Rick Nelson, a former official on the National Security Council under the George W. Bush administration who dealt with homeland security issues, said few other law enforcement agencies faced such pressure to continue operations while at the whims of political candidates prone to travel.

“They have a relatively small work force that doesn’t allow them to absorb and respond to the fluctuation,” said Mr. Nelson, a fellow at the Center for Strategic and International Studies. “They’re at higher risk than the general public because they can’t do their job if they’re social distancing.”

The Secret Service trains its officers and agents at a sprawling campus in Laurel, Md. The center has mock versions of Air Force One, Marine One, the president’s helicopter and the White House grounds. There is a large parking lot where agents learn how to drive the hulking armored limousine the president is driven in, which is