Nadler made the claim during a fiery interview on Wednesday night.
“This is a lawless administration. It is denying the American people the information they need by defying all subpoenas,” he told MSNBC’s Rachel Maddow, after his committee voted to hold the attorney general in contempt for defying a subpoena for Special Counsel Robert Mueller’s unredacted Russia report and underlying documents.
“It's the first administration you've ever seen where they say we'll deny all subpoenas from Congress whether it's on the mole investigation or on security clearances or in anything else they defy the law.
“The law very clearly says that upon request by the ways and means committee that the IRS should turn over anybody's taxes. They're simply ignoring that. So they're ignoring the law and they're stonewalling and hoping that they can get away with it we cannot have a situation where the president becomes a king or a dictator.”
He continued: “The American people understand that no. Nobody. Not the president, nobody may be above the law. And we have to enforce that.”
The explosive assessment came after Nadler earlier told reporters the country has reached a “constitutional crisis”.
"We've talked for a long time about approaching a constitutional crisis. We are now in it," Nadler told the press on Wednesday. Although he pushed back on impeachment as an option, he indicated that the United States was at a critical time of testing whether it could stay a republic or transition into a tyrannical form government.
Nadler's committee voted along partisan lines to hold Barr in contempt on Wednesday and angered the White House by not delaying the vote. At the same time, the president invoked executive privilege — refusing to comply with congressional subpoenas.
Nadler also compared Trump to former President Richard Nixon in that both refused to release information by citing executive privilege.
Nadler recalled the Supreme Court decision forcing Nixon to hand over tapes of his private conversations with advisers, which he said were the "most sensitive to executive privilege."
"The Supreme Court ruled eight to nothing that the interests of the public in justice and in accountability outweighed the interest of the president in privacy," he said.
The White House, in asserting executive privilege, accused Nadler of engaging in a "blatant abuse of power."
“Unfortunately, rather than allowing negotiations to continue, you scheduled an unnecessary contempt vote, which you refused to postpone to allow additional time for compromise,” Assistant Attorney General Stephen Boyd wrote to the committee.