Massachusetts Sen. Elizabeth Warren sent a letter to President-elect Donald Trump Tuesday accusing him of breaking his campaign promise to "drain the swamp" in Washington, D.C. by staffing his transition team with lobbyists, Wall Street bankers and industry insiders.

"The American people are watching to see if you were sincere in your campaign promises to look out for the interests of working families, rather than the interests of the rich and powerful," she wrote. "Now it is time to live up to those promises. Unfortunately, you already appear to be failing."

Warren - who has waged a bitter war of words with Trump, calling him a "pathetic coward" and worse on Twitter - called on the president-elect to remove "the lobbyists and special interest insiders" from his transition team.

"Let me be clear," she said. "Should you refuse, I will oppose you, every step of the way, for the next four years. I will champion the millions of Americans you will fail to protect. I will track your every move, and I will remind Americans, every day, of the actions you take that fail them. And I will not be the only one watching. The millions of Americans who voted for you - and the millions who didn't - will all be watching you."

The letter goes on to detail the special interest ties of more than 20 members of Trump's transition team, including veteran agribusiness lobbyist Michael Torrey, energy industry lobbyist Mike McKenna and David Malpass, who was Bear Stearns' chief economist in the years before the firm's 2008 collapse.

Warren's letter comes less than a week after she pledged to put aside her differences with Trump and work together to rebuild the economy.