Hardest hit have been southern and western states that pushed for early economic reopenings.
But the latest upsurge, pushing the nation's total of declared cases over 2.5 million and its death toll past 125,000, has prompted a growing chorus of calls for much tougher rules and enforcement.
"This is a very, very serious situation and the window is closing for us to take action and get this under control," Health and Human Services Secretary Alex Azar said on CNN.
'A unique position'
Azar was asked why Trump has refused to set an example by wearing a protective mask in public -- even when standing next to mask-wearing health advisors -- and reiterated the White House explanation that the president is tested daily and is "in a unique position" as a world leader.
But many Republicans who are normally reluctant to criticize the president have been calling more insistently for mask use, with some urging a clearer example from the nation's leader.
"If wearing masks is important, and all the health experts tell us that it is... it would help if from time to time the president would wear one to help us get rid of this political debate that says if you're for Trump, you don't wear a mask, if you're against Trump, you do," Republican Senator Lamar Alexander said Sunday on CNN.
In some areas, mask-wearing has taken on a political edge rather than being seen as a straightforward health choice.
Among Republicans calling strongly for mask-wearing are Senator Marco Rubio of Florida -- which has set daily records of new COVID-19 cases; Senate leader Mitch McConnell of Kentucky; and Governor Greg Abbott of Texas, another state that has seen a major uptick in cases after moving aggressively to reopen its economy.
In Phoenix, Governor Doug Ducey wore a mask on Thursday when he warned fellow Arizonans that "we are getting hit hard right now." The best defense against COVID-19, he told local media, is to wear protective masks.
Liz Cheney, a member of the House leadership, has tweeted a picture of her father, the former US vice president, wearing a mask under the words: "Dick Cheney says WEAR A MASK #realmenwearmasks"
Vice President Mike Pence has sometimes worn masks in public but stopped short of advocating mask-wearing when he chaired the first briefing in two months of the White House coronavirus task force on Friday.
Pence travelled Sunday to Texas for a briefing with Governor Abbott on the pandemic response, but has cancelled upcoming events in Arizona and Florida "out of an abundance of caution."
Asked why the administration hasn't pushed harder for masks, Pence told CBS on Sunday that "we believe that every state has a unique situation" and that "we want to defer to governors" to decide on rules for their own states.
Some Democrats want Trump not only to set an example but to mandate mask-wearing nationwide -- including Trump's White House rival Joe Biden who says he would seek to make masks compulsory in public spaces.
Asked on ABC if she would support such a move, House speaker Nancy Pelosi called it "definitely long overdue.
"We have the worst record of any country in the world. And the president says we're making progress."
She added, "The president should be an example. Real men wear masks. Be an example to the country and wear the mask."