On premiums alone, prices would rise by more than 20 percent for the oldest group of customers. By 2026, the budget office projected, “premiums in the nongroup market would be 20 percent to 25 percent lower for a 21-year-old and 8 percent to 10 percent lower for a 40-year-old — but 20 percent to 25 percent higher for a 64-year-old.”

But the change in tax credits matters more. The combined difference in how much extra the older customer would have to pay for health insurance is enormous. The C.B.O. estimates that the price an average 64-year-old earning $26,500 would need to pay after using a subsidy would increase from $1,700 under Obamacare to $14,600 under the Republican plan.

Perhaps unsurprisingly, the C.B.O. concludes that many, many fewer 64-year-olds will continue buying insurance in this market. By 2026, the uninsured rate for those 50 to 64 earning less than about $30,000 would more than double, from around 12 percent to around 30 percent. Those older customers who would lose out on insurance coverage are more likely than the young customers who would buy it to need help paying big medical bills.

Mr. Ryan has said that it is appropriate that the G.O.P. plan will cause more Americans to go without health insurance because it doesn’t have a mandate that people buy coverage or pay a penalty. “We’re saying the government’s not going to force people to buy something that they don’t want to buy,” he said on Fox News Monday afternoon. “And if we end an Obamacare mandate that says you must buy this government one-size-fits-all plan, guess what? People aren’t going to buy that.”