By Sheryl Gay Stolberg, New York Times–

Dr. Anthony S. Fauci, the nation’s top infectious disease expert and a central figure in the government’s response to the coronavirus, plans to deliver a stark warning to the Senate on Tuesday: Americans would experience “needless suffering and death” if the country opens up prematurely.

Dr. Fauci, who has emerged as the perhaps nation’s most respected voice during the worst public health crisis in a century, is one of four top government doctors scheduled to testify remotely at a high-profile — and highly unusual — hearing on Tuesday before the Senate Health, Education, Labor and Pensions Committee. He made his comments in an email to a New York Times reporter late Monday night.

“The major message that I wish to convey to the Senate HLP committee tomorrow is the danger of trying to open the country prematurely,” he wrote. “If we skip over the checkpoints in the guidelines to ‘Open America Again,’ then we risk the danger of multiple outbreaks throughout the country. This will not only result in needless suffering and death, but would actually set us back on our quest to return to normal.”

It is a message starkly at odds with the things-are-looking-up argument that President Trump has been trying to put out: that states are ready to reopen and the pandemic is under control.

In the Rose Garden earlier on Monday, Mr. Trump declared that “we have met the moment and we have prevailed,” though he later walked back the comments and said he only meant to say the country had prevailed on increasing access to coronavirus testing — an assertion public health experts say is not true.

Dr. Fauci, who has served under Republican and Democratic presidents for more than three decades and has worked to master the art of contradicting Mr. Trump without correcting him, echoed the language of Mr. Trump’s own plan, Opening Up America Again, which lays out guidelines for state officials to consider in reopening their economies.

But signs of opposition from parts of Mr. Trump’s party appeared almost immediately. Shortly after Dr. Fauci’s comments were published Monday night, Representative Andy Biggs, Republican of Arizona, pushed back on Twitter, and invoked another top scientist: Dr. Deborah L. Birx, Mr. Trump’s coronavirus response coordinator.

“Dr. Fauci has continually used his bully pulpit to bring public criticism on governors who are seeking to open up their states,” Mr. Biggs wrote. “The Fauci-Birx team have replaced faith w/ fear & hope w/ despair. The remedy is to open up our society & our economy. Trust & respect our freedom.”

The White House plan recommends, among other things, that before reopening states should have a “downward trajectory of positive tests” or a “downward trajectory of documented cases” of coronavirus over two weeks, while conducting robust contact tracing and “sentinel surveillance” testing of asymptomatic people in vulnerable populations, like nursing homes.

But the guidelines are not mandatory. Even as the death toll mounts — more than 80,000 Americans have lost their lives to Covid-19, the disease caused by the coronavirus — many states are reopening without adhering to them, seeking to ease the pain as millions of working people and small-business owners are facing economic ruin while sheltering at home.

In more than half of states easing restrictions last week, case counts were trending upward, the proportion of positive test results was rising, or both.

“We’re not reopening based on science,” said Dr. Thomas R. Frieden, a former director of the Centers for Disease Control and Prevention. “We’re reopening based on politics, ideology and public pressure. And I think it’s going to end badly.”

Tuesday’s hearing will be Dr. Fauci’s first appearance before Congress since Mr. Trump declared the coronavirus pandemic a national emergency on March 13, and it will offer a chance for him to address lawmakers and the public without the president by his side. The last time Dr. Fauci appeared on Capitol Hill, on March 11, when he was still permitted to testify before the Democratic-controlled House, he made headlines by bluntly telling the nation, “Things will get worse.”

His return to the Capitol, though virtual, will be must-watch TV in Washington — one of the strangest high-stakes hearings in recent memory. Dr. Fauci himself is now in “modified quarantine,” he has said, after what he described as a “low risk” exposure to someone infected with the virus.

He will appear alongside Dr. Robert Redfield, the director of the Centers for Disease Control and Prevention; Dr. Stephen Hahn, the commissioner of the Food and Drug Administration; and Adm. Brett P. Giroir, the assistant secretary for health. Drs. Redfield and Hahn are also in self-quarantine after exposure to the virus, as is the chairman of the committee, Senator Lamar Alexander, Republican of Tennessee.

All of the witnesses will testify remotely, and Mr. Alexander will lead the hearing from his home in Maryville, Tenn. But Dr. Fauci, who has been the director of the National Institute for Allergy and Infectious Diseases since 1984, will be the star. He has been largely out of public view for the past two weeks, ever since Mr. Trump canceled his daily coronavirus task force briefings.

At 79, Dr. Fauci has become both a sudden celebrity during the pandemic and a target for the far right. His face, with his wire-rim glasses and neatly parted gray hair, has been commemorated on sweatshirts, knee socks and mugs. A petition has circulated to name him People magazine’s “Sexiest Man Alive,” and someone who has actually garnered that title — the actor Brad Pitt — has portrayed Dr. Fauci on “Saturday Night Live.”

Some conservatives, though, see him as a media hound who is undermining the president. After Mr. Trump said drug companies would make a coronavirus vaccine ready “soon,” Dr. Fauci amended the president’s timetable, giving a more accurate estimate of at least a year or 18 months.

When Mr. Trump said a “cure” might be possible, Dr. Fauci explained that antiviral drugs were being studied to see if they might make the illness less severe. In March, he gave an extraordinarily candid interview to Jon Cohen, a writer for Science magazine, in which he confessed that he knew Mr. Trump’s assertions that he had slowed the pandemic by banning travel from China did not comport with the facts.

“I know, but what do you want me to do?” he said. “I mean, seriously Jon, let’s get real, what do you want me to do?”

After that interview, many in Washington thought Mr. Trump might fire Dr. Fauci, and the president stoked those fears by retweeting a conservative hashtag, #FireFauci.

In fact, it would be very difficult for the president to fire him because he is not a political appointee. And Mr. Trump himself has dismissed such talk; he has called Dr. Fauci “a wonderful guy,” and last month, he joked that Dr. Fauci, who is from Brooklyn, is so popular he could run against Representative Alexandria Ocasio-Cortez, the liberal firebrand Democrat from New York, and “win easily.”