Vermont Sen. Bernie Sanders called on President Barack Obama to intervene in the construction of the Dakota Access pipeline on Friday. In a letter, Sanders implored the president to reevaluate the environmental impact of the oil pipeline, which upon completion would transport hundreds and thousands of barrels of crude oil across four states.

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The letter comes a day after at least 140 protestors were arrested at a construction site near a Native American reservation in North Dakota. Activists and members of the Standing Rock Sioux tribe have worked to disrupt construction, arguing that the pipeline would desecrate sacred burial sites and threaten to contaminate water supplies in the event of a gas leak. Sanders said the pipeline would represent “a huge blow to our fight against climate change.”

This one line from Sanders' letter sums up the environmental concerns around the oil pipeline.

"If we have any hope of avoiding the worst consequences of climate change, we should not be building new oil pipelines that lock us into burning fossil fuels for generations to come. Rather, we should be building clean energy infrastructure to transform our energy system away from climate change causing fossil fuels and toward renewable energy sources."

While Sanders recognized the potential environmental consequences of the pipeline itself — citing one analysis that found that oil pumped through the pipeline would "produce carbon emissions equivalent to 21 million cars or 30 coal plants" — the line above offers a different perspective. It represents a broader, more long-term environmentalist argument against the project. Sanders accused the federal government of "putting the projects of the oil industry ahead of the treaty and sovereign rights of Native American communities."

If the pipeline ruptures — and that happens more often than you might think, according to Vox — then there would be a significant risk of agricultural damage and water contamination. (The pipeline would be constructed, in part, under the Missouri River, for example.) Opponents wrote that the pipeline "would fuel climate change, cause untold damage to the environment, and significantly disturb sacred lands and the way of life for Native Americans in the upper Midwest," in a petition that has received nearly 300,000 signatures.

Supporters of the project argue that transporting oil through a pipeline, rather than by train, is a safer alternative. Vox reported that incidents of trains derailing and spilling oil are more numerous than pipeline accidents.

Though the U.S. Army Corps of Engineers, the federal department constructing the pipeline, concluded in their environmental assessment last year that the pipeline "avoided impacts to tribally significant sites," other federal agencies (including the Environmental Protection Agency and the U.S. Department of the Interior) have raised questions about the project's proximity to water sources and tribal land, Mother Jones reported.

“Mr. President, you took a bold and principled stand against the Keystone pipeline,” Sanders wrote. “I ask you to take a similar stand against the Dakota Access Pipeline."