Sen. Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE (D-Mass.) is calling on former Energy Secretary Rick Perry Rick PerryOVERNIGHT ENERGY: Democrats push resolution to battle climate change, sluggish economy and racial injustice | Senators reach compromise on greenhouse gas amendment stalling energy bill | Trump courts Florida voters with offshore drilling moratorium OVERNIGHT ENERGY: Trump signs major conservation bill into law | Senate votes to confirm Energy's No. 2 official | Trump Jr. expresses opposition to Pebble Mine project Senate votes to confirm Energy's No. 2 official MORE to resign from Energy Transfer, a pipeline company he rejoined this month shortly after his departure from the Trump administration.

Warren wrote a letter to Perry dated Thursday calling the decision "unethical" and saying it would "discredit" his time in public office.

"This is exactly the kind of unethical, revolving-door corruption that has made Americans cynical and distrustful of the federal government," the Democratic presidential hopeful wrote.

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"You should rectify your mistake and immediately resign from your position on the Board of Directors," she added. "Doing so would send a strong and necessary message about government ethics and your own integrity."

Warren also wrote to the CEO of Energy Transfer, which is behind the controversial Dakota Access Pipeline, questioning when it or its general partner began discussions with Perry and asking about his compensation package.

“Secretary Perry’s rapid trip through the revolving door between public office and membership on your company’s board will lead to countless questions about whether Energy Transfer is purchasing access to high-level Trump administration officials and whether top Energy Department officials are making decisions that benefit the department’s former head,” the senator wrote to the company.

Energy Transfer told The Hill in a statement that it is "pleased to have Former Secretary Rick Perry as a board member of our general partner."

The company declined to answer questions from The Hill about Perry's appointment to the board of its general partner.

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He was appointed as director of LE GP LLC on Jan. 1, according to a Securities and Exchange Commission filing, which said the former Trump official will be eligible for "cash compensation" for his work.

Perry previously sat on the board of Energy Transfer until the end of 2016, according to ethics paperwork.

He left the Energy Department last month. President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE announced Perry's departure in October amid scrutiny over dealings with Ukraine.