Sen. Lindsey O. Graham announced Monday that he has tested positive for the coronavirus, a development that comes after he recently attended GOP and Senate functions without wearing a mask, including a gathering Saturday aboard Sen. Joe Manchin III’s boat.

Graham (R-S.C.), who was vaccinated against the coronavirus in December, said he started experiencing flu-like symptoms Saturday evening and saw the House physician Monday morning. He said he will be isolating for the next 10 days.

“I feel like I have a sinus infection and at present time I have mild symptoms,” Graham, the first known senator with a breakthrough infection from the virus, tweeted Monday afternoon, adding: “I am very glad I was vaccinated because without vaccination I am certain I would not feel as well as I do now. My symptoms would be far worse.”

It is unclear whether Graham’s infection could affect the Senate’s consideration this week of a massive infrastructure package that was the result of a recent bipartisan deal among centrist senators. Senate Majority Leader Charles E. Schumer (D-N.Y.) wants to pass the bill this week.

Graham’s announcement follows a week of rising tensions on Capitol Hill over a renewed call for mask-wearing by the Capitol physician after the Centers for Disease Control and Prevention updated its guidance in response to the growing number of infections across the country stemming from the delta variant of the virus.

The House and Senate have split over how to respond to the growing number of infections across the country.

House Speaker Nancy Pelosi (D-Calif.) last week reimposed a mask requirement on that side of the Capitol, threatening to fine members who do not wear masks in the House chamber, saying she was following the advice of Brian Monahan, the top doctor in the Office of the Attending Physician of Congress.

But the Senate adopted a more laissez-faire approach, with no guidance from Monahan released and senators and their staffs making their own decision about whether to wear a mask. That created a split-screen image in which the entire Republican side of the Senate chamber would be maskless, as the Democratic side was overwhelmingly masked up.

House Republicans reacted with hostility to the Pelosi’s mask requirement and used heated rhetoric to denounce it, such as likening the new order to authoritarianism rather than an attempt to protect members and staffers.

Covid-19, the disease caused by the coronavirus, has killed more than 613,000 Americans.

Several House GOP members staged a march Thursday across the Capitol to watch speeches by Senate conservatives deriding Pelosi and federal health officials. They took off their masks once they were on the Senate side of the Capitol and took up seats along the back wall of the upper chamber where staffers usually sit.

Some Senate Democrats, including Manchin (W.Va.), have mostly eschewed masks in the past week.

While Graham’s case is the first known breakthrough infection in the Senate, a White House staffer and a member of Pelosi’s staff who had been vaccinated tested positive for the coronavirus last month.

By 5:30 p.m. Monday, the Senate had begun voting on amendments to the $1 trillion bipartisan infrastructure package, which had been the inspiration for the Saturday’s boat ride on the Potomac River, and at least nine Republicans appeared on the chamber floor wearing masks.

Among the attendees who said they have since tested negative are Manchin, Sen. John Thune (R-S.D.) and Sen. Christopher A. Coons (D-Del.), although federal health guidelines stress that it could take up to five days for the disease to appear. Other senators among that group said they are following CDC guidelines, which recommends wearing a mask for 14 days or until receiving a negative test result.

Manchin said he did not regret hosting the event, which he said was designed to keep up the bipartisan spirit of the recent negotiations.

“There was no celebration,” he told reporters entering the Senate chamber Monday. “We were just trying to keep people together. We do everything in a bipartisan way, and that’s what we do.”

He said that the event was not a party and that he does not expect it to turnout to be a superspreader and alter the timing of the massive legislation, a key priority for the Biden administration.

“There’s no parties. Basically there’s gatherings we have — almost seven now … trying to get Democrats, Republicans together so we know each other and talk to each other,” he said. “We were outside, okay, and everybody’s been vaccinated. So, you know, I talked to Lindsey today. He’s fine. He sounds good.”

Among Democrats, Sen. Jon Tester (D-Mont.), who is part of the broad bipartisan negotiation but did not go to the Saturday event, was the only senator who never donned a mask Monday on the floor. Most Republicans wearing a mask, including Sens. Susan Collins of Maine, Lisa Murkowski of Alaska and Mitt Romney of Utah, had been part of the negotiations in recent weeks.

Thune, the minority whip — like the rest of Senate Minority Leader Mitch McConnell’s leadership team — did not wear a mask during the votes.

Several top aides of McConnell (R-Ky.) donned masks while on the chamber floor, in contrast with their practices last week.

While the summer spread of the delta variant has meant rising case numbers again in many parts of the country, data from the CDC shows that breakthrough infections remain rare. The potential for those breakthrough cases to carry viral loads similar to that of unvaccinated people means that the potential for transmission remains. A CDC report showed that in an outbreak linked to the July Fourth weekend in Provincetown, Mass., three-quarters of those infected had already been vaccinated.

Mike DeBonis contributed to this report.