Trump's big advantage: open primaries.

Tonight is "Western Tuesday," featuring primary elections in Arizona and Utah. Current polls on the GOP side suggest Donald Trump will win Arizona while Ted Cruz will win Utah.

Political commentators noticed something interesting early on about Donald Trump's surprising sweep through the Republican primaries — the mogul seems to do better in states that let almost any voter help pick the party nominee, not just those registered to a party.

In those open primaries, Trump has come out ahead in 13 out of 16 states. In states with closed primaries, Trump won only six out of 14 states. Overall, Trump has won closed states about half as often as he's won those open states.

Both primaries in Utah and Arizona are "closed," meaning only registered Republicans can vote. They're representative of what is ahead in the next few weeks: Of the 19 remaining primaries, 13 of them will be closed. That could be bad news for Trump if he wants to cruise to the convention uncontested.

Trump's four victories a week ago included three open states and one closed state. He lost in Ohio, which is also an open primary state, against a much better-than-expected showing by Ohio Gov. John Kasich (Trump's share was about in line with polling).

Why does Trump seem to do so much better in states with open primaries? The same is not true for Bernie Sanders, the "outsider" candidate on the left. There are several possible explanations, but none are as clear cut as they seem.

The most obvious possibility is that the reality TV star is pulling in independent voters, who are free to cast votes for the Republican nominee in open primary states but not in closed primary states. That would be convincing given the popular narrative of Trump's rise, if not that exit polling doesn't show Trump pulling in an outsized share of the independent vote.

Polling suggests that Trump would actually be better off if fewer independents showed up — they are more likely to vote for Ted Cruz or Kasich, as the Washington Post pointed out this month.

Updated CNN poll data show that there are only two states where more independents showing up would be good for Trump. He won those two states — Arkansas and South Carolina. But he also won Virginia and Vermont, where his support among Republicans is much stronger.

On the other hand, exit and entrance polls are often unreliable. The data seem to throw cold water on the theory that a surge of independents is responsible for Trump's dominance in open primary states, but it could be that Trump supporters show up as unaffiliated voters in open primary states and then tell pollsters that they're Republicans.

Republican Democrats and new voters

Data published by The Upshot point to other possible swing groups in open primary states: people who call themselves Republicans but are registered Democrats. Those people seem to be strong Trump supporters, and they could be showing up along with other Trump-loving Democrats in certain states.

Once again, however, the exit polls are hard to read on that issue. Only about 5 percent of voters in open primaries admit to pollsters that they're Democrats, and usually that's too small a portion to break out by candidate. In Ohio, about 8 percent of Republican primary voters were Democrats, and 39 percent said they supported Trump. That's more support than he got from Republicans (36 percent) or independents (35 percent), so it's possible that Trump won a few percentage points from enthusiastic Democrats in other open primary states as well.

Trump certainly does seem to be bringing some new faces into our political process. He received 30 percent of the vote from newcomers to the Iowa caucus and 38 percent from first-time primary voters in New Hampshire. Reuters polling over the last six months of 2015 found that about 27 percent of "very motivated" Republican and independent voters who have not voted in the last four national elections supported Trump, compared to about 23 percent of people who had voted before.

It's certainly possible some of those new voters are finding it easier to participate in primary elections in the open states, even if the mechanism isn't entirely clear. Trump's strength in open states is probably derived from some combination of those voting groups, and we'll have to wait to see if the pattern continues in the remaining primaries. Utah and Arizona will both hold closed primaries next week.