"We need people who are committed to serving the communities who elected them in the first place. And my background is one of service," Ms. Spanberger said at a recent campaign event."

The Candidates Who Came in From the Cold

Byron Tau, The Wall Street Journal 

SPOTSYLVANIA, Va.—Abigail Spanberger spent years undercover as a CIA case officer managing and recruiting spies. Now she is doing one of the most public things possible: running for Congress.

A Democrat, Ms. Spanberger is challenging the incumbent Republican Rep. Dave Brat in a GOP-friendly district that stretches from the outskirts of Richmond to the exurbs of Washington. Mr. Brat was an upstart candidate himself not long ago, a Tea Party-backed insurgent who in 2014 defeated former House Majority Leader Eric Cantor in the GOP primary.

Democrats have made unseating Mr. Brat a priority in the midterm elections, hoping the district’s educated, affluent voters will rally to their candidate, who is making her first run for public office.

“I wasn’t enthralled with, interested in or even comfortable with the idea of being in a public place with people publicly knowing who I am and knowing a lot about my background,” Ms. Spanberger said in a recent interview. “It’s a tremendous shift.”

This year, Ms. Spanberger is one of a crop of House candidates, most of them Democrats, with deep experience in national security who have emerged as a force on the campaign trail, touting backgrounds in espionage, counterterrorism and foreign policy.

“We need people who are committed to serving the communities who elected them in the first place. And my background is one of service,” Ms. Spanberger said at a recent campaign event at a winery overlooking a Civil War battlefield.

Nancy Soderberg, a former top national-security official in the Clinton administration, is running in the Aug. 28 Democratic primary for a Daytona Beach, Fla.-area House seat.

In Michigan’s Eighth Congressional District, former Central Intelligence Agency analyst Elissa Slotkin won the Democratic nomination this week and will face incumbent Republican Rep. Mike Bishop in November.

In Texas, former military intelligence officer Gina Ortiz Jones is running against incumbent GOP Rep. Will Hurd. Mr. Hurd, elected to Congress in 2014, is a former CIA operations officer who did tours in Afghanistan, Pakistan and India.

“Since I was undercover and I was undercover my entire time in the agency, I never said the three initials out loud. It was so strange basically saying it all the time,” Mr. Hurd recalled recently on a CBS News podcast about his first run for office and his transition from spy to congressman.

The national Democratic Party considers Ms. Spanberger, Ms. Ortiz Jones, Ms. Slotkin, Ms. Soderberg and Andy Kim, who is running for a House seat in New Jersey, top-tier candidates in competitive races. All have been included in the Democratic Congressional Campaign Committee’s “Red to Blue” program that lends organizational resources and support to candidates with a serious chance to win.

Earlier this year, former CIA intelligence analyst officer Jeff Beals mounted an unsuccessful bid in New York as a Democrat, losing in the primary. Scott Uehlinger, a former CIA operations officer, ran for the Republican nomination in a Pennsylvania House district and lost.

The public-affairs data and analytics firm Quorum has identified a handful of the 535 sitting members of Congress who have served intelligence agencies or in top national security jobs. Those include Mr. Hurd and Sen. Dan Sullivan (R., Alaska), who served on the White House National Security Council staff under former President George W. Bush.

A small number of others have shared similar profiles: Former GOP Rep. Porter Goss was in the CIA in the 1960s before running for Congress in 1988. He later became CIA director. Former President George H.W. Bush served as CIA director before his time in the White House.

Former members of the military at all ranks have long sought political careers after leaving the armed forces. But politics hasn’t been the typical career path for veterans of the spy services or former top national security staff members, especially those who spent most of their time working undercover or handling highly classified information.

Nicholas Dujmovic, a former CIA analyst who is now a professor of Intelligence Studies at Catholic University, said a public post-agency career, especially in politics, was once considered unthinkable for midlevel officers in the intelligence or national-security apparatus.

“CIA was really closed mouthed,” he said. “People would not do things that were public-oriented after retirement.”

But those who work for the CIA and in other national-security jobs haven’t been immune to broader trends in the labor market, including the rise of job hopping among younger employees.

On the stump, these candidates emphasize the nonpartisan nature of working in national security, contrasting their earlier work with what they say is dysfunction, polarization and partisanship of Washington, particularly Congress.

“I’m not somebody who saw myself as a politician or a partisan knife-fighter. said Mr. Kim, the New Jersey candidate.

Mr. Kim served in roles at the State Department, Defense Department and White House National Security Council in counterterrorism. He is challenging Republican Rep. Tom MacArthur.

Mr. MacArthur’s campaign has attempted to play down Mr. Kim’s experience, releasing an email where he was described as a “note taker” in a meeting with high level U.S. generals during his time overseas.

“Andy Kim is attempting to fool people into thinking he was a high-ranking military adviser directing foreign wars alongside our top generals, when the evidence seems to indicate that he was little more than a low-level bureaucrat charged with keeping notes,” a MacArthur campaign spokesperson said in May.

Mr. Kim’s campaign manager fired back: “If MacArthur wants to have a debate over who has done more for the security and safety of this country, Andy Kim is happy to engage him.”