President Biden, anticipating the grim milestone of one million American lives lost to Covid-19, said in a formal statement on Thursday that the United States must stay committed to fighting a virus that has “forever changed” the country.

“We must remain vigilant against this pandemic and do everything we can to save as many lives as possible, as we have with more testing, vaccines and treatments than ever before,” he said. “It’s critical that Congress sustain these resources in the coming months.”

The statement came hours before Mr. Biden is to convene his second Covid-19 summit, aimed at injecting new urgency into the global coronavirus response. Mr. Biden will also issue a proclamation on Thursday ordering flags at the White House and all federal buildings to be flown at half-staff until next Monday to mark the one million deaths.

As of Wednesday, the federal Centers for Disease Control and Prevention had reported more than 995,000 coronavirus deaths in the United States; a New York Times database put the figure at more than 997,000. But with heads of state, leaders of philanthropies and drug makers attending the virtual gathering, Mr. Biden was ready to mark the coming moment.

Well into the pandemic’s third year, Covid-19 has become the third leading cause of death among Americans, behind only heart disease and cancer. Its toll has extended deep into the lives of families across the United States. An estimated 250,000 children have lost parents or caregivers to Covid-19; of those, nearly 200,000 have lost one or both parents.

The pandemic has also defined the Biden presidency. Mr. Biden came into office vowing to conquer Covid-19, but hopes of achieving “herd immunity” through the combination of vaccination and exposure gave way to the harsh realization that vaccines’ protective power against infection could wane and that new variants made reinfection more common. Political divisions have thwarted mask and vaccine mandates.

Thursday’s meeting is an effort by the president to put Covid-19 back on the radar screen, both in the United States and around the globe. The White House has asked Congress for $22.5 billion in new emergency coronavirus aid, but the request is stalled on Capitol Hill, as is a compromise proposal for $10 billion in aid.

Senior Biden administration officials said Mr. Biden would use the summit to make a new plea for Congress to authorize the money.