With just 48 hours left before Iowans head to their caucuses, Bernie Sanders held a massive rally in Iowa City on Saturday night.

That is a massive amount of people.  Now, two caveats:

1. Vampire Weekend, the uber-popular band, played before Sanders spoke -- making this rally really more of a concert with a speech appended to it.

2. The event was in Iowa City, home of the University of Iowa, a huge state school. Sanders has been running extremely strongly with young people, so it makes sense he was able to draw big numbers on Saturday night.

By contrast, here's what a Hillary Clinton rally in Cedar Rapids on Saturday -- featuring Bill and Chelsea Clinton -- looked like:

Crowds are not determinative of results.  After all, the final Des Moines Register/Bloomberg poll released Saturday night showed Clinton at 45 percent to 42 percent for Sanders -- and contained lots of other nuggets of good news for her within it.

But crowd size does tend to be an indicator of organic energy and passion. (Would you go to an event on a Saturday night in late January in always-cold Iowa if you didn't genuinely believe in the cause?) And, throughout the final weeks of this race, it's been apparent that in Iowa, Sanders is the passion candidate.

That's what makes the caucus tomorrow so fascinating. It's Clinton's organization and infrastructure against the energy and enthusiasm of Sanders's.  And it's going to be close.