The event just as easily could ​have been canceled. It was planned back​ in October, when the organizers weren’t expecting a Trump election victory. Yet here they were last week​, five economists and an attorney, talking about the idea of a nationwide tax on carbon emissions.

Lots of people showed up. And they were conservatives, not liberal environmentalists. This panel discussion filled a large room at the conservative American Enterprise Institute in Washington, even though the Trump administration so far shows no interest in carbon taxes.

Participants poked a bit of fun at themselves – referring to their discussion as a "faculty club" chat.

Yet the event showcased an important reality: Even as climate change is essentially absent from the priority list of President Trump and the Republican-led Congress, a meaningful number of conservatives in America support the idea of reducing heat-trapping gasses in the atmosphere.

“You’re not going to convince 100 percent of the world,” Aparna Mathur, an American Enterprise Institute economist who was part of the panel, said after the event. But she sees a shrinking number of conservatives who oppose a carbon policy.

By some measures, she may be right. In March 2016, a Gallup survey found 40 percent of Republicans saying they worry a “great deal” or “fair amount” about the issue. That was up from 31 percent in a poll the previous year.

Even so, of course, support for climate action is hardly an influential viewpoint in GOP politics.

For all the inroads some libertarians and moderate Republicans thought they’d made with their carbon tax crusade, Trump’s ascendancy in last year’s election campaign suggests how little climate change matters to the bulk of conservative voters. He dismissed the issue, championed fossil fuels, and ran into no primary-election barriers.

It looks like a long road ahead to remake the GOP into something more environmentally friendly.

But Ms. Mathur and some other conservative climate advocates see some wind at their backs.

Many cities and states are moving on climate, even if Washington isn’t. Businesses, which compete in a global economy, are doing it as well. And Millennials, who believe in climate change more than older generations do, are a rising influence in both major parties. And they point to polls finding Republican support for clean energy and at least some government climate action.

A conservative think tank hosting a debate on a carbon tax underscores the interest the idea has already generated on the political right. Taxing carbon is potentially both effective and bipartisan – a conservative approach in the sense that it leans on marketplace incentives to reducing the burning of fossil fuels.

In fact, some conservatives even hold out hope a carbon tax could be implemented under Trump, wiggling its way into a broad overhaul of the federal tax code. Economists like Mathur, along with Bob Inglis, a former South Carolina Republican congressman who now pushes for a revenue-neutral carbon tax, have proposed using the revenues to cut the corporate income tax rate. A portion of revenues could be used to offset energy price increases for low-income households most affected by a carbon tax.

“I believe there are a lot of Republicans who actually might be in support of this policy, they are just hesitant to come out and talk about it because at the end of the day we’re talking about a new tax,” Mathur says.

That’s the catch. Trump appears loath to adopt anything that would be portrayed as a tax on business.

More broadly, many Republicans fear the idea of a new revenue beast that Democrats might ultimately use in ways Republicans never envisioned.

“If you give them a tool they can abuse of their own gain, we have to expect they will abuse it for their own gain,” said David Kreutzer, an economist who is now advising the Environmental Protection Agency, said at the event.

The Republicans now in charge of Washington appear keen on unraveling carbon-reduction policies, not creating them. At the event, Mr. Kreutzer and Benjamin Zycher, an economist at the American Enterprise Institute, took repeated jabs at the so-called “social cost of carbon.” That’s a formula to assign monetary value to health and other damages from carbon emissions.

Conservatives argue the Obama administration used an inflated figure. Changing that value could undercut the case for carbon regulations – or diminish the size of a carbon tax.

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Still, carbon tax evangelists say the GOP Congress will need something to replace regulations they’re aiming to scrap, such as an Obama administration rule limiting electricity sector emissions. Republicans might find themselves in a place similar to what they’re facing with Obamacare – repeal in search of replacement, says Joseph Majkut, climate science director at the libertarian Niskanen Center, which supports a carbon tax.

For years Republicans simply opposed President Obama, Mr. Majkut says. “Now they’re faced with the question: ‘What are you going to do about greenhouse gas reductions?’ ”