President Donald Trump suggested Wednesday his former campaign chairman Paul Manafort — on trial on federal bank fraud and tax charges — is being treated "worse" than "Public Enemy Number One" Al Capone.

Trump's comparison of the treatment of Manafort and the notorious gangster, part of tweet storm attacking special counsel prosecutor Robert Mueller, is a sharp departure from longstanding practice by presidents of not weighing in on a criminal case because of the risk of swaying a jury or causing a mistrial.

"It's inappropriate ... for the president of the United States to be making comments like this on Twitter on an ongoing trial," said Richard Serafini, a former federal prosecutor now in private defense practice in Florida. "It's just another violation of a norm, a longstanding norm," Serafini said. "I mean, it's just not done."

In that same online diatribe, Trump questioned why the "government" hadn't told him when he hired Manafort in 2016 to run his campaign that the longtime Republican strategist was under investigation.

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Manafort was sent to jail in June, when a judge revoked his bond after allegations he tried to tamper with witnesses. He's been held in solitary confinement since then.

Mueller is investigating possible collusion by Trump campaign officials with Russians trying to interfere with the 2016 election. But the charges against Manafort, to which he has pleaded not guilty, are unrelated to that issue. Instead he is charged with crimes related to income earned for consulting work performed in Ukraine from 2005 to 2014.