WASHINGTON: Tempers flared in the U.S. Congress on Wednesday after its chief physician urged lawmakers to resume wearing masks to slow the spread of the highly contagious Delta variant of COVID-19, with the top Democrat labeling Republican opposition as "moronic."

A high-ranking aide to House of Representatives Speaker Nancy Pelosi stopped short of confirming a report based on garbled audio that Pelosi called her Republican counterpart "such a moron" because of his opposition to the new directive.

"The Speaker believes that saying a mask requirement is 'not a decision based on science' is moronic," Drew Hammill, deputy chief of staff for House Speaker Nancy Pelosi, said in a tweet.

Hammill was referring to a tweet by McCarthy https://twitter.com/GOPLeader/status/1420209661735804934 in which he said, "Make no mistake - The threat of bringing masks back is not a decision based on science, but a decision conjured up by liberal government officials who want to continue to live in a perpetual pandemic state."

The high-level spat came as COVID-19 cases in recent days have been rising, along with deaths, across the United States.

Since early in the pandemic, mask-wearing and vaccinations have been U.S. political flashpoints, with Republicans, encouraged by former President Donald Trump, resisting and Democrats urging compliance with medical advice.

Many Republicans have complained that such government edicts infringe on individual liberties.

Late on Tuesday, Dr. Brian Monahan, the attending physician for Congress, required the use of masks indoors where people are congregating. It followed a similar move by the White House after the Centers for Disease Control and Prevention issued new mask recommendations to stem the spread of the new variant.

"Mask and vaccine mandates: Bullying, Controlling, Unconstitutional, Threats to Liberty!" Republican Representative Jody Hice of Georgia tweeted https://twitter.com/CongressmanHice/status/1420365111747911683 on Wednesday morning.

Some 57.6% of Americans have received at least one dose of COVID-19 vaccine, with the lowest rates across the heavily Republican U.S. Southeast. Four of the five U.S. states with the lowest vaccination rates have Republican governors: Mississippi, Idaho, Wyoming and Alaska, according to a Reuters COVID tracker. The governor of the fifth state, Louisiana, is a Democrat.

The top Senate Republican, Mitch McConnell, is rolling out a campaign ad https://www.reuters.com/world/us/mcconnell-strives-counter-bad-advice-boost-us-republican-vaccination-rate-2021-07-28 in his home state of Kentucky to counter what he called "bad advice" prompting some Americans to opt not to get vaccinated.

"For the Congress, representing a collection of individuals traveling weekly from various risk areas (both high and low rates of disease transmission), all individuals should wear a well-fitted, medical-grade filtration mask ... when they are in an interior space," Dr. Monahan said in a memo late Tuesday.

The rule applies to all House of Representatives office buildings, in the hall of the House and in committee meetings, he said.

Even before the recommendation, many congressional Democrats had resumed wearing masks in the Capitol this week.

At her weekly news conference, Pelosi attempted to cool passions somewhat by refusing to comment directly on whether McCarthy's position was "moronic." Instead, in response to a reporter's question, Pelosi said, "To say that wearing a mask is not based on science, I think is not wise."

Throughout the pandemic, the 100-member Senate and the 435-member House have taken different precautions to contain COVID-19 infections in the sprawling Capitol.

Monahan's latest directive did not require renewed mask-wearing on the Senate side of the Capitol - a decision that did not escape McCarthy.

"If she (Pelosi) knows so much about science explain to me where the science changes in the Rotunda," McCarthy said of the massive room that separates the House and Senate.