FRESNO, Calif. — As the operations manager of an outreach center for the homeless here, Paul Stack is used to seeing people down on their luck. What he had never seen before was people living in tents and lean-tos on the railroad lot across from the center.

“They just popped up about 18 months ago,” Mr. Stack said. “One day it was empty. The next day, there were people living there.”

Like a dozen or so other cities across the nation, Fresno is dealing with an unhappy déjà vu: the arrival of modern-day Hoovervilles, illegal encampments of homeless people that are reminiscent, on a far smaller scale, of Depression-era shantytowns. At his news conference on Tuesday night, President Obama was asked directly about the tent cities and responded by saying that it was “not acceptable for children and families to be without a roof over their heads in a country as wealthy as ours.”

While encampments and street living have always been a part of the landscape in big cities like Los Angeles and New York, these new tent cities have taken root — or grown from smaller enclaves of the homeless as more people lose jobs and housing — in such disparate places as Nashville, Olympia, Wash., and St. Petersburg, Fla.