Amid that, the bureau has begun to lay plans for restarting parts of the count that have been stopped in their tracks. National field offices, shuttered in March, are slated to start reopening on June 1. Deliveries of forms and instructions to those five million households lacking addresses are now set to resume in mid-June, three months late. In-person visits by census takers to group quarters like nursing homes, once scheduled to begin at the start of this month, have been rescheduled to start July 1.

Some other aspects of the census — a count of roughly a half-million homeless people, pop-up centers that help people complete the census at places like grocery stores, the count of all 1.7 million households in Puerto Rico — remain on hold until the bureau can figure out how to conduct them safely.

Most important, the strategists are betting that the virus’s grip will weaken enough by mid-August to safely deploy hundreds of thousands of temporary field workers to track down the millions who still have not sent in forms. Without the success of that exercise — known in census-speak by the acronym NRFU (“ner-foo”), for nonresponse follow-up — the census will be compromised.

Experts say that effort, which is set to run through October, is likely to be the diciest aspect of the entire reboot. The census is supposed to be a snapshot of the nation at the beginning of April; the door-knocking was originally supposed to begin in May. But by autumn, the national mosaic will have reshuffled.

“The farther you get from April 1, the less accurate the data is,” said Jeri Green, a veteran Census Bureau employee who now is the senior adviser on the census for the National Urban League. “Imagine in October that a household gets a knock on the door and someone in a mask asks who lived there on April 1. In some communities people may be one stimulus check from getting off someone’s couch. Weddings are coming up. People are going to move out of their parents’ homes.”

There are other concerns as well: The rescheduled follow-up would take place at the peak of summer heat in the Southwest and hurricane season in the South. A prolonged economic collapse could trigger huge migrations of job-seekers. And, of course, the pandemic may not abate enough to allow hordes of door-knockers to trek through neighborhoods, much less persuade residents to open their doors.

Mr. Cook, of the Census Bureau, said the agency was ready to change plans again if the need arises. Veterans of past censuses say doomsday scenarios are most likely just that. The pandemic could also fade. Local campaigns to drive up response could resume. And there are ways to discern who lives in households that do not respond — data from neighbors, government records and the makeup of nearby households — that would allow the bureau to at least make an educated guess.