Rep. Darrell Issa (R-Calif.) is in a competitive reelection race for the first time in his career, in large part because of Latino voters. (Olamikan Gbemiga/AP)

One by one, Doug Applegate’s fellow veterans stepped up to the microphone and named the threat to America: Donald Trump. One of them shook with rage, discussing Trump’s joke about the “captured” Sen. John McCain (R-Ariz.). Others asked how Applegate’s opponent, Rep. Darrell Issa (R-Calif.), could support Trump and claim to be a friend of veterans.

“You have a presidential candidate insult women — you’re talking about women who are for the first time being drafted,” said Nancy Cook, 43, a Gulf War-era veteran, her voice quavering with emotion.

Just hours had passed since FBI Director James B. Comey’s announcement Friday that he would examine more emails that might be related to whether Hillary Clinton had let classified information slip through her private server as secretary of state. Applegate, a retired Marine colonel and Democrat who has given Issa the first real challenge of his career, was sticking to the facts that undergirded that challenge: Issa supported Trump, and the 49th District of California and its growing Latino voter base did not.

“Demographics have changed in the 49th,” Applegate said in an interview. “I knew this was a Marine district. I knew one thing the Democrats never have tried is to run a Marine. And I know that in the military, if you say anything that’s racist or misogynistic, 9 out of 10 times you’ll be disciplined for it.”

Donald Trump’s campaign is sparking a surge in citizenship applications and voter registrations among Hispanics fearful of his immigration policies. Since January, California alone has seen a boost of 218 percent in Democratic registration, and among Hispanics, registration is up 123 percent. (Alice Li/The Washington Post)

Every factor that cuts against Trump’s Republican Party cuts deeper in California. Last week, House Speaker Paul D. Ryan (R-Wis.) barnstormed the state’s Central Valley to help incumbents who had won easily in 2014. This year, they will need to overcome more than 1 million new Democratic voter registrations — and a Trump candidacy that in one poll was cruising along with support from less than 30 percent of voters.

Four Republican-held seats in California are seen as competitive, in large part because of Latino votes. In a decade, demographic change in the rest of the country could affect dozens more seats now thought to be safely red.

In ads and on the stump, Applegate asks voters to consider Issa a fumbling congressman and an apologist for the top of his party’s ticket. He was, Applegate says, “Trump before there was Trump.” Issa’s years as House oversight committee chairman, which included the first investigation of the 2012 Benghazi, Libya, attack, are cited as proof that he has ignored the district.

Until the Comey letter, which prompted Issa to tweet that the FBI could “get it right this time,” Issa’s campaign had focused on problems with the Veterans Affairs hospital system and the National Guard bonus scandal. Most of Issa’s lawn signs say he is fighting for veterans; Applegate’s signs eschew his first name for his title, colonel.

But it is the changing voter base, not any public campaign, that has Democrats salivating at the thought of beating Issa. Many of the new registrations came during Sen. Bernie Sanders’s ambitious but unsuccessful Democratic primary bid; more came afterward, helped by a Latino backlash to Trump and several well-funded campaigns to weaponize it. More were added because of a new law that automatically registers voters when they obtain driver’s licenses.

Applegate, 62, had never run for office, and the 49th District had never looked like fertile territory for a Democrat. Before 2011, and California’s first nonpartisan redistricting, much of what became the 49th District was reliably Republican, with Camp Pendleton and the conservative “North County” of San Diego overwhelming the liberal towns on the Pacific Coast.

Democrats appear to have an outside chance of taking back the House, which they lost after the 2010 election.

“We assumed this race would be like the last few years, where we had well-intentioned candidates running in an unwinnable seat,” said Francine Busby, chairwoman of San Diego’s Democratic Party.

Ten years earlier, Busby found out for herself why greater San Diego County was unwinnable, at least then. She ran three losing races for Congress; a close special election went against her when an opponent exploited an on-camera moment when Busby told Latino volunteers they didn’t “need papers” to support her. The Minuteman Project, which urged concerned — and armed — citizens to join slapdash border patrols, had a beachhead in the district.

Latino voters weren’t coming out in numbers sufficient to challenge that. Many didn’t register. Once, Busby recalled, a Latina candidate for local office in Escondido found that die-hard supporters did not cast votes because they didn’t realize they needed to come out each time she was on the ballot.

“Every single year, people asked the same thing: How do we reach out to Latino voters?” Busby said. “The Democrats are on the right side of the issues, but we’d never found a way to compel them to vote in great numbers. I knew we needed Latino organizers to make it work, and now that’s happened. They’re frightened about Trump.”

This year, they’ve had help. Tom Steyer, the hedge fund manager who has poured money into pro-Democratic causes since the Citizens United Supreme Court decision, helped fund a campaign called Yo Voy a Votar, ¿Y Tú? — “I will vote, and you?” — supplementing what was on the ground. Patricia Serrano, chairwoman of the North County Immigration Task Force, said her group registered nearly 1,000 Latino voters after a post-primary volunteer push.

Arcela Nunez-Alvarez, director of the National Latino Research Center at the University of California at San Marcos, said there has been fresh engagement on campus since Trump became a plausible presidential candidate. On the campus — built on old Mexican chicken farmland, Nunez-Alvarez pointed out — nearly 10,000 students walked past reminders to cast early ballots or join the Democratic Party.

“There’s an interest in public policy I’d never seen before in North County,” she said. “We have cities where the Latino population comprises half of the total, and 90 percent of the political representation is white.”

New voters were among the thousands who turned out for the June primary — and made Democrats realize that Issa could lose. California’s “top two” system, in which candidates of all parties run in a primary and the top two vote-getters advance, had never threatened Issa. This year, it gave him a narrow 5.3-point win. The Democratic Congressional Campaign Committee dove into the race, finding Issa in a tie for the general election.

Issa’s behavior has proved their theory. He has restricted his media appearances; questions about how to talk to him in recent days were answered with questions about why the media was not focusing on Applegate’s messy divorce records. Recently, the Huffington Post published an Issa mailer, aimed at Democrats, in which Issa thanked President Obama for “the Survivor’s Bill of Rights, which I co-sponsored.”

After it went viral, Obama, who had been accused of massive corruption by Issa, could not resist a counterpunch.

“That is the definition of chutzpah,” he said at an Oct. 23 fundraiser in the district. “That is shameless!”

In a statement, Issa could not resist grabbing the old gavel. He was “disappointed but not surprised” by the president’s snark; Obama, he said, had denied “accountability for the serious scandals that happened under his watch where Americans died overseas and veterans have died here at home.”

Issa has joined almost every Republican in cheering on the FBI for asking new questions about Clinton’s emails.

On Friday, as the veterans event ended, Applegate said that he needed to hear “more about what’s going on” with the emails and argued that the months-long Republican battering of Comey for not charging Clinton had always been ridiculous.

“He’s not going to cover for any president, least of all a Democrat — least of all a Clinton,” Applegate said.

To Republicans, who spent October in defensive crouches, the Comey letter was manna. At the least, it was a chance to tell voters in swing-state races that the GOP nominees would hold Clinton accountable and the Democrats would be a rubber stamp.

Applegate scoffed at that idea.

“Anything that Darrell Issa’s done in investigating has never produced anything,” he said. “Not a report. Not a finding. It’s cost a lot of money, and it’s given Darrell Issa an opportunity to hold a news conference and accuse the president of having the most scandalous administration in history. And now he’s sending out mail with the president’s picture on it.”