Bernie Sanders had a great weekend with big wins in the Alaska, Hawaii and Washington caucuses on Saturday. Since it is delegate math that matters, he reduced Clinton’s lead by 35 to 268 delegates won so far.

We’ve now completed caucuses/primaries in well over half the states and patterns have started to emerge. Much has been made of the fact that Sanders does well with young and white voters while Clinton has won the support of Black and Hispanic voters. After these weekend wins for Sanders, Harry Enten wrote that he outperforms expectations in states that hold caucuses instead of primaries. What is interesting to note about that is that the only two caucuses Clinton has won have been Iowa and Nevada – where she was able to take the time to build the kind of organization that is required.

But in looking at the results so far, there is another difference between the two campaigns that doesn’t get mentioned very often. In the states where CNN has done exit polling, Sanders won Independents in all but two states (Georgia and Mississippi), while Clinton won Democrats in all but two (Vermont and New Hampshire).

Given that more people in this country now identify as Independents, that is hardly surprising. But it could be a factor in upcoming contests. As you may know, the Democratic Party in each state sets rules about who can/can’t participate in caucuses/primaries. Of the states Sanders has won so far, all but 4 either held “open” primaries/caucuses where anyone could vote – or were open to Democrats only, but allowed voters to change their affiliation on the day of the vote. Many of the young people and Independents who support Sanders have been able to simply show up and vote.

What is interesting to note is that Sanders has won 3 primaries: Michigan and Vermont – both of which were open for anyone to vote, and New Hampshire – which allows voters to change their party on primary day. Meanwhile, Clinton has won the three closed primaries in Florida, Louisiana and Arizona. Given other areas of strengths/weaknesses for the candidates, the comparison of Michigan (open – big win for Sanders) and Florida (closed – big win for Clinton) is instructive along these lines.

If this pattern holds, Sanders should outperform the polls in Wisconsin, which holds an open primary. But by mid/late April he’ll run into a brick wall of closed primaries in New York, Maryland, Connecticut, Delaware and Pennsylvania – with almost 500 delegates up for grabs. If Sanders stays in the race through to the California primary, with its 400 delegates, he will face a “semi-closed” primary there, which requires voters to be registered as either Democrats or “no party preference” (similar to New Hampshire but without the neighbor state advantage and a mostly white electorate).

Just as Democrats are currently discussing caucuses vs primaries and the role of superdelegates, this presidential primary could spur a conversation about open vs closed primaries. That would be a worthwhile discussion. But it would affect future elections. For Sanders to hang onto his claim of momentum in this race, he’ll need to find a way to win with registered Democrats.

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Nancy LeTourneau

Follow Nancy on Twitter @Smartypants60.