In 2012, Mitt Romney and almost all the other Republican candidates for president raised doubts about the causes and even the existence of global warming, and argued that the government should do nothing to alleviate the problem without further research.

This was strange. Only a few years earlier, Romney, along with Newt Gingrich, John McCain and other prominent Republicans, agreed that human activity did contribute to climate change, and even favored government action to reduce the emissions that cause it.

Why did they deny what they once acknowledged? The scientific consensus around human-induced climate change has grown stronger through the years, so it could not have been new scientific knowledge.

No, they changed their position for purely political reasons: if they did not waffle on that critical issue, they would not win the hard-line Republican base or, perhaps more importantly, the corporate donors they needed to win the Republican nomination.

Now, four years later, one sees a similar dynamic.

Most Republican presidential candidates continue to favor obfuscation and delay. The largest Republican donors, particularly the Koch Brothers, continue to make waffling a prerequisite for financial support.

Meanwhile, many retired Republicans, who no longer need such contributions, have come out strongly for more government action, including George Schultz, Secretrary of the Treasury under Richard Nixon and Secretary of State under Ronald Reagan; Paul O’Neill, Secretary of the Treasury under George W. Bush; Greg Mankiw, key economic adviser for Mitt Romney’s campaign; and even Mitt Romney himself.

In this new environment, candidates waffling on climate change may win the Republican nomination but lose the general election. People no longer see climate change as a future threat but as a contemporary problem. A Gallup Poll from March 2015 reported that 55 percent of Americans believe climate change is already happening.

The costs of natural disasters on U.S. communities have risen dramatically in the past few decades, despite persistent efforts to improve infrastructure to weather such storms.

Of course, no one can tell precisely how much climate change contributes to the devastating droughts in California and the Texas panhandle, the increased flooding in the South and Midwest (and elsewhere), or the severity of such superstorms as Hurricanes Sandy and Katrina. But those events have corresponded closely with the predictions of climate change models for years.

Even if human activity does contribute to climate change, the naysayers object, the costs of cutting emissions are just too great. Besides, they continue, if the U.S. cuts emissions while China and India do not, then the U.S. will still suffer the effects of climate change, but will lose its competitive edge to adapt to it.

Those arguments no longer hold water. A number of recent studies, including one by the International Monetary Fund, have found that the costs of reducing greenhouse gas emissions are far lower than the costs of doing nothing at all. Germany, despite its aggressive environmental policies, remains one of the strongest economies in Europe.

Meanwhile, China has been investing heavily in nuclear, wind and solar power in an effort to reach peak emission levels by 2030.

Given China’s interest, global demand for renewable energy technology is likely to surge in the next decades. If U.S. policy continues to subsidize carbon rather than alternative fuels, it will be unprepared for the shift in markets and suffer in economic competitiveness.

And with the United States, China and the European Union all seeking to diminish their dependence on fossil fuels, all other countries in the world will feel pressure to join in or be left behind.

Finally, many conservatives, moderates and liberals agree on a policy that promises to reduce emissions while sustaining growth: “putting a price” on carbon that asks fossil fuel producers to bear part of the costs that global warming is inflicting on the planet.

For example, a revenue-neutral carbon tax would place a fee on the carbon content of fossil fuels and then return the revenues to the population at large. Such a tax would require few regulations and few administrative costs, but would encourage businesses to economize on energy use and search actively for alternative sources. Even major oil companies such as Royal Dutch Shell and BP have come out in favor of limiting greenhouse gas emissions.

Thus, all the pieces are there for a new direction in climate policy. All that is needed is leadership to put them together.

Maine has a long tradition of sending leaders to the U.S. Senate from both parties. Today, Maine is fortunate to have two highly effective Senators who not only acknowledge the human contributions to climate change but have actively supported public action to reduce greenhouse gas emissions.

Because neither are Democrats, they are both well-placed in this highly partisan environment to break the logjam.

Now is the time, and this is the issue, to make history.

James Richter is a professor of politics at Bates College in Lewiston.