Highways, along with mortgage guarantees for white homeowners in the suburbs and a Supreme Court ruling that protected white families from school desegregation there, also encouraged the flight of taxpayers out of cities.

“When our public policy at really every level — including federal — abandoned these cities, they took the wealth with them and left them with the costs,” said Barbara Samuels, a fair housing attorney with the A.C.L.U. in Baltimore. “It drives me crazy when white people sit smugly in their suburban houses and say, ‘Oh, the Democrats, or black people, or whoever, are running cities into the ground.’”

Baltimore today has thousands of vacant homes left behind by this process, but lacks money to tear them down. Ms. Samuels would like to see the federal government invest more to help shrinking postindustrial cities, just as the federal government’s Hardest Hit Fund has directed resources to the places most affected by the housing crash.

In many other ways, particularly since the 1980s, the federal government has pulled back aid to cities. It allowed public housing to deteriorate, and stopped building new public housing projects. It ended other programs supporting private development of subsidized housing. It rolled sewer and water funds into Community Development Block Grants, which have fallen in value by 80 percent since their peak in 1979.

Now Baltimore has sinkholes opening on city streets. It needs new school buildings. Its Police Department needs computers in police cars.

“The result of all this is we spend almost nothing on parks and recreation,” Ms. Samuels said. “While that sounds like it might be a frill, it’s not, because it’s what keeps kids busy and active. We do basically nothing for the youth of Baltimore, and that’s our biggest problem."

There are other policies the president could pursue today that don’t require vast infrastructure money.