Harvard Law professor Alan Dershowitz on Friday said former Trump campaign chairman Paul ManafortPaul John ManafortDem warns Trump: 'Obstruction of justice' to fire Rosenstein Ex-White House official revises statement to Mueller after Flynn guilty plea: report Former White House lawyer sought to pay Manafort, Gates legal fees: report MORE's guilty plea earlier that day was a "big win" for special counsel Robert MuellerRobert Swan MuellerSasse: US should applaud choice of Mueller to lead Russia probe MORE in his investigation.

"Potentially, [Manafort's guilty plea] opens up lots of doors that probably haven't been opened before," Dershowitz, who is a columnist for The Hill, said on MSNBC on Friday. 

"I think from the point of view of Mueller, this is a big win, this is a big gain because he gets access to somebody," he added.

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Manafort pleaded guilty on Friday to two federal charges in a deal that includes cooperation with Mueller, who is investigating whether there was collusion between President TrumpDonald John TrumpSunday shows preview: Trump sells U.N. reorganizing and Kavanaugh allegations dominate Ex-Trump staffer out at CNN amid “false and defamatory accusations” Democrats opposed to Pelosi lack challenger to topple her MORE’s campaign and Moscow during the 2016 election. 

According to the plea agreement filed in court Friday afternoon, Manafort must cooperate “fully, truthfully, completely, and forthrightly” with the Justice Department in any and all matters that the government finds his cooperation to be relevant. 

Dershowitz, who has said he's a liberal Democrat and voted for Hillary Clinton in the 2016 election, has frequently criticized Mueller's probe and defended Trump on cable news appearances.  

Manafort’s cooperation with Mueller could be significant to the special counsel investigation, given his work on the Trump campaign. But, Dershowitz said, the significance of his cooperation will depend on what Manafort knows and is willing to share.

Manafort is the fourth Trump associate to plead guilty in Mueller’s investigation. 

Updated at 3:03 p.m.