As climate-change activists wait for the State Department's environmental analysis on the Keystone XL pipeline, it's worth considering what the movement has gained from this debate -- and what it risks losing.

For environmentalists, Keystone has been a godsend: a cause to rally the troops, a poster child for corporate indifference to the planet and a warning that environmentalists still have the political strength to delay, and maybe scuttle, the most cherished plans of the energy industry.

But what's been the cost of those gains? Even if the movement succeeds at defeating Keystone -- in fact, especially if it succeeds -- environmentalists may have undermined their broader goals, by further alienating the people they need most: moderate Republican voters.

That's because whatever happens to Keystone, the U.S. won't make a meaningful dent in its carbon emissions without putting a price on carbon. The Environmental Protection Agency's proposed limits on emissions from coal-fired power plants are necessary, but far from sufficient. A carbon tax, in turn, requires congressional action.

And congressional action, in turn, requires one of two things: a Democratic comeback in the House that gerrymandered districts render unlikely; or a greater willingness among House Republicans to entertain a carbon tax.

Wait: Isn't Republican support for environmental protection oxymoronic? It wasn't always that way. In 1992, when the Pew Research Center asked people whether there needed to be stricter laws and regulations to protect the environment, 93 percent of Democrats said yes -- and so did 86 percent of Republicans. When Pew asked the same question last year, Democrats' support was still at 93 percent; Republicans' had fallen to 47 percent. Interestingly, that drop predated Barack Obama becoming president.

Republicans' changing views on environmental protection initially look like one more example of the party's long-term shift toward strict conservatism. But that's not the whole story. As Pew wrote, "Views on the importance of environmental protection have arguably been the most pointed area of polarization" over the past 20 years.

I asked two former Republican lawmakers, both moderates who were in office during that shift, what might account for it. Jim Kolbe, an Arizona congressman who retired in 2007, said Republican support for environmental protections has fallen as the cost of regulation increased. Bob Inglis, a congressman from South Carolina who lost his seat to a primary challenger in 2010, cited Republicans' increasing antagonism toward more government.

Kolbe also pointed to something else that environmentalists should be concerned about: the view that moral browbeating has left more Republicans willing to ignore the climate-change movement. "Aren't we terrible stewards of the earth," is how he described the tone of the debate. "For some Republicans, they get tired of it."

Jeremy Carl, a climate-change expert at Stanford University's conservative-leaning Hoover Institution, made a similar point, arguing that "the moral climate around this issue has really poisoned it."

It's hard to know how seriously to take arguments like this. If you oppose doing anything to safeguard the planet against potentially devastating disruptions in weather patterns, you deserve to be called to account for it, even if that means your feelings get hurt. But if the goal is getting some Republicans back into the fold, then maybe there's reason to take a different approach.

Which brings us back to Keystone. The pipeline debate raises the question of just how useful the environmentalists' intransigence, which has been crucial to their opposition, can be as a long-term strategy. There are no compromises offered, and no consideration for the validity of any of the arguments made by Keystone's proponents. The concerns of people who stand to get work by building the pipeline aren't just outweighed; they're irrelevant. You're either for the pipeline or against it.

Case in point: When reports surfaced a few months ago that Canadian Prime Minister Stephen Harper offered unspecified offsetting carbon emissions in return for a U.S. decision to approve Keystone, opponents of the pipeline didn't exactly embrace the possibility. Yet if the goal is reducing emissions, that would have been a good deal. If the goal is just killing a pipeline, it wouldn't. But what good is that goal all by itself?

There are short-term political ramifications to these decisions. If President Barack Obama rejects Keystone, does it get the environmental movement closer to a carbon tax? Or does it just make Republicans more resentful, and give the next Republican president more cover to disregard their demands?

Maybe environmentalists are willing to take that risk, writing off Republican support in the expectation that Democrats will hold government. (Kolbe, when asked how climate-change activists should pursue their agenda, answered this way: "Elect Democrats to Congress.") But that isn't a strategy; it's a gamble. And if a gamble seems like the wrong way to save the world, maybe environmentalists should make their next campaign something more meaningful and even more challenging: winning over more Republicans.

(Christopher Flavelle is a member of Bloomberg View's editorial board. Follow him on Twitter.)