WASHINGTON — Joe Biden pledged Tuesday to bring the coronavirus pandemic under enough control to open most of the nation's schools during his first 100 days as president — going much further on the issue than he has in the past, even while warning that the U.S. is facing a "dark winter."

The president-elect said that promise is dependent on Congress providing sufficient funding to protect returning students, teachers and campus staff. He made it during an event in Delaware to introduce a team of health experts set to help the new administration combat a virus that has already killed more than 285,000 Americans.

The first coronavirus vaccine, from drugmaker Pfizer, is expected to be endorsed by a panel of Food and Drug Administration advisers as soon as this week, with delivery of 100 million doses — enough for 50 million Americans — expected in coming months.

Biden said he'd call for all Americans to wear masks for 100 days and would distribute at least 100 million vaccines during his first 100 days in the White House, in addition to seeking to reopen most of the nation's schools over the same period.

"It should be a national priority to get our kids back into school and keep them in school," Biden said. "If Congress provides the funding, we need to protect students, educators and staff. If states and cities put strong public health measures in place that we all follow, then my team will work to see that the majority of our schools can be open by the end of my first 100 days."

There were 130,930 K-12 schools in the U.S. in the 2017-18 school year, according to the latest data from the Education Department. At least half of those would have to resume in-person learning by the end of April for Biden to make good on his pledge, assuming sufficient funding is approved.

Biden provided few details on how he will achieve that goal except to say that officials will prioritize getting vaccines to educators "as soon as possible" after health care personnel and people in long-term care facilities have gotten them.

While still a candidate, Biden released a plan in July for resuming in-person classroom instruction during the pandemic that promised to send Congress an emergency funding package to help schools reopen with a price tag worth up to $30 billion.

That plan said final decisions on reopening schools would fall to state and local officials but nonetheless promised to enlist federal agencies to establish "basic, objective criteria" for doing so. Those included districts securing necessary funding to reconfigure classrooms for better social distancing, procuring protective equipment and devising plans to accommodate at-risk teachers and students.

Biden's comments won praise from the nation's two major teachers unions. Much of what he's proposed aligns with an approach previously offered by the American Federation of Teachers, which has called for a national mask mandate, wider virus testing and funding to make buildings safer.

"All I can say is hallelujah," said Randi Weingarten, the union's president. "We finally have a president-elect who understands that we need resources and strong public health measures in order to reopen school buildings."

With proper funding and strong public health measures, it would be "reasonable" to reopen schools within 100 days, Weingarten said, although students would still need to be spaced apart in classrooms until a vaccine is widely distributed.

The national superintendents association, known as AASA, welcomed Biden's plan but said it hinges on his ability to secure funding from Congress.

"It's a great idea and I very much support it, but the conditions that were made are absolutely essential, No. 1 being funding," executive director Dan Domenech said. "We've been asking for that money for the entire year, and it hasn't come forth."

Meanwhile, the task of reopening schools has continued to become more difficult as the virus continues to spread, Domenech said. As cases climb across the country, most of the nation's schools are operating online, he said, with only a third offering classes in-person.

Indeed, Biden's schools pledge was an optimistic note in a speech that otherwise included a lot of dire warnings. The president-elect said that, after nearly nine months of living with the pandemic, Americans are "at risk of becoming numb to its toll on us" and resigned to accepting "the death, the pain and the sorrow."

"We're in a very dark winter. Things may well get worse before they get better," Biden said. "A vaccine may soon be available. We need to level with one another. It will take longer than we would like to distribute it to all corners of the country." ___

Associated Press writer Collin Binkley contributed to this report from Boston.