In an open letter to the 45th President, the Democratic Senator from Massachusetts criticised him for breaking his promise to rid Washington DC of special interest groups and the influence of Wall Street bankers.
She highlighted that his transition team includes a former Goldman Sachs executive, a paid consultant for the telecommunications giant Verizon and a climate change denier backed by the oil industry.
Senator Warren wrote: "Based on public reports, your transition team and your potential cabinet include over twenty Wall Street elites, industry insiders, and lobbyists making decisions that could have huge implications for their clients or employers.
"They include, among many others, a former Goldman Sachs executive who is rumored to be a Treasury Secretary pick...and a climate-change-denying, oil industry-paid think tank fellow who is leading your environmental team's transition."
The close ally of Hillary Clinton reminded Donald Trump of the importance of the transition team on how it will shape the next four years of his presidency.
She said:"You made numerous promises to the American people in your election campaign, none bigger than the promise to ‘drain the swamp' of Washington D.C. special interests rigged against the middle class.
"The decisions you make with your transition team will shape the next four years of this nation.
"They will also reflect the strength of your character and your ability to truly lead - not just follow the marching orders of the special interests and Wall Street bankers you purportedly oppose."
Senator Warren warned the billionaire businessman that if he continued to renege on his election promise to "drain the swamp" - she would oppose him "every step of the way".
She concluded by saying: "Let me be clear. 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."
Mr Trump has been already criticised for the prominence of family members in his transition team and his attempts to get cleared to attend daily intelligence briefings.
His appointment of far-right White Nationalist Steve Bannon as chief White House strategist and senior counsellor has caused serious opposition from both Democratic leadership and the Republican establishment.
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