Michael Cohen, US President Donald Trump's former lawyer, is due to testify against his former boss on February 7 in front of the House Oversight and Government Reform Committee before beginning a three-year prison term for multiple charges.
Cohen is a pivotal figure in investigations by special counsel Robert Mueller into potential co-ordination between Russian Federation and the Trump campaign and by federal prosecutors in NY into campaign finance violations related to payments to buy the silence of a porn actress and a former Playboy Playmate who say they had sex with Trump.
Following his sentencing hearing last month, Cohen's attorney Lanny Davis said he was prepared to "state publicly all he knows about Mr. Trump", including providing answers to "any appropriate Congressional committee interested in the search for truth and the difference between facts and lies". He went on: "I look forward to having the privilege of being afforded a platform with which to give a full and credible account of the events which have transpired".
Given Cohen's prolonged time spent in proximity to Trump, his family and the Trump Organization, some insiders consider his cooperation with authorities to be one of most serious potential legal threats to confront the president.
Federal prosecutors said in their sentencing memo for Cohen that he made the payments "in coordination with and at the direction of" Trump, establishing him as an unindicted co-conspirator in the case.
The announcement of Cohen's testimony is the latest sign that House Democrats plan to make oversight of every aspect of Trump's life a key part of their control of the House.
Cohen once said he would take a bullet for Trump, who called his former close confidante a "Rat" on Twitter in mid-December. Trump has denied that he directed Cohen to make the payments.More news: US official says troop withdrawal from Syria has started
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The Democrats promised to use their new majority in the House of Representatives to delve deeper than ever before into Donald Trump's pre-presidential background.
Democrats won back the House past year promising to hold Trump and his administration to account and have begun laying the groundwork for a long run of investigations targeting his administrator's policies and ethics lapses, Trump's businesses and his campaign's interactions with Russian Federation.
On the subject of the payments, Trump insisted he only found out about them after they were made, despite the release of a 2016 recorded conversation in which Trump and Cohen can be heard discussing a deal to pay McDougal for her story of a 2006 affair.
Elijah Cummings, the committee's chair, said the panel would avoid interfering with Mueller's investigation.
But because he pleaded guilty he was never cross-examined on details of his dealings with Mr Trump.
It's not yet clear whether Cohen will agree to speak to any other committees.
Rep. Adam Schiff, D-Calif., the Intelligence Committee chairman, said in an interview last week that he was in touch with Cohen's lawyer about a possible appearance.