Their electric cousins are easier to ride than the gas-powered monsters. They have no clutch or gearshift, so riders do not need to coordinate all their extremities to operate the controls. They just twist the grip and go. There’s no hot exhaust pipe to burn a leg. And they project a friendlier, more eco-conscious image. They’re whisper-quiet.

There are hopeful signs for the industry.

While sales are flagging, motorcycle ridership is at an all-time high, according to the Motorcycle Industry Council in Irvine, Calif. Almost 29 million riders swung a leg over a bike at least once in 2018. What’s getting the industry in trouble is that the pre-owned market is three times the size of the market for new bikes, the group says.

Electrics could change that. Almost 70 percent of millennial riders in the council’s survey of owners said they were interested in electric motorcycles. But so far, no Tesla of bikes has emerged.

Zero Motorcycles, based in Scotts Valley, Calif., entered the market in 2008. Hollywood Electrics is its No. 1 dealer globally. Still, the shop has sold just 500 of its bikes over the last decade.

For companies like Zero, which have seen Brammo, Alta Motors and other electric motorcycle start-ups come and go, Harley-Davidson’s entrance into the category provides legitimacy.

“I hope Harley’s serious,” said Sam Paschel, chief executive of Zero Motorcycles.

Mr. Paschel said he was worried that Harley would jump into the market, “stumble and go back to the combustion engine business model they’ve been running.”