WASHINGTON – For the past year, the U.S. Secret Service has shuttled hundreds of agents to a training site just outside the capital as it prepares for a costly, contentious presidential campaign.

Tuesday night, the need for such preparation became clearer when protesters rushed the stage where former Vice President Joe Biden celebrated a string of primary election victories.

Biden was not harmed by the sign-waving protesters, who were repelled by his wife, Jill Biden, and a campaign aide. But the jarring scene prompted a House committee Wednesday to urge the Department of Homeland Security to consider assigning protective details to Biden and Bernie Sanders as they battle for the Democratic nomination.

Both campaigns have been operating without government protection, but the Secret Service's involvement appears more likely as the presidential campaign heats up.

Decisions on protection assignments are made by the Secretary of Homeland Security in consultation with congressional leaders. Late Wednesday, the Department of Homeland Security said it was "ready to execute" if requests were made by the candidates or a bipartisan group of congressional leaders. Biden appeared closer to making a formal request Thursday when he told NBC's Today that Secret Service protection is "something that has to be considered."

Since last year, nearly 500 Secret Service agents have completed weeklong training sessions to ready themselves for the rigors of the campaign season, including the parties' political conventions in Milwaukee and Charlotte, North Carolina.

Instructors have drilled agents on scenarios ranging from urban combat to manning rope-lines and securing stages like the one protesters rushed Tuesday night.

Major presidential and vice presidential candidates automatically receive Secret Service protection within 120 days of the general election. But many candidates receive it much earlier in the campaign cycle.

For the 2020 campaigns, the agency had prepared to provide protection for as many as eight candidates at one time after the initial roster of Democratic hopefuls swelled to more than 20.

A spate of mass shootings has made preparations more daunting. Secret Service officials were briefed by local law enforcement and medical authorities involved in the response to the 2017 Las Vegas attack — the largest mass shooting in American history — as the agency began setting its protective strategy for campaign venues across the country.

Officials said the mass shootings underscore the risk both to candidates and their thousands of supporters.

"The scope is enormous," said Ken Valentine, who served as chief of the agency's Dignitary Protection Division until his retirement last week. "Part of the reason it's so enormous is the ramifications of this not going well are enormous. And so the preparations are intense."

Secret Service agents worked for free after hitting pay caps

The raucous 2016 campaign pushed the agency to its limits as more than 1,000 agents maxed out on overtime and salary months before the election.

Despite new legislation that boosted agents' maximum annual compensation by about $25,000, hundreds of agents are expected to reach their pay caps before the end of this year. That means they will end up working long hours for free.

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"A lot of the journeymen (senior special agents assigned to the campaigns), the ones who are doing a lot of the hard work in this thing, will far surpass" the pay cap, he said.

In 2018, President Donald Trump signed legislation that raised combined salary and overtime limits from $164,200 to $189,000. That highlighted the agency's crushing workload for the 2016 election and during the first years of the Trump administration.

Congress intervened to boost pay after USA TODAY reported in August 2017 that about 1,200 agents and officers already had hit the mandated pay caps for the year.

In the early days of the Trump administration, the service protected up to 42 officials, including 18 Trump family members. That number has since dropped to 38, according to the agency, which does not identify those it protects.

By comparison, 31 people received protection under the Obama administration.

Though the pay measure passed, Rep. John Katko, R-N.Y., a sponsor and member of the House Homeland Security Committee, has said the additional money wasn't a permanent fix.

"This is a short-term solution to a long-term problem," the congressman said then. "We'll take what we can get right now."

Valentine acknowledged that compensation levels continue to shadow the agency, but asserted its mission has not been compromised.

"I would love to pay everybody more," he said before his retirement. "I'm a lot more focused on making sure they've got what they need. When we're in the thick of it, the money thing is not even talked about. It's a non-factor."

Preparing for the 'worst case'

The Secret Service motorcade was churning toward its downtown destination when a gunman driving a late-model minivan made his move.

A burst of gunfire from the driver's automatic rifle disabled the presidential candidate's limousine, leaving the candidate dangerously vulnerable as the suspect's van blocked the path forward.

The advantage didn't hold for long.

Within seconds, teams of heavily-armed agents swept up both sides of the street before unleashing an ear-piercing fusillade. The threat was eliminated.

Since last spring, similar "worst-case" scenarios have played out in a mock village built on the tranquil grounds of a Beltsville, Maryland, training center just off the busy Baltimore-Washington Parkway.

Agents have prepared for the campaign season by engaging in simulated street combat and preparing for more common situations like candidate rope-lines.

"As an agency, we certainly saw a higher level of aggressiveness from a lot of the crowds" in 2016, said Brian McDonough, the assistant agent in charge of the Protective Detail Training Section.

At some venues, bike racks were used as a buffer between candidate and crowd. But people jumped the barriers for better vantage points.

"We were actually having to physically detain" people, said McDonough, who has served on details for Biden and President George W. Bush. "Probably, for a lot of agents, it became a little less academic because there was so much of it going on."

McDonough said clips of the 2016 confrontations are now incorporated in the training regimen.

"I think it benefited what I do by making sure that people aren't going to get complacent, because all I have to do is insert a video from three years ago into my PowerPoint presentation and it really drives the issue home."

Trained in everything from food allergies to mass shootings

Apart from physical protection exercises, each weeklong training session includes hours of briefings on search and seizure laws. Intelligence analysts provide backgrounders on potential threats. A full day is dedicated to emergency medicine.

Once teams are assigned to candidates, agents receive detailed medical briefings for them and their families, from food allergies to chronic illnesses.

The first of the 16 training classes began in March 2019. Instructors also have traveled to more than a dozen field offices to prepare agents for deployments on advance teams wherever they might be needed.

As agents begin traveling with candidates, the operations will be monitored from the fifth floor of an office building in downtown Washington, D.C., where supervisors will provide logistical, technical and other support. Giant televisions will allow managers in Washington to track campaign events as they unfold and monitor agents' schedules as they move from city to city.

"We are seeking a 360-degree sphere of protection around anyone we undertake to protect," Valentine said.

That mission must account for the possibility of a mass shooting.

The challenge is not an unfamiliar one. In the run-up to the 2016 party conventions, the country was jolted by a pair of deadly shootings that targeted police officers. Five officers were killed in Dallas and three died in Baton Rouge in separate ambushes on the eve of the Republican National Convention in Cleveland.

While the Secret Service's preparations have included mass shootings for years, the Las Vegas attack brought a dreaded worst-case scenario to chilling reality.

"It's not just keeping the candidate safe," Valentine said. "Our job really extends out to the crowd. We're responsible for every person."