As protesters prepare for nationwide demonstrations against Donald Trump’s environmental agenda, Bernie Sanders on Thursday backed legislation that would phase out the use of fossil fuels in the US by 2050.

“Mr President, start listening to the people and the scientists, not the greed of the fossil fuel industry,” Sanders said at a rally on Capitol Hill, where he was flanked by environmental advocates and two of his Democratic colleagues, Senators Jeff Merkley of Oregon, who authored the bill, and Ed Markey of Massachusetts.

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“As you all know,” the Vermont senator and former presidential candidate told the crowd, “Donald Trump is wrong about a lot of things. But there is no area where he is more wrong than on the issue of climate change.”

Sanders added: “No, Mr President, climate change is not a hoax. Climate change is real.”

At the event, held just outside the US Capitol, Sanders, Merkley and Markey unveiled legislation which seeks to completely phase out the use of fossil fuels through a transition to 100% clean and renewable energy by the middle of this century. But the bill, called the 100 by ’50 Act, is unlikely to be considered by a Republican-controlled Congress, where many GOP lawmakers are still unwilling to acknowledge the science behind global warming and contribution of human activity.

The partisan nature of the debate was not lost on the audience, as one attendee interrupted the proceedings to ask: “Where are the Republicans?”

“We’re going to have to recruit those colleagues across the aisle,” said Merkley, who ranks among the Senate’s most progressive voices. “This is not a red or a blue issue,” he said. “There is no excuse for failure. This is the biggest test facing human civilization and we have to respond and win this battle.”

Demonstrators are set to gather in Washington for the People’s Climate March on Saturday, which will also mark the 100th day of the Trump administration. Similar marches are expected in major cities across the US, as part of a broader effort to oppose Trump’s efforts to roll back the climate initiatives put in place by Barack Obama.

The president has signaled his intentions to tear up the landmark Paris climate agreement, forged by more than 195 nations in late 2015, to reduce greenhouse gas emissions by 26 to 28% over a decade.

Trump’s top aides, who are split on whether or not the US should remain part of the Paris accord, were scheduled to meet on Thursday afternoon to discuss the deal’s future. Trump’s chief of the Environmental Protection Agency, Scott Pruitt, is a vocal proponent of pulling out immediately.

Trump has vowed to withdraw from the Paris agreement, and has already taken steps through executive action to begin the process of overturning Obama-era environmental regulations.

Among them was a directive to the EPA to dismantle the Clean Power Plan, one of Obama’s signature climate policies to reduce carbon pollution from power plants. Trump also signed an executive order that aims to unravel the Obama rule designed to protect American waterways from pollution, and proposed in his budget draconian cuts to the EPA.

At the rally, Markey said Trump’s actions were precisely why hundreds of thousands of people felt compelled to take to the streets.

“Donald Trump [is] still saying that he’s going to issue a new executive order allowing for more oil drilling on the public lands of the United States of America,” he said, drawing jeers for the president from the crowd. “That is just completely and totally unacceptable.”