I actually don't think polls are overestimating Trump by much, more that they're underestimating Haley. And I think it really comes down to undecided voters. Trump is a known quantity for pretty much everyone in America. So anyone who isn't sure if they're going to vote for him isn't unaware of his issues, but probably has some doubts about him. So my theory is that undecided voters skew, if not anti-Trump, at least Trump-skeptical and are disproportionately choosing Haley. You can see an inkling of that in the preliminary exit poll data: In Virginia, for example, voters that decided their vote choice earlier than this year went overwhelmingly for Trump (78 to 21 percent), but voters that made their decision later chose Haley (54 to 42 percent).

—Mary Radcliffe, 538

Polling nowadays is incredibly hard. Seeing response rates of under 2 percent is all too common. So even very slight differences in how fired up people are to take polls can translate into meaningful differences between polls and results. Primary polling is especially hard because you can't use partisan identification to benchmark how reasonable the sample looks. That all said, in New Hampshire and in South Carolina, I wonder if Haley may have benefited from the lack of a competitive Democratic primary, and the fact that some who might have otherwise voted in a Democratic primary were able to vote for Haley.

—Dan Hopkins, 538 contributor

What's everyone's favorite theory about why polls have seemed to consistently overestimate Trump support? (I know this was discussed some on the podcast.)

—Julia Azari, 538 contributor

Tonight I'll also be watching my home state of Arkansas, where polls just closed. There hasn't been recent polling on the GOP presidential primary there, but Trump remains popular in the conservative state, which he won 62 percent of the state in the 2020 general election. His former press secretary is the governor, Gov. Sarah Huckabee Sanders, and she endorsed him in the fall.

There's not much suspense downballot either. Rep. Steve Womack, who represents the northwest corner of the state—home to the University of Arkansas, Walmart, and other big businesses and booming towns—is fending off an unlikely challenger from his right and is the only Republican incumbent to face a challenger. That challenger, State Sen. Clint Penzo is against vaccines, the war in Ukraine, government spending and has criticized Womack's votes during the House speaker fight last fall. But Womack has represented the district for 13 years and all the advantages of incumbency.

On the Democratic side, all four candidates for the House are running uncontested.

Perhaps of a bit more interest — earlier, I wrote about the state Supreme Court races and how the outcomes there could result in a game of musical chairs leading the very conservative Sanders to make one or two appointments to the court.

—Monica Potts, 538

As Monica just noted, about the only drama in the presidential primary contests tonight can be found in Vermont, where Haley actually might defeat Trump in a traditional, state-run primary (unlike the party-run primary she won in Washington, D.C., over the weekend).

So, what's going on in Vermont? Fundamentally, the Republican primary electorate just isn't as conservative in Vermont. I've been using the 2016 Republican nomination race as a back-of-the-napkin guide to the makeup of the GOP electorate this year, and it's been useful. At this point, we could split the GOP into two parts: those who voted for Trump, Ted Cruz and Ben Carson, who tend to make up the Trumpier part of the GOP now; and those who backed Marco Rubio and John Kasich, who aren't as inclined to back Trump. Looking at how the Super Tuesday states voted in 2016, you'll notice that Vermont is the only state voting today in which the combined vote share for Rubio and Kasich outdistanced the Trump, Cruz and Carson cohort.

Back in 2016, the types of voters who backed Rubio and Kasich tended to be more moderate and more likely to hold a four-year college degree. In that sense, they attracted parts of the GOP coalition (and beyond) that make up much of Haley's coalition this year. As we already discussed, some Democratic-leaning voters may have helped Haley in every contest, and Vermont happens to also be a very blue state full of those voters.

—Geoffrey Skelley, 538