To date, four people have pleaded guilty to charges brought against them by special counsel Robert S. Mueller III and his team. Two were ancillary characters: a businessman who apparently sold bank account numbers to Russian trolls; and a lawyer who had worked with former Trump campaign chairman Paul Manafort and his longtime partner Rick Gates.

Two were closer to the campaign. One is campaign adviser George Papadopoulos, whose connections to Russia-linked characters helped spur the initial investigation in July 2016 into whether President Trump’s campaign had aided Russian interference efforts. And then there’s Michael Flynn, a campaign aide who wound up as Trump’s national security adviser. Both Papadopoulos and Flynn have apparently agreed to work with Mueller’s team in its investigation, but it’s not clear how much either knew about what the campaign was doing. Flynn’s role was the same on paper as Papadopoulos’s, but he was clearly closer to Trump.

Neither, though, is known to have had intimate familiarity with what the Trump campaign effort looked like. Which makes Friday’s news of an intent by Gates — both a campaign staffer and, before that, a Manafort partner — to cooperate with Mueller potentially very significant.

Mueller’s team dug deep on Gates and Manafort and uncovered a slew of questionable business practices. Between the two, they face 58 criminal counts, including conspiracy charges, bank fraud, lying to federal officials, income tax charges and more. Gates faces more charges than Manafort, giving him all the more reason to try to reduce the burden of any criminal penalty.

Remember that Mueller’s mandate isn’t only to suss out what Russian interference in 2016 looked like. It’s specifically to determine the answer to the question first prompted by Papadopoulos: Did anyone from Trump’s campaign help? While Gates was involved in the campaign more directly than Papadopoulos — he was deputy chairman — his work almost certainly paled next to how significant Manafort’s was.

That’s the clear utility for Mueller in Gates cooperating. Gates was privy to a big chunk of Manafort’s financial transactions that are being questioned by prosecutors, and his willingness to aid their cause makes it much, much more possible that they’ll be able to convict Manafort. Just as the admission of guilt by that attorney, Alex van der Zwaan, may have made it more possible that Gates would be convicted, upping the pressure on him to flip, knowing that Gates would take the witness stand against him would give Manafort a lot of incentive to figure out a deal with Mueller’s team, too.

And getting Manafort to agree to cooperate would be huge. Save flipping a member of Trump’s family, like son-in-law Jared Kushner, there are few people who were higher in the Trump campaign infrastructure during 2016. Manafort is much more likely to be aware of efforts to shift the direction of the campaign, including any ways in which those shifts crossed ethical or legal lines.

We hasten to note that there is no public evidence at this time that Trump campaign staff directly sought to aid Russian interference efforts. But significant moments at which the campaign drifted close to that point involve Manafort directly.

Most notably, there’s that meeting in June 2016 at Trump Tower, which included Donald Trump Jr., Kushner and Manafort. This is the meeting with the Kremlin-linked attorney and the Russian lobbyist that was predicated on the promise of incriminating information about Democratic presidential nominee Hillary Clinton delivered by the Russian government. (“I love it,” Trump Jr. said when it was offered.) Manafort has reportedly denied knowing what the meeting was about, skimming past an email subject line from Trump Jr. that read “FW: Russia — Clinton — private and confidential” before seeing his invitation to attend the meeting. Trump Jr. has said that Manafort spent the whole meeting looking at his phone, implying that the meeting was so useless and boring that Manafort could barely be bothered to pay attention. We later learned, though, that Manafort was taking assiduous notes.

So that’s one question: What would Manafort who’s cooperating with Mueller’s team say about that meeting? About Trump Jr.’s efforts to set it up? About whether Trump knew it was happening, which he’s denied? The evening after the meeting was set, Trump gave a speech in which he pledged “a major speech on probably Monday of next week, and we’re going to be discussing all of the things that have taken place with the Clintons.” That speech didn’t happen — nor did the promised dirt on Clinton materialize.

Manafort is also believed to have tried to leverage his position with the campaign to bolster his relationship with Russian oligarch Oleg Deripaska. Manafort offered private briefings on the campaign to Deripaska, who’s close to Russian President Vladimir Putin. Emails obtained by Mueller suggest that Manafort saw his prominent position as a way to “get whole” with Deripaska, opening up a number of questions about how Manafort’s position may in fact have been used.

With Gates cooperating, Mueller’s team gains important leverage over someone who certainly can answer a number of outstanding questions about the campaign and the people working on it. It’s still very much the case that Trump himself may never be implicated in aiding the Russian effort and exonerated of having had any knowledge about it. But Manafort is one of the few people who might know what Trump knew — or Manafort might be able to himself put pressure on people, like Kushner or Trump Jr., who are even closer to the president.

“Collusion” has not been proved. Manafort is innocent until proved guilty. But moving Gates from hostile to friendly certainly puts a lot more pressure on Trump and his senior team than existed 24 hours ago.