This year, Mitch Mallin bought a metallic blue Tesla Model 3 sedan equipped with heated seats and a bevy of other options, all for $56,000 — twice as much as he had ever paid for a car before.

The final cost will be lowered by a $7,500 federal tax credit, a bonus available to buyers of electric vehicles.

Mr. Mallin, a retired actuary from Farmington Hills, Mich., said he probably would have bought the car even without the tax credit because he wanted to “participate in the movement” to fight global warming and drive zero-emission cars.

Still, the credit “was factored in,” he said. “It’s almost like a rebate.”

But not for much longer. Under federal rules, the full tax credit is available only on the first 200,000 cars that a manufacturer sells in the United States; two quarters later, the credit is reduced. Tesla reached that threshold in July. As a result, the credit available to Tesla buyers will fall to $3,750 for cars delivered on or after Jan. 1, then will be halved again on July 1 and go away completely in 2020.