Former Trump campaign adviser Paul Manafort agreed Friday to cooperate with special prosecutor Robert Mueller’s investigation as part of a plea deal — and a former presidential investigator said the negotiation could be a “significant breakthrough” for the investigation into the Trump campaign’s potential Russian ties.

Ken Starr, who served as the independent prosecutor investigating President Bill Clinton in the ’90s, said in an interview Sunday on ABC’s This Week that while “we don’t know” how significant Manafort’s information will be, the plea deal could be very helpful to Mueller’s investigation.

“It is very likely that Paul Manafort has indicated through his counsel and directly that he can provide very helpful information, useful information to get to the bottom of what Bob Mueller and his team have been charged to do,” Starr said about the deal. “So it is a very significant breakthrough.”

In a separate interview Sunday on CNN, Starr added the deal means “we’re much closer to getting the truth than we were before this plea.”

“It’s so terrific for the investigation, and frankly for the American people, that we’re moving forward and we’re getting someone who may be knowledgable,” Starr said.

The plea deal, which also required Manafort to forfeit his real estate holdings to the government — including a condo in Trump Tower — requires Manafort to answer questions about Mueller’s investigation “fully” and “truthfully” and turn over documents to the investigators. Manafort was already separately found guilty on eight charges of tax and bank fraud in August.

Manafort’s testimony could prove especially helpful regarding Donald Trump Jr.’s meeting during the presidential campaign with a Russian lawyer, as the former campaign manager was present at the meeting.

“Manafort is a Sherpa for the key moments,” former White House ethics chief Norm Eisen said Sunday on ABC. “For the first time we have somebody who was at the infamous Trump Tower meeting. We have his notes, he can explain his notes, he can talk about the run-up to the meeting, the afterwards.”

Despite noting the potential significance of Manafort’s testimony to the investigation, however, Starr said Sunday he hoped the information would not lead to Trump being removed from the White House.

“Impeachment is hell,” Starr said on CNN. “The country should not be taken through that. ... Unless there is a growing national consensus that impeachment is proper. It’s doomed to fail, and it’s just the wrong way to go.”

“I can understand full well the interests of the American people, and then a significant number of senators who said ... this is not directly related enough to his conduct with the presidency,” Starr said about Clinton’s impeachment proceedings in 1998. “And that may happen [with Trump]. We’ll see.”