The political war over climate science is flaring up again on Capitol Hill this week as the U.S. Senate debates a bill that would approve the highly controversial Keystone XL pipeline. Pipeline opponents, many of whom oppose the project because they say it would accelerate climate change, are trying to secure votes on a number of largely symbolic amendments affirming the Senate’s belief that climate change is real and human-caused and that policymakers should address it.

The pipeline opponents, including a number of Democratic senators and one independent, hope that their push will put Republicans in a politically perilous position: Either they block votes on the climate amendments and get accused of dodging the issue, or they allow votes and have to take a position on a sensitive issue with a core Republican voting bloc—self-described conservatives who don’t believe human activity is seriously affecting Earth’s climate system. The Keystone debate, which is expected to last several weeks, comes shortly after government researchers concluded that 2014 was the planet’s hottest year on record.

Senators Bernie Sanders (I–VT), Brian Schatz (D–HI), Jeff Merkley (D–OR), Tim Kaine (D–VA), and Sheldon Whitehouse (D–RI) all have filed climate-related amendments. And Democrats aren’t hiding the reasoning behind their strategy. “We're going to have a vote to find out who the climate change deniers in the U.S. Senate really are,” said Senator Charles Schumer (D–NY), the third-ranking Senate Democrat, during a Capitol Hill press conference today. “Do they deny that human activity has helped create climate change? Stay tuned. We'll see.”

Schumer noted that Senate Majority Leader Mitch McConnell (R–KY) had pledged an “open” process to let both parties offer amendments. “We intend to hold him to that promise,” Schumer said.

With floor debate on some nonclimate amendments kicking off today, however, it’s still unclear which of the amendments, if any, will get votes—much less whether they can pass. McConnell said last week that he wouldn’t block a vote on one of the climate amendments.

The amendments, if adopted, would alter S.1, a mostly Republican-sponsored bill that would approve a key permit for building a segment of Keystone XL, which would bring crude oil to the United States from Canada’s oil sands region. Whereas Republicans overwhelmingly back the permit’s approval, most Democrats oppose it; some Democrats oppose the pipeline outright, while others say Congress should leave the decision to the executive branch, which has been pondering the issue for several years amid legal and political wrangling over the pipeline’s route. The White House has threatened to veto a version of S.1 that has already passed the House of Representatives, but Republicans have suggested that they might try to attach the Senate bill to must-pass legislation, such as a spending bill, that President Barack Obama might be reluctant to veto.

Many Democrats worry that the pipeline’s construction would significantly worsen global warming by encouraging Canadian oil companies to expand mining in the oil sands, an energy- and water-intensive process that produces more carbon emissions than conventional oil drilling. Sanders, an independent who is allied with Democrats, argued last week that the Senate should be cutting greenhouse gas emissions, boosting energy efficiency and renewable energy use, or investing in more research and development or rail transportation. “This bill does none of that,” he said on the Senate floor on 13 January. “In fact, what the Keystone pipeline does is move us in exactly the wrong direction.”

Whether Keystone XL would actually significantly speed up climate change depends on how much it promotes further development of the oil sands, studies suggest. (The scientific community has been divided on the issue, as Jeff Tollefson reported in Nature in 2013.) An environmental assessment conducted by the U.S. State Department found that the pipeline probably wouldn’t have a major impact. Environmental groups such as the Natural Resources Defense Council dispute that analysis, arguing that, without Keystone XL, falling oil prices could make it uneconomical for oil companies to keep mining the oil sands.

The amendments

The proposed climate amendments—just a few of at least 50 that Senators would like to get votes—address a number of issues.

Sanders’s amendment would declare the “sense of Congress” that lawmakers are “in agreement with the opinion of virtually the entire worldwide scientific community” that climate change is real, that it’s human-caused, that it’s already causing problems domestically and globally, that the United States needs to transition to cleaner forms of energy, and that time is running out before the world suffers “irreparable harm.”

One of Schatz’s three amendments asks whether it’s “the sense of the Senate that climate change—(1) is real; (2) is caused by humans; (3) is urgent; and (4) is solvable.” A second asks whether the Senate agrees that global temperatures have risen, that the rise is already affecting the climate, that the United States should shift to cleaner energy, and that the United States should cut greenhouse gas emissions and encourage other countries to follow suit. His third amendment and Kaine’s amendment make similar statements to the other two.

And an amendment offered by Merkley asks lawmakers to endorse statements made by a variety of scientific organizations and government agencies saying that climate change is real and that humans are driving it. The groups include the National Academy of Sciences, AAAS (publisher of ScienceInsider), the American Geophysical Union, the American Meteorological Society, the National Oceanic and Atmospheric Administration, and NASA.

Whitehouse’s amendment takes a veiled jab at Senator James Inhofe (R–OK), the new chair of the Senate Environment and Public Works Committee, who has called climate change a “hoax.” The amendment states that it’s “the sense of the Senate that climate change is real and not a hoax.”

If votes on any of the amendments were to occur, it wouldn’t be the first time that members of Congress have had to take a stance on climate science. Early in 2014, a House committee rejected, on a party-line vote, a similar amendment offered by Representative Jan Schakowsky (D–IL). In 2011, former Representative Henry Waxman (D–CA) tried to attach a climate science amendment to a GOP-sponsored bill that would have struck down the Environmental Protection Agency’s power to regulate greenhouse gas emissions. The amendment failed on a mostly party-line 240 to 184 vote.

With the 2016 presidential race looming and several Republican senators mulling presidential runs—including Ted Cruz (R–TX), Marco Rubio (R–FL), and Rand Paul (R–KY)—any climate vote could carry additional political importance. Democrats and their environmental allies are sure to use a “no” vote against any Republican senator who becomes the GOP nominee. But a “yes” vote could be perilous for candidates running in Republican candidate-selection primaries and caucuses, which often hinge on winning over tea party and conservative Republicans. These voters, according to polling data, are more likely to deny climate change than their liberal or moderate GOP counterparts.