The House speaker, Nancy Pelosi, on Wednesday heaped disdain on the Republican minority leader’s criticism of Congress’s new mask requirement – a reversal of policy that reflected growing number of cases and fears about the highly-transmissible Covid-19 Delta variant at the Capitol.

“He’s such a moron,” Pelosi said of the House minority leader Kevin McCarthy, after he tweeted that the mandatory mask policy was not based on science but a political decision by Democrats. (Consensus among public health officials and scientists is that masks significantly lower transmission of Covid-19, especially in indoor settings.)

The unusually abrupt remark from Pelosi came as the attending physician of the Capitol reimposed the mask requirement for the House of Representatives. At least two House lawmakers and a fully-vaccinated aide to Pelosi have tested positive for the coronavirus, while the Senate homeland security and governmental affairs committee postponed a hearing after some of its members were partially exposed.

The House had dropped its months-long mask policy six weeks ago in a demonstration of its optimism that the US had largely defeated the pandemic, shortly before Joe Biden declared America’s so-called independence from Covid-19.

The number of Covid-19 cases has also skyrocketed across the country, and the seven-day rolling average of new infections reached 40,246 on Wednesday – around four times the level just three weeks ago – fuelled largely by vaccine holdouts and the Delta variant.

Capitol physician Dr Brian Monahan said in a memo to House lawmakers and congressional aides that he was reimposing the mask policy based on new CDC guidance and the unique nature of the Capitol, where thousands of people from across the country congregate each week.

“All individuals should wear a well-fitted, medical-grade filtration mask (for example an ear loop surgical mask or a KN95 mask) when they are in an interior space,” Monahan said. “For meetings in an enclosed US House of Representatives controlled space, masks are REQUIRED.”

The top doctor in Congress separately dispensed the same advice for the Senate but stopped short of pushing for a mask mandate. The Senate is far smaller than the House of Representatives, and most senators voluntarily adopted masks during the pandemic.

But like the measures being considered by Joe Biden to increase vaccination rates in the US, the reintroduction of the mask requirement in the House has inflamed Republicans, who have seized on such policies as a supposed egregious overreach of government power by Democrats.

The House minority leader McCarthy reacted testily to Pelosi’s remark on Tuesday, again questioning the basis of the mask mandate: “If she’s so brilliant, can she explain to me where the science in the building changes between the House and the Senate,” he said.

House rules say that any member who refuses to wear masks on the floor of the House chamber and in specified areas of the Capitol can be fined $500 for a first offence and $2,500 for a second, with the penalties paid out directly from their salaries.

Monahan’s memo says members will not be required to wear masks when they are alone in the Capitol complex, or when they are recognised to speak on the House floor. .

Still, several Republicans have already vowed to defy the mask requirement, and firebrand congresswoman Lauren Boebert caused consternation when she walked onto the House floor without a mask and threw one back at a staffer when offered her a spare, CNN reported.