Retired Supreme Court Justice John Paul Stevens said this week that he is concerned President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE is "exercising powers that do not really belong to him."

Stevens, 99, who retired in 2010, told The Wall Street Journal there are "things we should all be concerned about."

When pressed for specifics, Stevens said he thinks Trump is exercising powers that are not within a president’s authority.

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"I mean, he has to comply with subpoenas and things like that," Stevens, an appointee of former Republican President Ford, told the outlet.

Stevens's comments came after Trump vowed last month to fight "all the subpoenas" issued by House Democrats as they seek to investigate his campaign, business and administration.

Trump has sought a preliminary injunction to stop Deutsche Bank and Capital One from handing over financial records pertaining to him, his family and his private businesses that were requested in subpoenas issued by House Financial Services Committee Chairwoman Maxine Waters Maxine Moore WatersPowell, Mnuchin stress limits of current emergency lending programs Pelosi: House will stay in session until agreement is reached on coronavirus relief Omar invokes father's death from coronavirus in reaction to Woodward book MORE (D-Calif.) and House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffPelosi, Democrats unveil bills to rein in alleged White House abuses of power Chris Matthews ripped for complimenting Trump's 'true presidential behavior' on Ginsburg Trump casts doubt on Ginsburg statement, wonders if it was written by Schiff, Pelosi or Schumer MORE (D-Calif.).

Attorneys for the Democratic-controlled House Financial Services and Intelligence committees, meanwhile, have accused the president of attempting to stop lawmakers from carrying out legitimate investigations.

House Democrats in court documents said they "are investigating serious and urgent questions concerning the safety of banking practices, money laundering in the financial sector, foreign influence in the U.S. political process, and the threat of foreign financial leverage, including over the President, his family, and his business."

Trump is also suing to stop a congressional subpoena issued by House Oversight and Reform Committee Chairman Elijah Cummings Elijah Eugene CummingsBlack GOP candidate accuses Behar of wearing black face in heated interview Overnight Health Care: US won't join global coronavirus vaccine initiative | Federal panel lays out initial priorities for COVID-19 vaccine distribution | NIH panel: 'Insufficient data' to show treatment touted by Trump works House Oversight Democrats to subpoena AbbVie in drug pricing probe MORE (D-Md.) for financial records from the accounting firm Mazars.

Stevens was appointed to the high court in 1975 and was confirmed 98-0 by a Democratic-led Senate to succeed a liberal justice. He retired from the bench as the head of the liberal minority, the Journal noted.

His new book, "The Making of a Justice: Reflections on My First 94 Years," will be released May 14.