It puts him in direct conflict with Senator Bernie Sanders of Vermont and others who back a single-payer “Medicare for all” plan. Mr. Sanders has led the charge to eliminate most private health insurance as part of his proposal, though other Democrats running for president have said they would seek to retain a private-insurance structure.

In Des Moines, where Mr. Biden addressed an audience of about 200 Iowa AARP members who had gathered on the campus of Drake University, the former vice president said he would not “criticize my opponents” before leaning into a harsh critique of the system favored by Mr. Sanders.

Expanding the Medicare universe from 60 million to 300 million people, Mr. Biden said, would eliminate the health insurance program primarily used by older Americans.

“Medicare goes away, it’s a new Medicare system,” Mr. Biden said. “It may be as good, you may like it as well, it may or may not, but the transition of dropping 300 million people on a totally new plan, I think is a little risky at this point.”

The stop in Des Moines was the first event of a three-day Iowa swing for Mr. Biden, who has led in national and state polling despite appearing in the state far less than his top competitors. He appeared at just eight public events in Iowa during the first six months of this year, while 12 candidates have been at 30 or more, according to The Des Moines Register’s candidate tracker.