As labor advocates hail 2021 as the “year of the worker,” Sen. Bernie Sanders (I-Vermont) is encouraging workers to maintain the momentum by using their collective leverage to organize.

On Wednesday, Sanders issued a warning to corporations and executives on Twitter. “For the ruling class, greed is a religion. They will cut back on workers’ wages, on workers’ benefits, on workers’ safety on the job — all for the sake of having more, more, more,” he wrote. “I’ve got news for the ruling class: you cannot have it all. The working class is fighting back.”

Sanders’s statements come at the end of a consequential year for the labor movement. Over 100,000 workers authorized strikes this year, with workers at John Deere, Kellogg, and more – along with health care workers across the country – securing wins in contract negotiations with employers. This past year also saw workers organizing in solidarity against fierce anti-union retaliation from companies like Amazon and Starbucks, which have employed illegal union-busting tactics in response to workers’ demands.

Sanders has repeatedly celebrated this year’s major labor moves – and on Thursday, he once again encouraged employees to organize their workplaces. “It’s true that corporate profits have never been higher. And still, they want more,” he said. “But there is another more important truth: there are a lot more of us than there are of them. Now is the time to organize. Now is the time to end their greed.”

As the labor movement underwent a roaring resurgence in 2021, Sanders rallied with striking workers; in November, he sent pizza and a donation to striking Kellogg workers and joined them in Battle Creek, Michigan during the 11th week of their strike. He also praised Starbucks workers in Buffalo when they formed the company’s first-ever union, and slammed John Deere when it took drastic action to break its 10,000-worker strike.

This week, Sanders sent a letter to billionaire Warren Buffett asking him to compel executives to offer a fair contract to workers in a manufacturing plant owned by Buffett’s company, Berkshire Hathaway. About 450 metal manufacturing workers for Precision Castparts in Huntington, West Virginia have been on strike for nearly three months, since October 1.

The workers, who make specialty parts used in aircrafts and other equipment, say that they’re pushing for better working conditions, health care and wages – but that the length of the strike has been taking its toll.

The union expects to return to the bargaining table next week. Chad Thompson, president of the United Steelworkers Local 40 union, told The Associated Press that the company hasn’t been negotiating in good faith, and that the union would be open to agreeing to a fair contract. But “we don’t think they’re even being in the ballpark of fair,” Thompson said.

Sanders condemned the company for treating its workers poorly. “At a time when this company and Berkshire Hathaway are both doing very well, there is no reason why workers employed by you should be worrying about whether they will be able to feed their children or have health care,” Sanders wrote to Buffett. The senator then asked the billionaire to intervene in negotiations and to present an offer to workers that is better than the ones they’ve previously rejected.

“There is no reason why the standard of living of these hard working Americans should decline. I know that you and Berkshire Hathaway can do better than that,” Sanders continued.

Buffett rejected Sanders’s request. “Our companies deal individually with their own labor and personnel,” he wrote back.

According to Forbes, Buffett is the eighth richest person in the world, with $109.3 billion in wealth. Like other billionaires in the U.S., his fortune has only grown since the pandemic began; per Bloomberg, Buffett’s wealth has increased by $21.4 billion this year alone.