Then Sanders lost New Jersey. And then, with votes rolling in, Sanders trailed Hillary Clinton in, of all places, South Dakota. A bit before midnight Eastern, the AP called the race for her.
That's ... odd. Sanders won North Dakota on Tuesday night, a state that's demographically very similar to its neighbor to the south. He's won Nebraska, Minnesota and Wyoming. It's a region where he's done well. But while he romped in North Dakota, he lost South Dakota. How?
Easy. The northern Dakota is a caucus state. The southern holds a primary.
We've noted over the course of the campaign how Sanders does much better in caucuses than primaries. In caucus states, he's averaging over 60 percent of the vote. In primaries, he averages just under 43 percent. He's won 71 percent of caucuses; Clinton has won 72 percent of primaries (excluding South Dakota, which hasn't yet been called).
Her weakness in caucuses was a problem for Clinton in 2008, too. At the end of March, we plotted the contests this year and eight years ago, to date. Barack Obama won a lot of primaries that year, but dominated in caucuses.
The same thing happened in 2008 happened on Tuesday: Obama won North Dakota's caucuses and Clinton won South Dakota's primary.
It's a reminder that for all of the attention we pay to candidate positions and demographics, something like format can make a big difference in how an election turns out.