When CBS debate moderator John Dickerson asked the Democratic presidential candidates how many Syrian refugees they thought the US should let into the country, Bernie Sanders refused to answer — saying there was "no magic number."

In the context of the refugee issue, that was an odd answer.

Sanders wasn't anti-refugee, but he didn't appear to know much about the issue

Sanders was generally supportive of taking in Syrian refugees. But, he said, he wasn't willing to commit to a "magic number" because "we don't know how big the problem is."

That's only true in the very, very broad sense that we don't know how many people will ultimately be displaced by the Syrian civil war. But there are decent estimates out there of how many Syrians are displaced right now. And the UN Office of the High Commissioner for Refugees has a clear goal for how many refugees they want to resettle permanently in other countries by the end of 2016: 130,000.

The US has historically resettled half or more of the world's refugees who end up living permanently in a third country. That's why advocacy groups, and O'Malley and Clinton, settled on 65,000 Syrian refugees as the target they wanted the Obama administration to hit.

Related How the US screens and takes in Syrian refugees

It's not just that Sanders wasn't able to pull out the 65,000 number on a whim, though (because it's been such a well-established line from advocates) it would make sense for him to be prepped with it. It's his unwillingness to give any frame of reference for refugee policy: the US's annual refugee caps, the numbers of Syrians fleeing the country, etc. It appeared as if he cared about caring for refugees as a moral issue, but wasn't sure about what that implied for policy.

That's generally in tune with where Sanders is on immigration right now. He occasionally treated immigration as another threat to American workers early in his campaign, but he's clearly making an effort to align himself with progressives on the issue — including hiring immigration activists on staff. But he's simply not as familiar with the issue as he is with economic issues, for example, and it appears he's still learning about the policy that goes with the rhetoric.

Martin O'Malley wants to cast himself as the true progressive on immigration

In contrast to Sanders, O'Malley didn't just talk about the specific 65,000 refugee resettlement goal. He was armed with a too-cute analogy about making room for more spectators at a baseball game. The strongest impression he left was that he was prepared to talk about Syrian refugees — arguably more so than he was to talk about foreign policy directly.

That's not surprising. Immigration is the one issue (or one of two issues, if you count gun control) on which O'Malley has a more consistently progressive record than his opponents. (Clinton, for her part, also supported admitting 65,000 refugees, but emphasized that the US needed a very careful security screening process — which embodies the line she's consistently tried to straddle to both please immigrant rights groups and appease white centrists.) And it's one of two issues — along with guns — on which O'Malley's campaign clearly thinks he can outflank Sanders with progressive voters and portray himself as the true progressive alternative to Clinton. In a campaign that's usually being treated as Sanders versus Clinton in the press and polls, the debate stage is the one place O'Malley can do that.

In Saturday night's debate, O'Malley was miles more enthusiastic and confident talking about immigration than he was when talking about any other issue. There's a reason for that. It's the issue on which he and his campaign really think his answers matter.