What if there were some issue, some cause, some job that Donald Trump could take on as president, something he could unite many people behind, a mission even his bitterest critics could applaud?

And what if this effort did not benefit cronies or corporations, but instead protected the lives and dignity of vulnerable, disadvantaged Americans, members of one of the few such groups Mr. Trump did not demean during the campaign?

And what if this goal were realistic, involving something Mr. Trump is actually good at, playing to his professional strengths?

The job is this: building housing for homeless veterans in Los Angeles. That city and county have long had one of the largest concentrations of homeless veterans in the country. They also have one of the country’s largest Veterans Affairs medical centers — on 387 prime acres in West Los Angeles — but not one unit of permanent supportive housing to help destitute veterans leave the streets for good.

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Donald Trump took questions from veterans at the Commander-in-Chief Forum in New York in September.CreditEric Thayer for The New York Times

This disconnect has persisted for many decades. It’s truly a debacle, and not just because the property was deeded to the government for the specific purpose of housing veterans — in 1888.

As homeless men and women have survived on Los Angeles County’s streets and sidewalks, the sprawling V.A. site was taken by corporate and other leaseholders for unrelated uses — for baseball diamonds, theater stages, hotel laundries, rental car and bus storage, even oil wells and a dog park.

The string of politicians and Veterans Affairs officials who share responsibility for this failure is long and bipartisan. Democrats in the local congressional delegation paid the issue lip service for many years. It wasn’t until veterans advocates sued, and a settlement was reached last year, that any progress seemed possible.

The V.A. now has plans to build 1,200 apartments, with social and mental-health services, including a child-friendly “village” for women, including those who have survived sexual trauma. Congress passed legislation to enable the development, and President Obama signed it in September.

But the project has not gotten moving yet, which is where — next year — President Trump comes in. Some New Yorkers remember the Wollman skating rink in Central Park, a botched municipal renovation that Mr. Trump took over and finished. It’s one line on Mr. Trump’s résumé where he can claim to have cut through government red tape and achieved an actual concrete-and-steel victory for the public good.

Maybe he can do the same in Southern California.

“He can marshal a team of generals, V.A. folk, private-sector and nonprofit real estate experts and managers and build this housing immediately,” said Bobby Shriver, a former mayor of Santa Monica who led the lawsuit. “He can promise Los Angeles veterans that he will use his construction expertise to house them now, not next year or the year after.”