WASHINGTON — Top federal health officials on Tuesday defended the Biden administration’s efforts to protect Americans from the highly contagious Omicron variant, facing withering accusations from senators about the scarcity of coronavirus tests and confusing guidance on how soon people who test positive for the virus can return to normal life.

In a nearly four-hour hearing, lawmakers charged that the administration remained woefully unable to meet the demand for at-home tests, noting that the White House would fulfill its pledge to send 500 million of them to American households for free only after the current surge had peaked.

The health officials testified before the Senate Committee on Health, Education, Labor and Pensions during one of the most trying weeks yet in the administration’s struggle with the pandemic. Infection rates are skyrocketing nationwide, and hospitals set a single-day record on Sunday for the number of patients with the virus, surpassing last winter’s peak.

While Democratic senators offered only gentle criticism, Republicans were unsparing, claiming that President Biden and his pandemic response team had mangled public health strategy and messaging.

“Most Americans can’t make heads or tails of anything coming out of this administration,” Senator Tommy Tuberville, Republican of Alabama, said. “I’m getting texts as we speak, sitting here, asking, ‘Where do I get the test?’ We spent billions on this.”

The officials who testified said the administration had mounted an all-out effort to test, treat and vaccinate Americans in the middle of a shape-shifting pandemic that had suddenly reached a new inflection point with the Omicron variant.

“It’s hard to process what’s actually happening right now, which is, most people are going to get Covid,” said Dr. Janet Woodcock, the acting commissioner of the Food and Drug Administration, offering one of the federal government’s most pointed acknowledgments of Omicron’s impact since the variant arrived in the United States.

“What we need to do is make sure the hospitals can still function, transportation, you know, other essential services are not disrupted while this happens,” she added.

Dr. Anthony S. Fauci, Mr. Biden’s chief medical adviser, said the virus had “fooled everybody all the time, from the time it first came in, to Delta, to now Omicron,” adding, “We’re doing the best we possibly can.”

The hearing came as the Omicron variant, coupled with the Delta variant, has strained hospital systems and caused businesses to struggle to stay open because of staff shortages. An average of more than 735,000 infections are being reported in the United States each day, according to a New York Times database.

On average over the last seven days, more than 135,000 people were hospitalized with the virus, an 83 percent increase from two weeks ago. The hospitalization totals include people who test positive for the virus incidentally after being admitted for conditions unrelated to Covid-19, but there is no national data showing how many people are in that category.

Modeling scenarios cited in an internal government document dated Jan. 5 and obtained by The New York Times suggest there will be more than a million confirmed infections per day by the end of the month.

That number is widely viewed as a vast undercount because of the scarcity of tests and the widespread failure of people to report positive results from at-home tests to government authorities. The Institute for Health Metrics and Evaluation, an independent health research center at the University of Washington, estimated last week that daily infections had already risen to six million, and it predicted that more than half of Americans would be infected by the Omicron variant over the next six weeks. Public health experts have said that many cases will be mild or asymptomatic.

Senior Biden administration officials said in interviews on Monday that infections and hospitalizations were expected to peak nationwide by the end of January and then plummet sharply. But Dr. Fauci warned on Tuesday that peaks and valleys would not look the same across the United States.

The hearing took a detour early on with a fierce back-and-forth between Dr. Fauci and Senator Rand Paul, Republican of Kentucky. The senator accused Dr. Fauci of working to undermine scientists with opposing views on the virus, something Dr. Fauci strongly denied.

His voice raised, Dr. Fauci said personal attacks from Republicans had put his safety and his family’s safety at risk. He held up a copy of a fund-raising webpage for Mr. Paul that featured a “Fire Dr. Fauci” graphic, and he said the senator had targeted him to score points with conservatives.

After nearly a year of concerted effort to tame the pandemic, Mr. Biden is facing an exhausted public and a new burst of alarming headlines. Asked by reporters on Tuesday whether he was worried about the nation’s fight against the virus, the president said that he was “concerned about the pandemic, just because worldwide it’s not slowing up very much.” He added that federal officials were working to help states and hospitals.

Last week, a group of former pandemic advisers to Mr. Biden published a series of articles calling on the administration to reset its response to Covid in a way that would acknowledge the “new normal” of living with the virus indefinitely.

Federal officials on Tuesday also hinted at that. Dr. Rochelle P. Walensky, the director of the Centers for Disease Control and Prevention, told lawmakers that it was more important for Americans to use rapid tests to guide their behavior than to report every positive result to government agencies.

“One of the really important purposes of these rapid tests, even if we don’t count them, is to empower the public to do the right thing through this pandemic” and not infect others, she said.

One idea the former Biden advisers proposed was the distribution of free N95 masks, which could better protect against the Omicron variant than cloth or surgical masks. The C.D.C. was set to update its mask guidance to better reflect how some masks offer different levels of protection, an agency official said on Tuesday.

At the hearing, Dawn O’Connell, the assistant secretary for preparedness and response at the Department of Health and Human Services, said there were 737 million N95 masks in the government’s strategic national stockpile, and that additional contracts for such masks would most likely be finalized by February. The government is asking potential contractors to make 141 million of the masks each month at a “surge capacity,” she said.

Over and over, senators from both parties returned to the administration’s efforts to meet the demand for tests, and its sometimes conflicting recommendations on when to use them.

Senator Patty Murray, Democrat of Washington and the committee’s chairwoman, praised the administration’s work to supply vaccines and treatments to Americans, but she said that health workers were still spread far too thin and that schools were “worried they’ll have to shut down again if they can’t get the support for testing they need.”

Senator Richard M. Burr of North Carolina, the panel’s top Republican, criticized the administration’s promise to deliver 500 million rapid tests to the homes of Americans, saying that Mr. Biden had pledged to do so without having the tests in hand.

“Try to get the administration to refrain from making these proclamations until we’ve got the product,” Mr. Burr said to Ms. O’Connell.

Ms. O’Connell said that when federal health officials saw Omicron sweeping across South Africa and Europe, “we immediately reached out to our manufacturers to understand any supply constraints they had and to evaluate their surge capacity” for producing tests.

“We have also met daily with them to make sure that they have what they need from their suppliers,” she said, adding that the Defense Production Act had been used in recent weeks to help free up supplies and manufacturing capacity.

She said that in the fall, the administration had invested $3 billion to support manufacturing rapid tests, leading to an increase in availability, but she acknowledged that “that’s not enough.”

Ms. O’Connell said that while some of the 500 million tests the government had bought would be sent to Americans by the end of January, it would take two months to distribute the rest. By then, as one senator noted, the Omicron surge will likely have long peaked.

Just 50 million of the 500 million promised tests have been purchased so far, Ms. O’Connell said, though more agreements will be announced in the coming days. Separately, seven million tests have been shipped to community health clinics and food banks, out of 50 million that the administration has committed to sending, she said.