On Thursday, Bernie Sanders and four other senators sent a letter to President Obama asking him to require a full environmental and cultural assessment of the controversial pipeline project, which would carry fracked oil from North Dakota to Illinois.

The letter calls for a halt the pipeline’s construction while the review is carried out, arguing that the project is “a violation of tribal treaty rights” and would “have a significant impact on our climate.” It doesn’t call for an outright rejection of the pipeline, but it does call for a stringent review and approval standards that would make rejection pretty darn likely.

Sanders himself has been calling for a complete rejection of the pipeline for months, starting as far back as January, when he was running for the Democratic nomination — long before pipeline protests made national news. Last month, Sanders gave an anti-pipeline speech to a crowd of protestors.

Meanwhile, Hillary Clinton is not saying what she thinks of Dakota Access. Her campaign chair, John Podesta, gave a vague answer when Grist’s Ben Adler asked him about it during an interview on Wednesday.

Donald Trump, for his part, has invested in the company that’s building the pipeline.