Mr. Trump’s guidance to governors and local officials envisions robust testing that does not yet exist. The testing would help isolate the sick so the well could return to work. Mr. Trump said in a conference call to governors on Thursday, “You’re going to call your own shots.” He is pushing them off in a canoe with a smile — and saying good luck finding a paddle.

They cannot do it alone. The United States cannot return to normal without mass testing, and the existing system is inadequate. New York Gov. Andrew M. Cuomo, a Democrat, said correctly, “This has to be figured out. I understand that this is a problematic area, and the federal government’s not eager to get involved in testing. I get that, but the plain reality here is we have to do it in partnership with the federal government.”

Judging by his briefing Thursday evening, Mr. Trump does not get it. But his own advisers seem to realize what they are up against. The Post reports that Mr. Trump’s team was thinking of requiring that most workers get tested before going back to work but eventually settled on seeking testing for a far more limited set of four key groups: health-care workers, food-safety workers, those in nursing homes and people with chronic diseases. That alone may require millions more tests that don’t exist. And what about everyone else?

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Before the pandemic, the U.S. workforce was about 157 million people. The total tests to date in the United States, 3.4 million as of Friday, would have to be multiplied roughly 23 times just to reach half of the pre-pandemic workforce. Even if the dimensions are less because of job losses, this would seem to be a colossal problem demanding Mr. Trump’s leadership.

Aside from the current scattershot U.S. approach to testing, another major challenge is supply shortages. The diagnostic tests, no matter who does them, require specialized chemicals, known as reagents. The pandemic created a global supply shock. Much of the supply came from China, and it will take time to build infrastructure for more. A separate type of testing, serology, which checks for antibodies in those people who have recovered from infection, currently has sufficient supply, but a shortage is coming there, too. Swabs are a critical element in testing and also scarce.

If Mr. Trump were really in a take-charge mood, he would convene a task force to figure out how to overcome these very real obstacles. The toll taken by leadership failure is growing by the hour.