VIDEO - Dershowitz: Manafort cooperation is a 'big win' for Mueller | TheHill

Harvard Law professor Alan Dershowitz on Friday said former Trump campaign chairman Paul Manafort Paul John ManafortTrump: Republicans' and my poll numbers would be higher if not for Mueller's 'witch hunt' Manafort’s plea deal – the clear winners and losers Sunday shows preview: Trump faces fallout after Manafort flips MORE 's guilty plea earlier that day was a "big win" for special counsel Robert Mueller Robert 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.

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 Trump Donald John TrumpPaul McCartney slams Trump in new song: 'We've got a mad captain sailing this boat' George Conway rips Trump over tweet about Obama's '57 states' gaffe Pompeo doubles down on criticism of Kerry: The Iran deal failed, 'let it go' 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.