It seems a simple enough proposition: Give people the choice to buy into Medicare, the popular federal insurance program for those over 65.

Former Vice President Joseph R. Biden Jr. is one of the Democratic presidential contenders who favor this kind of buy-in, often called the public option. They view it as a more gradual, politically pragmatic alternative to the Medicare-for-all proposal championed by Senator Bernie Sanders, which would abolish private health insurance altogether.

A public option, supporters say, is the logical next step in the expansion of access begun under the Affordable Care Act, passed while Mr. Biden was in office. “We have to protect and build on Obamacare,” he said.

But depending on its design, a public option may well threaten the A.C.A. in unexpected ways.

A government plan, even a Medicare buy-in, could shrink the number of customers buying policies on the Obamacare markets, making them less appealing for leading insurers, according to many health insurers, policy analysts and even some Democrats.