit more.