But remember: It’s just one poll, and we talked to only 502 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 a Republican survive a national wave of rebellion in the suburbs? We made 29261 calls, and 502 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:

Mr. Wallace, who is embracing progressive positions, says Mr. Fitzpatrick hasn’t done enough to distance himself from President Trump and is beholden to corporate donors. He argues that his own personal wealth and ability to self-fund his campaign keeps him immune from the influence of big donors.

Republicans are portraying Mr. Wallace as a carpetbagger; he moved back to this mostly suburban district near Philadelphia last year from Maryland. Mr. Wallace has also been attacked for his family foundation’s donations to the Boycott, Divestment and Sanctions movement, a campaign to boycott Israel. He has disavowed the grants, saying they were made by another board member, and says he is “unequivocally pro-Israel, pro-peace, and pro-democracy.”

This suburban district is a classic battleground in presidential elections, and it ought to be one of the top Democratic pickup opportunities in the country. But Mr. Fitzpatrick, a former F.B.I. agent, has a moderate reputation and earned the endorsement of the Pennsylvania A.F.L.-C.I.O. and of the gun control advocate Gabrielle Giffords. He voted for the Republican tax overhaul but against the effort to repeal the Affordable Care Act.

After the redrawing of the Pennsylvania congressional map in February, the new First District preserves the core of Pennsylvania’s Eighth and keeps Bucks County intact. But it now adds more Democratic territory from central Montgomery County, so going by 2016 results, it’s now a Clinton-leaning district instead of a Trump-leaning one.

is an incumbent, first elected in the Eighth District two years ago, succeeding his brother. 53% favorable rating; 36% unfavorable; 11% don’t know

is a wealthy philanthropist whose grandfather was a major figure in the the administration of Franklin Roosevelt. 46% favorable rating; 39% unfavorable; 15% don’t know

Each dot shows one of the 29261 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 502 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. Our poll under different turnout scenarios Who will vote? Est. turnout Our poll result The types of people who voted in 2014 233k Fitzpatrick +7 People whose voting history suggests they will vote, regardless of what they say 288k Fitzpatrick +2 Our estimate 291k Fitzpatrick +1 People who say they are almost certain to vote, and no one else 300k Fitzpatrick +4 People who say they will vote, adjusted for past levels of truthfulness 308k Fitzpatrick +2 The types of people who voted in 2016 369k Fitzpatrick +6 Every active registered voter 461k Fitzpatrick +7 Just because one candidate leads in all of these different turnout scenarios doesn’t mean much by itself. They don’t represent the full range of possible turnout scenarios, let alone the full range of possible election results.

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 2 4 0 5 4 6 1 in 52 9% 9% 30 to 64 1 7 2 8 9 2 8 5 1 in 61 57% 60% 65 and older 6 2 9 9 1 7 1 1 in 37 34% 31% Male 1 0 5 2 2 2 2 2 1 in 47 44% 48% Female 1 5 4 7 7 2 8 0 1 in 55 56% 52% White 2 0 2 1 1 3 9 8 1 in 51 79% 78% Nonwhite 2 6 7 7 4 4 1 in 61 9% 10% Cell 1 3 0 5 4 2 7 1 1 in 48 54% — Landline 1 2 9 4 5 2 3 1 1 in 56 46% — 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, education 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. Here are other common ways to weight a poll: Our poll under different weighting schemes Our poll result Don’t weight by education, like many polls in 2016 Wallace +1 Weight using census data instead of voting records, like most public polls Fitzpatrick +1 Our estimate Fitzpatrick +1 Don’t weight by party registration, like most public polls Fitzpatrick +3