As Sen. Sherrod Brown (D-Ohio) took the Senate floor on Monday, he had one request for Sen. Dan Sullivan (R-Alaska). “Please wear a mask,” he asked Sullivan.

The Republican presiding over the Senate abruptly cut off Brown.

“I don’t wear a mask when I’m speaking, like most senators,” Sullivan said. His red, white and blue mask sat atop his desk. “I don’t need your instructions.”

The brief but tense exchange between the senators, which went viral on Twitter in a clip viewed more than 1.8 million times by Tuesday morning, marked the latest partisan clash between congressional leaders, who have often argued over basic coronavirus safety and health measures inside the Capitol.

On Twitter, Brown argued the incident showed a lack of concern about spreading the virus among GOP senators.

“Once again, I asked my Republican colleagues to stop endangering all the Senate workers — and simply wear a mask when presiding over the Senate,” he tweeted with a link to the video. “Once again, they refused.”

But some Republicans accused Brown of exaggerating his concerns.

“This is idiotic. @SherrodBrown is being a complete ass,” Sen. Ted Cruz (R-Tex.) tweeted, claiming Sullivan was more than 50 feet away from Brown. “He wears a mask to speak — when nobody is remotely near him — as an ostentatious sign of fake virtue.”

A spokesperson for Sullivan told The Washington Post that the senator believes in the use of masks to prevent the spread of the coronavirus.

“He wears a mask in public, removing it in limited circumstances, like to speak on the Senate floor,” the spokesperson said in an email.

Earlier this summer, The Post reported, congressional leaders could not come to an agreement on how to rapidly test members, as well as their staff, for the novel coronavirus. Congressional leaders could not even reach an accord on which type of thermometer to use.

Whether lawmakers must wear a mask, among other health and safety coronavirus guidelines, depends on where they are in the building. Masks are mandatory on the House floor, but only recommended in the Senate.

In March, Sen. Rand Paul (R-Ky.) became the first senator to test positive for the coronavirus, raising concerns about the safety and health of the rest of the senators. Soon after Paul’s office disclosed the senator’s diagnosis, Republican Sens. Mike Lee and Mitt Romney of Utah announced they were self-quarantining because they had recent contact with him.

Most senators, including Senate Majority Leader Mitch McConnell (R-Ky.), and staff members have worn masks since the Senate returned to session in May, The Post reported. Brian P. Monahan, the Capitol’s attending physician, issued a seven-page memo in May encouraging senators to don masks as “a service to the community to decrease the risk of infection overall.” Those who do not wear a face covering “can maintain the six-foot separation guidelines,” Monahan added.

Although the majority of the senators wear their masks when on the Senate floor, there are exceptions. Paul has repeatedly refused to wear a mask, arguing he is “immune” to the virus after contracting it in the spring. There is not sufficient scientific evidence, experts have said, to prove whether someone who contracted the virus is actually immune to the disease.

In Monday’s exchange, after Sullivan refused to put on a mask while speaking, Brown said the incident was proof “there isn’t much interest in public health” at the Senate.

“We have a majority leader who calls us back here to vote on an unqualified nominee and, at the same time, to vote for judge after judge after judge, exposing all the people who can’t say anything … and expose all the staff here.” Brown said in the video clip. “The majority leader just doesn’t seem to care.”

Sullivan wore his mask while Brown addressed the floor, but took it off again after Brown was done to adjourn the Senate.