US President Donald Trump has been accused of sounding like a mafia kingpin after he called his convicted former lawyer a "rat".
Democratic Senator Chris Coons told CNN on Monday that Mr Trump sounded "more like a mob boss than president of the United States".
Mr Trump had said a day earlier that Michael Cohen "became a 'Rat'", and he accused the FBI of illegal actions.
Cohen was jailed for campaign finance violations during the 2016 election.
He also admitted lying to Congress and tax evasion.
Former FBI Director James Comey was quick to defend the agency.
Mr Trump on Sunday tweeted that his former right-hand man Cohen only turned on him after "unthinkable" actions by the FBI.
"They BROKE INTO AN ATTORNEY'S OFFICE!" Mr Trump tweeted, questioning why the agency did not also "break into" the Democratic National Committee or Hillary Clinton's offices.
Speaking on CNN on Monday, Mr Coons said the notion that agents broke into the Trump attorney's office "runs right up against the foundation of our law".
"They were executing a warrant issued with the approval of a judge," he said.
"His use of the term 'rat' for Michael Cohen and mischaracterising this as a break-in to his attorney's office frankly makes him sound more like a mob boss than president of the United States."
The president has not responded to the Delaware senator's remarks.
The FBI had raided Cohen's office in April for confidential documents on a "referral" from Special Counsel Robert Mueller.
Cohen was sentenced last week to three years in prison for campaign finance violations, lying to Congress and tax evasion - and blamed some of his actions on Mr Trump's bad influence.
Mr Trump has accused the FBI of partisanship before, and has been in a vocal feud with its former director, Mr Comey, since he sacked him last year.
Following Mr Trump's online rebuke of the law enforcement agency, Mr Comey took to Twitter on Sunday to dispute the president's characterisation of FBI actions.
"This is from the President of our country, lying about the lawful execution of a search warrant issued by a federal judge," he said on Twitter. "Shame on Republicans who don't speak up at this moment."
Speaking to reporters on Capitol Hill on Monday, Mr Comey once again defended the FBI, saying Mr Trump's attacks on the agency and "rule of law" were "a tragedy".
"The president of the United States is undermining the rule of law calling someone who has co-operated 'a rat'," Mr Comey said.
Media captionEx-FBI director James Comey said Trump's rat insult "undermines the rule of law"
"This is not about Republicans and Democrats, this is about what does it mean to be an American," he emphasised. "Somebody has to stand up and speak for the FBI, the rule of law."
Former chief assistant US attorney Andrew McCarthy, a Fox News contributor, suggested on Twitter to Mr Trump that he should avoid using the term "rat".
It is not the first time Mr Trump has been accused of sounding like a "mob boss".
Earlier this summer, opponents lashed out at the president when he praised his now convicted ex-campaign chair Paul Manafort for refusing "to break" and "make up stories in order to get a deal".