By Paul Kiernan

WASHINGTON--Millions of Americans who have missed rent payments due to the coronavirus pandemic could be at risk of being evicted in the coming months unless government measures to protect them are extended, economists and housing experts say.

Nearly 12 million adults live in households that missed their last rent payment, and 23 million have little or no confidence in their ability to make the next one, according to weekly Census Bureau data.

About a third of the country's renters are protected by an eviction moratorium that covers properties with federally insured mortgages. That expires July 25. Many renters are jobless and depend on supplemental weekly unemployment benefits of $600 that are due to end on July 31.

A number of cities and states have broader protections that will remain in place longer. Boston has banned evictions from public housing through the end of the year. Pennsylvania recently extended its moratorium against evictions for nonpayment of rent until Aug. 31.

The White House is negotiating with Republicans and Democrats in Congress to pass another round of economic relief during the last week of July. Treasury Secretary Steven Mnuchin has said supplemental jobless benefits have created a disincentive to return to work as the economy starts to reopen and should be reduced.

House Democrats voted in May to expand the eviction moratorium to cover all residential dwellings and extend it by a year. The bill included an extension of enhanced jobless benefits and $100 billion in rental assistance. The office of Sen. Mike Crapo (R., Idaho), chairman of the Senate Committee on Banking, Housing and Urban Affairs, declined to comment on any GOP plan to keep people in their homes because negotiations are under way.

Paying rent is a struggle for Americans whose jobs evaporated when government-imposed lockdowns closed businesses ranging from barber shops and restaurants to stores and fitness centers. More than 18 million people were receiving unemployment insurance during the last week of June.

"They all had jobs, and they had economically viable jobs, but we told them they couldn't work--to protect us--and now we're going to kick them out of their houses," said Shamus Roller, executive director of the National Housing Law Project.

Depending on the jurisdiction, landlords must give notice if they plan to evict tenants, often 30 days in advance. Housing experts say some landlords are willing to offer flexibility to tenants in financial distress, given the difficulty of finding new tenants in a downturn, and about 1.5 million adults had their last rent payment deferred, according to census data. But such help isn't always available.

Among those at risk of eviction is 58-year-old Aileen Collins of Dallas, who says she didn't start receiving unemployment benefits until eight weeks after she lost her job at a nonprofit in March.

After missing a rent payment in April, she said she was threatened with eviction from her two-bedroom apartment. She got a reprieve when she showed the manager that the building's mortgage was covered by the federal moratorium.

Without extra jobless benefits, Ms. Collins said her income would go down to $200 a week--from about $800 when she was working--too little to cover monthly living expenses of $2,000. Her savings will last two months, and Ms. Collins fears becoming homeless again, an experience she lived through eight years ago.

"Some days, I sit up here and I just watch the news, and I say, 'Where's the plan?' There's no plan," she said.

Molly Lynch of Henderson, Nev., is worried her family will be evicted when the state's moratorium is lifted on Sept. 1. Ms. Lynch has a 5-year-old daughter and is due to have a baby in November.

"I can't go back to work because she's not in school; the schools are closed," Ms. Lynch said. "So I don't see how we're supposed to get out of the hole we're in."

Ms. Lynch and her partner, a forklift driver who was laid off in March, fell behind on their rent in April after the state's backlogged unemployment system failed to pay him benefits, she said.

Ms. Lynch's family has managed to get by, thanks to an inheritance from her grandmother. But that won't last more than a few more months. If the inheritance is used up before some form of government protection kicks in, she said, "we would be living in our cars, which is also illegal in the state of Nevada."

"If we have an eviction on our record, nobody's going to rent to us," she said.

Homelessness rises with unemployment, according to a model by White House economist Kevin Corinth, based on data from the 2007-09 downturn. He found that each percentage-point increase in the unemployment rate forces about 21,500 people nationwide into homelessness. The U.S. jobless rate in June was 11.1%, up from 3.5% in February.

"The obvious next question is: How is this time different from then?" said Brendan O'Flaherty, a Columbia University economist. "The moratoria and pandemic unemployment insurance are one part of the answer," he said. "But we don't know what's going to happen to them in a couple of weeks."

Researchers at Princeton University's Eviction Lab documented a sharp decline in eviction filings from March to April in the 11 cities they track after the federal moratorium and many state-level bans took effect.

Evictions have since begun to creep up in cities where state moratoriums have expired, the group's data show. Eviction filings in Milwaukee shot up to 1,447 in June, after Wisconsin's moratorium expired, from 61 in May.

"Milwaukee may be the canary in the coal mine," said Alieza Durana, a spokeswoman for Eviction Lab.

Even with eviction protection and unemployment benefits, Americans like Jeffrey Lello, 42, have fallen through the safety net. In late March, his hours at the Outback Steakhouse where he waited tables in Orlando, Fla., were reduced to zero because the franchise closed its dining rooms.

He soon burned through his savings and filed for unemployment benefits. So far, he said, he has received just one week's worth of benefits totaling $875.

Mr. Lello said he had to leave the townhouse he shared with housemates because he was behind on his share of the rent. For the past several weeks he has lived in a tent in a wooded area near a food bank.

Elizabeth Watts, a spokeswoman for Bloomin' Brands Inc., which owns the Outback Steakhouse brand, said Mr. Lello was told the company would "support him and help him however we can until we can get him back to his normal hours." She said the company had provided "relief pay," which Mr. Lello said amounted to less than $300, and several free meals.

"People meet me now and they'll see that I'm unemployed and they'll see that I don't have a place to live, and then they'll make judgments about who I am now," Mr. Lello said, adding that he sees himself as a responsible, hard worker. "I know who I am. But nobody sees that anymore."

Write to Paul Kiernan at paul.kiernan@wsj.com