In a series of late-night tweets, Trump labelled Cohen a "liar & fraudster," but also slammed the Democrat-led House Oversight Committee for ordering him to testify at the same time as Trump's summit with Kim in Vietnam.
Trump ex-lawyer Cohen again in Congress next week: US lawmaker
The summit ended abruptly and ahead of schedule on Thursday, without agreement, despite hopes both sides might make progress on North Korea's denuclearisation and the easing of sanctions on Pyongyang.
"Sometimes you have to walk and this was just one of those times," Trump said at the time.
On Sunday, he said the hearing may have contributed to the breakdown.
"For the Democrats to interview in open hearings a convicted liar & fraudster, at the same time as the very important Nuclear Summit with North Korea, is perhaps a new low in American politics and may have contributed to the "walk,'" he tweeted.
North Korea has no economic future if it has nuclear weapons: Trump
"Never done when a president is overseas. Shame!"
In his testimony, Cohen - who is to report to prison on May 6 to begin serving a three-year sentence for fraud, tax evasion, illegal campaign contributions and lying to Congress - expressed regret for his years of devoted service to Trump.
"He is a racist. He is a conman. He is a cheat," Cohen said of his former boss.
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