Nearly 900 Secret Service employees were infected with the virus, a watchdog group finds.

The rigors of protecting a president, a vice president and their families in an election year amid a pandemic placed a heavy burden on the Secret Service, with nearly 900 employees testing positive for the coronavirus, a watchdog group said this week.

The group, the Citizens for Responsibility and Ethics in Washington, faulted former President Donald J. Trump for continuing to hold large campaign rallies, which it said had contributed to the infections.

It obtained the Secret Service data from the federal government as part of a public records request under the Freedom of Information Act. The cases were recorded from March 2020 to March of this year, according to the group, but the data did not include details of the assignments of the agents who were infected. The government also did not disclose what percentage of the total number of Secret Service employees had contracted the virus.

The employees who tested positive included 477 special agents, 249 members of the uniformed division and 131 staff members working in administrative, professional and technical positions, according to the group. The Secret Service is the main federal law enforcement agency charged with protecting U.S. political leaders, including the president, and the families.

“Throughout the pandemic, then-President and Vice President Trump and Pence held large-scale rallies against public health guidelines, and Trump and his family made repeated protected trips to Trump-branded properties which the then-president was making millions of dollars a year,” the group said on Tuesday in a post on its website. The group also blamed the former president for riding in a vehicle with Secret Service protection while he was under treatment for a coronavirus infection last October, “further putting agents in danger,” it said.

Representatives for Mr. Trump did not immediately respond to a request for comment on Wednesday night.

A spokeswoman for the Secret Service said in an emailed statement on Wednesday that the agency had distributed masks, gloves and other protective gear to employees, and conducted a robust virus testing program. She added that the agency’s mission “required significant public interaction during a public health crisis,” and that it “was fully prepared and staffed to successfully meet these challenges.”

Last November, the Secret Service’s uniformed officer division experienced a coronavirus outbreak, according to several people who were briefed on the matter at the time. The outbreak was at least the fourth to strike the agency since the pandemic began, with at least 30 uniformed Secret Service officers testing positive for the virus over several weeks. About 60 officers had been asked by the agency to go into quarantine.

In the final months of Mr. Trump’s presidency, the virus permeated the West Wing. Several of his top aides tested positive, including Hope Hicks, his adviser; Kayleigh McEnany, the White House press secretary; and two of Ms. McEnany’s deputies, Chad Gilmartin and Karoline Leavitt.