A quarter century ago, Toyota unveiled a revolutionary vehicle: the Prius.

The Prius was the first of a new category of cars, marrying an electric motor to a gasoline engine to dramatically increase fuel economy and reduce emissions.

Today, hybrid vehicles remain popular with many drivers — enduringly popular, across a growing number of vehicle segments. That's even as a new revolution is unfolding, with automakers investing many billions of dollars in all-electric vehicles, which don't use gas at all.

Car buyers still like hybrids because they can save on gas and cut their carbon footprint without having to deal with range anxiety or the need to charge at home.

But even as hybrids go mainstream, they are losing traction among their original enthusiasts: environmentalists.

Many say it's time for hybrids to fade into history; that they are at best a detour, and at worst an obstruction, in the fight against climate change.

"Right now we are facing a climate crisis, and we absolutely need to reduce our dependency on fossil fuel cars," says Katherine Garcia, who directs the Sierra Club's Clean Transportation for All campaign.

Here's what to know about the environmental debate over hybrids.

Hybrids are a popular choice with shoppers

Hybrids may not get as much buzz as EVs, but they're flying off dealership lots.

Take Steve Bond, who was eyeing some of the hybrids on display at the Washington, D.C., auto show last month. "I think it's time to go from straight gasoline for a hybrid model for the better mileage," he said, explaining why he thought a hybrid Toyota Highlander would be his next vehicle.

He's thought about an electric vehicle, but he says the lack of fast chargers for long-distance travel are keeping him from buying one today.

He summed up his view on EVs in four words: "Yes — but not yet."

Over in the Subaru exhibit, Carla Grenell also said she'd be much more likely to buy a hybrid than an EV.

"If I were able to plug in at my home, I would do that," she said. But as a city dweller she wasn't sure if she could charge at home — many people can't.

Michelle Krebs, executive analyst with Cox Automotive, says these concerns are common. And as a result, hybrids are much more popular with shoppers than all-electric vehicles.

"About 11% of all new car shoppers look at EVs," she says. "About 20% of all new car shoppers shop for hybrids."

Actual sales figures are lower, for both types of vehicles, because there aren't enough vehicles to meet demand. But the shopping figures show that a lot of people want hybrids, if they can get their hands on them.

There are now many more hybrid options

Initially, the Prius dominated the hybrid market. But today hybrids have expanded, with options in virtually every segment, as Toyota and other automakers continue to invest in them.

There are still smaller vehicles, like the Prius, which just got a well-received redesign. (Sample review quote: "We can't believe we're writing this in a Prius review, but — the new Prius is ... fun to drive!" ) And it still gets a cool 57 miles to the gallon.

But SUVs outsell small cars across the market, including among hybrids. Toyota's hybrid RAV-4 crossover (41 miles to the gallon) and hybrid Highlander SUV (36 mpg) far outsold the Prius last year.

Kia, Hyundai and Honda all offer hybrid SUVs in a variety of sizes. And Toyota's minivan, the Sienna, only comes as a hybrid (36 mpg).

Then there are pickup trucks. Ford sells a hybrid version of its F-150, and the popular Maverick — a smaller pickup designed to appeal to urban dwellers with a hankering for a truck — comes as a hybrid in its standard configuration. The Maverick gets about 42 miles to the gallon.

And on the sportier side of the spectrum, Corvette just introduced its first hybrid: the E-Ray. The gas mileage isn't impressive. But with an electric motor about the size of a Folgers coffee can tucked between the front wheels, it's the fastest Corvette ever, capable of going from zero to 60 in 2.5 seconds.

But are hybrids good for the planet at this moment?

Right now, there is a debate unfolding across the auto industry — between companies, advocates and government regulators — about whether hybrids are helping or hurting in the current fight against climate change.

To state the obvious, a hybrid vehicle is better for the climate and for human health than a gas guzzler.

But as they look into the future, climate advocates are no longer emphasizing the need to make gas-powered cars more efficient. They want to reduce the number of car trips taken, by investing in public transit and walkability.

And they want all new cars on the road to be zero-emissions vehicles, which right now, means all-electric vehicles, powered by a greener electric grid. Such a transition poses daunting challenges — from building vehicles to sourcing battery minerals.

Garcia, with the Sierra Club, says it's essential to pull off the transition to zero-emissions vehicles as fast as possible. Every year of delay locks in carbon emissions for years to come given the increasingly long lifespan of vehicles on the road.

"Every year that goes by we are going to be extending the pollution from transportation," she says. "The truth is, we should have transitioned 20 years ago."

These advocates are concerned that all the money and engineering know-how invested into hybrids is money and know-how that's not going into pure electric vehicles, slowing down the transition.

Toyota makes the environmental case for hybrids

On the other side of the debate, you have Toyota.

While it revolutionized hybrids, the Japanese automaker has long been vocally skeptical of EVs. As a result, it remains well behind its rivals in the race to bring them to market.

Today, Toyota has a new CEO who has indicated more interest in all-electric vehicles. But the company continues to argue, to anyone who will listen, that hybrids can and should be a central component of fighting climate change.

"I guess we're taking a pragmatic approach to this," says Cooper Ericksen, a senior vice president at Toyota Motor North America responsible for battery electric vehicles.

He points out that hybrids are significantly cheaper than EVs to produce and to buy, and they don't require a charger. A car shopper can buy a hybrid instead of a traditional gas vehicle without having to change their driving behavior at all, making it an easier sell.

But even if all drivers were totally ready to leap to EVs, Toyota argues hybrids would still make sense. Because their batteries are so much smaller, a company can build far, far more hybrids than EVs from the same battery resources.

How will regulators regard hybrids?

The debate over hybrids is not hypothetical. Regulators have to decide if their policies need to focus on switching to EVs as fast as possible, or on encouraging hybrids and their better gas mileage.

And in Europe, California and a number of other states, policymakers have adopted the more radical position, essentially mandating that all new cars be zero-emission by 2035. That may leave room for plug-in hybrids, but would not allow conventional hybrids, which require gasoline to run.

To understand how regulators' views have shifted, consider what's parked in the driveway of Margo Oge, the former director of the EPA's Office of Transportation and Air Quality.

When the Prius first came out, she was thrilled. "I was one of the first buyers," she says. "I was excited because I want to walk the talk."

These days, however, she's driving an all-electric vehicle. And she's calling for her old agency to set "ambitious" national vehicle standards to accelerate the shift away from all vehicles with a gas tank — including the Priuses she used to love.

Hybrids may once have been revolutionary, but Oge says her problem with them is simple: They still run on fossil fuels.