After more than a decade as the keeper of Donald Trump's secrets, Michael Cohen has been spilling the beans about the president's private business deals, foreign interests and alleged mistresses to federal prosecutors in Washington and New York.

But apart from brief comments in the courtroom where he pleaded guilty to several crimes and received a three-year prison sentence, the president's former lawyer and fixer has not spoken publicly about what he now calls Trump's "dirty deeds."

That is likely to change Wednesday when Cohen testifies before the House Oversight Committee in a hearing that could be the most damaging for a president since former White House Counsel John Dean helped bring down Richard Nixon during the Watergate scandal.

"He has the potential to be a very devastating witness," said Dean, 80, who completed his own transformation from criminal presidential aide to star witness more than four decades ago.

Cohen was at Trump's side or working as a senior executive in the Trump Organization during a period now under intense scrutiny by both prosecutors and congressional investigators. Lawmakers are planning to ask him about Trump's campaign finances, still-secret tax returns and now-closed charitable foundation, among other issues.

Rep. Elijah Cummings (D-Md.), chair of the House Oversight Committee, told reporters that the hearing "may very well be a turning point in our country's history."