Mike Lindell, the MyPillow C.E.O. and a Trump conspiracy advocate, is barred from Twitter.

Twitter has permanently suspended MyPillow chief executive Mike Lindell — one of President Trump’s most conspicuous remaining public defenders — for peddling debunked conspiracy theories about voter fraud in the 2020 elections.

Mr. Lindell’s Twitter account, which had nearly 413,000 followers, was permanently suspended “due to repeated violations of our Civic Integrity Policy,” Lauren Alexander, a Twitter spokeswoman, said in an email.

Mr. Trump’s own account was permanently closed earlier this month for much the same reason — setting off a chain of high-profile bans imposed amid concerns that Mr. Trump and his supporters would use the platform to incite more violence, like the storming of the Capitol earlier this month.

After the Capitol attack, Twitter said it had updated its rules to more aggressively police false or misleading information about the presidential election. As part of that move, Twitter has moved to suspend the accounts of more than 70,000 people who have promoted content related to QAnon, a fringe pro-Trump group that the F.B.I. has labeled a domestic terrorist threat.

Many of Mr. Trump’s most erstwhile defenders backed away from him in the days following the Jan. 6 riot at the Capitol, which was stoked by the former president’s fiery and false speech to supporters claiming massive voter fraud. Not Mr. Lindell.

A few days after the riot, he visited Mr. Trump in the White House — where photographers captured images of his notes, which while only partly visible seemed to suggest the president impose “martial law if necessary” to remain in office.

Dominion Voting Systems, the target of his unsubstantiated claims of massive, intentional voter fraud, threatened to sue Mr. Lindell last week, describing him as a leader of a “misinformation campaign” that has resulted in significant business losses and threats of violence against Dominion employees.

On Monday, Dominion Voting Systems filed a $1.3 billion defamation lawsuit against Trump’s personal attorney, Rudy Giuliani, claiming he had personally profited by using his attacks against the company to promote commercial sponsorships.

Mr. Lindell, 59, filed a lawsuit of his own on Monday, suing the British tabloid The Daily Mail, over a recent report that he was having an extramarital affair, seeking $75,000 in damages.

His high-profile defense of Mr. Trump has earned him a devoted following on the right, but might have significant implications for his business. Bed Bath & Beyond and several other chains have said they plan to stop selling MyPillow products.