But remember: It’s just one poll, and we talked to only 496 people. Each candidate’s total could easily be five points different if we polled everyone in the district. And having a small sample is only one possible source of error.

Can Republicans hold a diverse district that voted for Hillary Clinton? We made 27871 calls, and 496 people spoke to us.

This survey was conducted by The New York Times Upshot and Siena College.

Hey, I’m Alex Burns, a politics correspondent for The Times. I’ll give you the latest reporting and intel on the midterms and take your questions from the campaign trail.

It’s generally best to look at a single poll in the context of other polls:

California’s 39th has the largest Asian-American population of any battleground district, and Asian voters were a big part of why the district swung so strongly to Hillary Clinton in 2016. Ms. Kim’s added appeal to Asian-Americans could be an important part of why Republicans have a chance to hold on in such a diverse district.

Ms. Kim, who was endorsed by the retiring incumbent Ed Royce, emphasizes her connection to the district through her two-decades-long work in Mr. Royce’s office. Ms. Kim has shared her own immigration story to highlight her immigration views, saying she supports increased border security as well as compassion for children brought to the U.S. without legal documentation.

Mr. Cisneros, a political newcomer, is a former Republican — he switched his party affiliation in 2008 because he felt the G.O.P. was becoming too ideological. After winning the California Mega Millions lottery in 2010, he created and became involved with several arts, education and veterans philanthropic efforts. He has raised almost $10 million versus Ms. Kim’s $2.2 million as of the most recent reporting period , but $8 million of that is his own money he loaned to his campaign.

With the longtime Republican incumbent retiring, Democrats are targeting the district as one of the most competitive in Southern California.

The district, which went for Hillary Clinton in 2016, includes parts of Los Angeles, Orange and San Bernardino counties, including towns like Yorba Linda, Diamond Bar and Hacienda Heights.

is an immigrant from South Korea who has served as a state representative and as an aide to the current representative. 46% favorable rating; 35% unfavorable; 19% don’t know

Each dot shows one of the 27871 calls we made.

If sampling error were the only type of error in a poll, we would expect candidates who trail by one point in a poll of 496 people to win about two out of every five races. But this probably understates the total error by a factor of two .

One reason we’re doing these surveys live is so you can see the uncertainty for yourself.

As we reach more people, our poll will become more stable and the margin of sampling error will shrink. The changes in the timeline below reflect that sampling error, not real changes in the race.

Our turnout model There’s a big question on top of the standard margin of error in a poll: Who is going to vote? It’s a particularly challenging question this year, since special elections have shown Democrats voting in large numbers. To estimate the likely electorate, we combine what people say about how likely they are to vote with information about how often they have voted in the past. In previous races, this approach has been more accurate than simply taking people at their word. But there are many other ways to do it. Assumptions about who is going to vote may be particularly important in this race. Our poll under different turnout scenarios Who will vote? Est. turnout Our poll result The types of people who voted in 2014 164k Kim +5 People whose voting history suggests they will vote, regardless of what they say 192k Cisneros +1 Our estimate 193k Cisneros +1 People who say they will vote, adjusted for past levels of truthfulness 209k Cisneros +4 People who say they are almost certain to vote, and no one else 220k Cisneros +13 The types of people who voted in 2016 260k Cisneros +4 Every active registered voter 351k Cisneros +10 In these scenarios, higher turnout tends to be better for Democrats.

The types of people we reached Even if we got turnout exactly right, the margin of error wouldn’t capture all of the error in a poll. The simplest version assumes we have a perfect random sample of the voting population. We do not. People who respond to surveys are almost always too old, too white, too educated and too politically engaged to accurately represent everyone. How successful we were in reaching different kinds of voters Called Inter-

viewed Success

rate Our

respon­ses Goal 18 to 29 3 0 6 2 7 3 1 in 42 15% 13% 30 to 64 1 6 4 7 3 2 7 4 1 in 60 55% 56% 65 and older 6 5 4 2 1 4 8 1 in 44 30% 32% Male 1 0 5 2 9 2 7 0 1 in 39 54% 48% Female 1 5 5 5 8 2 2 6 1 in 69 46% 52% White 1 1 3 6 9 2 3 8 1 in 48 48% 49% Nonwhite 1 3 0 8 2 2 2 0 1 in 59 44% 45% Cell 1 9 6 2 0 3 9 2 1 in 50 79% — Landline 6 4 6 7 1 0 4 1 in 62 21% — Pollsters compensate by giving more weight to respondents from under-represented groups. Here, we’re weighting by age, party registration, gender, likelihood of voting, race and region, mainly using data from voting records files compiled by L2, a nonpartisan voter file vendor. But weighting works only if you weight by the right categories and you know what the composition of the electorate will be. In 2016, many pollsters didn’t weight by education and overestimated Hillary Clinton’s standing as a result. Even after weighting, our poll does not have as many of some types of people as we would like. Here are other common ways to weight a poll: Our poll under different weighting schemes Our poll result Don’t weight by party registration, like most public polls Cisneros +2 Don’t weight by education, like many polls in 2016 Cisneros +1 Our estimate Cisneros +1 Weight using census data instead of voting records, like most public polls Even