Touring a vast convention hall that was to be turned into a makeshift hospital, Mr. Cuomo said on Tuesday that the state needed 30,000 ventilators in the coming weeks — not months — to cope with skyrocketing infections. “I don’t need ventilators in six months,” he said. “And I don’t need ventilators in five months, four months or three months.” After he spoke, Vice President Pence said the federal government would send 4,000 more to New York.

Besides the slow timetable, experts on medical devices questioned the wisdom of bringing in an unrelated industry like the automakers to manufacture a sophisticated medical device.

“It’s not like making a sedan or S.U.V.,” said Dr. Hugh Cassiere, a pulmonologist who sits on a Food and Drug Administration panel for anesthesia and respiratory devices.

“Sounds good as a sound bite, but the practicalities may be very difficult,” added Dr. Cassiere, who is helping oversee care for more than 40 severely ill coronavirus patients on ventilators at North Shore University Hospital on Long Island.

Experts in the field say there are other ways to rapidly boost the ventilator supply for overwhelmed hospitals. Most ventilators are sophisticated machines that keep critically ill people alive by delivering oxygen into the lungs through a tube inserted in the windpipe. Full-feature machines can cost as much as $50,000, but doctors say even basic models that cost less than $2,000 can sustain a patient in acute respiratory distress. The most grievously ill may need to remain on the machines for weeks until they can breathe on their own.

There are tens of thousands of no-frills resuscitators used by paramedics and the military. Plastic surgeons and outpatient surgery centers also have ventilators in their offices. With elective procedures indefinitely on hold in much of the country, those machines are now sitting idle.

“I think it will be faster to throw a couple ventilators on a truck and drive them to a hospital than wait for new ones to arrive,” said Dr. Adam Rubinstein, a plastic surgeon in Miami who has been compiling a list of doctors willing to loan out their ventilators in a pinch. “So far, no one has said no.”