What's interesting about the new Selzer poll is why the race is so close. In theory, Clinton should have run away with it. This was the thinking a year ago today, when Clinton had a 54-point lead over Sanders. But over time that lead narrowed and narrowed and narrowed, leading us to this point. The reason? The economy.

Selzer asked Democrats who they thought would be better at handling a number of issues that will face the next president. Clinton came out on top on issues of foreign policy; Sanders on economic issues.

On the flip side, the pollsters also asked what bothered voters about each of the candidates. A sizable majority of Democrats aren't worried about the investigation into Clinton's private email server as secretary of state, but about half are bothered by her apparently flipping her position on the Trans-Pacific Partnership trade deal. And more than half are bothered by the money she took to give speeches to Wall Street.

For Sanders, there's not much worry about his identifying as a democratic socialist or his near-singular focus on the economy. But his lack of foreign policy know-how does give 6 in 10 Democrats pause.

Despite that foreign policy liability, though, Sanders is up by one.

The reason? Democrats have much different policy priorities than do Republicans. In Pew Research's annual survey of priorities for voters, Democrats are consistently more worried about economic issues than the issue of terrorism, while Republicans have gone back and forth. What's more, Democratic concern about terrorism has stayed relatively flat over the past decade, save for 2010.

So while it would seem that the recent terror attacks in Brussels might boost support for Clinton, the candidate that Democrats see as more capable on the issue (the poll was conducted mostly before Tuesday's attacks, but included responses on the day of them), it probably won't — because Democrats are also much less likely to worry about terror. After the attacks in Paris last November CNN and its polling partner ORC asked how worried Americans were about being the victims of a terror attack. Democrats were far less concerned than Republicans.

Clinton's strength on terrorism is an asset in a general election. But among Democrats, it's not much of one. Sanders's ability to hammer Clinton on economic issues and tie her to Wall Street is clearly a large part of why he's hung with her in this race, as this new poll demonstrates.

We've noted before that there are big splits in the Democratic party by age, race and gender. Here, it seems, is another.