I just want to underscore Elliott's point about how hard polling has gotten with a quick story I wrote up in a paper with Tori Gorton called "On the Internet, No One Knows You're An Activist." We were trying to survey lower-engagement voters, so we drew a sample of 9,937 registered Pennsylvania voters who hadn't consistently voted 2012-2018. Then, we matched them to Facebook accounts and served ads to 1,321 inviting them to take a survey. 66 people clicked on our ad, 7 began the survey, 6 completed it, and 1 person left an email for follow-up. I of course want to express my deep gratitude to that one respondent. But beyond that, a 0.4% response rate magnifies sampling errors, because very small differences in people's willingness to take polls can add up to very large errors in vote margins. That's especially true in primaries, where pollsters can't use partisan identification to stabilize the results.

—Dan Hopkins, 538 contributor

We've been tracking Trump's non-incumbent endorsees (because endorsing the incumbent is kind of a gimme) tonight, and so far he's 4 for 4 of the races called. Addison McDowell, Trump's pick in North Carolina's GOP primary for the 6th District, is also leading. But in California, state Assemblyman Vince Fong, who received Trump's endorsement for the Republican primary in the state's 20th District (Kevin McCarthy's old seat), is trailing at 26 percent with 23 percent of the expected vote reporting.

—Kaleigh Rogers, 538

It has become somewhat of a theme of our live blogs this primary calendar to note that Trump's challengers are beating their polls. This first looked to be the case in New Hampshire, where polls significantly underestimate support among Republican likely voters for Haley. But the pattern is more robust than one state: In fact Haley has been beating her polls by an average of five points (in vote share) once you account for undecided voters in these surveys. The consistency of this pattern is striking enough to warrant a post on the matter.

As Mary said earlier, polling primaries is hard! That is betrayed by the fact that POTUS primary polls are by far the least accurate type of poll in 538's pollster rating database going back to 1999. But error is different than bias, and when most of the polls are off in the same direction, something has gone awry.

That "something" is likely the difficulty in obtaining opinions of moderate Republicans from samples of "likely Republican primary voters." Remember that fewer than one percent of people called for a poll actually complete the interview. That means the ones that do are statistical "weirdos" (excuse the technical language). Pollsters adjust for this by weighting their samples to known population benchmarks — like the percent of all adults who are white, over 65, have a college education etc. But in primaries, such benchmarks do not actually exist; pollsters are just making educated guesses about them.

My theory is that most of these primary polls pulling samples of voters from voter registration lists are missing moderate crossover partisans and first-time voters. Additionally, we know that people who are highly motivated to participate in polls (the "weirdos") also happen to be the most politically and ideologically extreme Americans. That's a recipe for polling bias in primaries, where weighting to party, past vote and polarized demographic benchmarks does not control for the partisan consequences of overrepresenting politically engaged Americans.

Now, none of this is to say that polling is "broken." It's just hard to precisely sample a population that doesn't really exist. The polls for the 2024 primary still have below-average error, historically speaking, so we shouldn't go throwing out the baby with the bathwater. But this could nevertheless be a sign of pollsters having a hard time reaching moderate "normie" voters. And if that persists, it could have consequences for general election polls too.

—G. Elliott Morris, 538

In California's 40th District, with 46 percent of the expected vote in according to the Associated Press, engineer Allyson Damikolas (endorsed by EMILY's List), is trailing former fire captain Joe Kerr (endorsed by the Congressional Progressive Caucus PAC). The 40th is a key target for Democrats, who are looking to win back some of the ground they gained (and then lost) in the Orange County area. The 40th District spans Riverside and San Bernardino Counties, too.

—Meredith Conroy, 538 contributor

In several of California's open congressional districts, tonight could be the end of any suspense about who the next representative will be. Or, it could be the start of an eight-month intraparty slog.

That's because California's unique all-party primary system — where the top two primary vote-getters advance to the general election — allows for either a traditional contest between a Democrat and a Republican or a fight between members of the same party in the general election, and it's not always obvious which one you're going to get. That's the case in at least four open-seat races today — three Democratic and one Republican.

In California's 16th District, there are nine Democrats but just two Republicans vying to replace Democratic Rep. Anna Eshoo. While limited polling of the district suggests that former San Jose Mayor Sam Liccardo or Santa Clara Supervisor Joe Simitian, both Democrats, are the front-runners, if one of the two Republicans consolidates most of the GOP vote and the Democratic vote scatters among the many candidates, a Republican could finish in the top two. In the 2022 primary, the Republican candidates didn't meet that threshold, splitting a combined 22 percent of the vote; the four GOP candidates in the state's Senate race won a combined 24 percent in the district. A Republican advancing would ensure that whichever Democrat also finished ahead would win the general election in November, given the district's partisan bent. But if two Democrats finish in the top two, it'll be a real race.

Same goes in the 30th District, where Adam Schiff is leaving to run for Senate. Twelve Democrats and just two Republicans will appear on the ballot there. In the 2022 primary, four GOP House candidates totaled 21 percent, and Senate GOP candidates combined for a smaller 22 percent. Democratic state legislators Anthony Portantino and Laura Friedman, and Democratic former city attorney Mike Feuer are the front-runners for this seat; if two of those three make it to the general election, it will be an expensive and sharp-elbowed race to November, but if one of the Republicans squeezes in, it's basically over.

In the 31st District, where Democrat Grace Napolitano is retiring, six Democrats and two Republicans are running. In 2022, the one Republican running for the seat, Daniel Martinez, advanced to the general by winning 37 percent of the vote in the primary. Former Rep. Gil Cisneros, state Sens. Susan Rubio and Bob Archuleta, former Monrovia Mayor Mary Ann Lutz and attorney Greg Hafif are all running well-funded Democratic campaigns, creating the possibility that Martinez, who is running again, could finish in the top two instead of two Democrats, effectively ending this race tonight.

And in Kevin McCarthy's old seat, the 20th District, it's a similar story with the parties reversed. There, the two Democrats will probably combine for around 30 percent of the primary vote (in 2022, the House candidates combined for 30 percent and the Senate candidates for 32 percent), while six Republicans fight over the remaining 70 percent (a seventh dropped out but remains on the ballot).

—Jacob Rubashkin, Inside Elections