Paul Manafort was once one of the most significant players in Donald Trump’s world, and now he’s cooperating with federal prosecutors in the ongoing Russia investigation. That could be a big deal.

Manafort’s brief tenure as the head of Trump’s campaign overlapped with concerns about Russia’s interference in the 2016 election, he’s got high-level connections to Russia in his own right, and he’s a key link to the president as one of the highest-ranking Trump campaign officials ensnared in the investigation.

Manafort pleaded guilty Friday to tax fraud, lobbying fraud and obstruction of justice charges. In exchange for potential sentencing leniency, he will be a witness for special counsel Robert S. Mueller III, who is leading the investigation into Russia election interference.

White House press secretary Sarah Huckabee Sanders quickly distanced Trump from Manafort. “This had absolutely nothing to do with the President or his victorious 2016 Presidential campaign," she said in a statement. "It is totally unrelated.”

We don’t yet know the terms of Manafort’s cooperation.

Whatever Manafort shares, President Trump may know what’s coming. His personal attorney, Rudolph W. Giuliani, said earlier this week that Trump basically has an open line to Manafort through a joint-defense agreement that allows the two men’s attorneys to talk under the protection of attorney-client privilege. Giuliani also didn’t rule out the possibility Trump would pardon Manafort if Manafort flipped.

Given the murkiness of what Manafort might share, it’s worth laying out what Manafort might know, based on what we know about his status in Trump’s campaign and his many Russia connections.

1. Did the Trump campaign have any heads-up about alleged Russian hacking of Democrats? Manafort was Trump’s campaign chairman for several key months in 2016, around the time Democratic emails allegedly hacked by Russians got dumped into the public sphere. Mueller’s investigation seems interested in this moment: They have interviewed allies of Roger Stone, a longtime adviser to Trump who has acknowledged exchanging messages with an online persona that Mueller’s team says was operated by those Russian hackers. If someone in the campaign knew about it, it’s hard to see who would have a better vantage point than its chief.

2. Did Trump know about that meeting with a Russian lawyer? When Donald Trump Jr. was told the Russian government wanted to “support” his father — and oh by the way, do you want to meet with a Russian lawyer who has dirt on Hillary Clinton? — The younger Trump didn’t go alone. He brought along Manafort and Jared Kushner, Trump’s son-in-law.

It appears Manafort took detailed notes of that meeting, and those notes could be key evidence if there are any collusion-related charges.

That meeting’s circumstances are “as close as you can get to a smoking gun” on whether the Trump campaign colluded with Russia, white-collar lawyer Jeffrey Jacobovitz and other legal experts told The Fix last summer when news of the meeting broke. Now a key question is whether Trump knew about the meeting. He and Trump Jr. have denied it.

3. What do Manafort’s other ties to Russia mean? Among them: A Russian aluminum magnate. A pro-Russian former president of Ukraine. A Republican congressman who advocates for close ties between the United States and Russia. A business associate from his time in Ukraine who once served in the Russian army and had dinner with Manafort during the campaign. Manafort offered to give a Russian billionaire close to the Kremlin private briefings on the Trump campaign. And the FBI accused the Russian manager of Manafort’s lobbying office in Ukraine of having ongoing ties to Russian intelligence, though Konstantin Kilimnik has denied that.

Of all the Trump campaign officials, Manafort has the most known connections to Russia.

His Russia ties helped end his official work with Trump, too. During the campaign, the New York Times uncovered ledgers in Ukraine for secret, under-the-table payments to a pro-Russian party’s allies. Manafort’s name was in them, though he denied any wrongdoing. The news eventually led to Manafort’s resignation from Trump’s campaign, three months after he was elevated to the top job.

We don’t know what Manafort will share with prosecutors, just as we don’t know what the other four former Trump campaign officials and allies who are cooperating with the investigation have shared. But perhaps no one is better able to shed light on whether the campaign had dealings with Russia than Manafort.