* Republican lobbyist is most prominent aide to plead guilty
* Manafort chaired Trump 2016 campaign at critical moments
* Potential political blow to Trump ahead of November elections (Adds context, details of charges)
By Nathan Layne and Karen Freifeld
WASHINGTON, Sept 14 (Reuters) - Donald Trump's former campaign chairman Paul Manafort will cooperate with the federal investigation into Russian meddling in the 2016 election, prosecutors said on Friday, a dramatic turnaround that deals the U.S. president a political setback.
After months of refusing to assist Special Counsel Robert Mueller's inquiry into Russian interference and possible coordination between Trump campaign members and Moscow, Manafort took a plea deal and agreed to cooperate in return for reduced charges.
It was not immediately clear what information about Trump that Manafort, a longtime Republican political consultant who ran the campaign as it took off in mid-2016, could offer prosecutors.
But Friday's announcement was a political blow to Trump's presidency ahead of Nov. 6 congressional elections that will determine whether or not Republicans maintain control of Congress.
Manafort, 69, pleaded guilty in a federal court in Washington on Friday to counts of conspiracy against the United States and conspiracy to obstruct justice, becoming the most prominent former Trump campaign official to plead guilty in Mueller's investigation.
He had other charges dropped but could still face 10 years in prison on the two charges in Washington. In August, Manafort was convicted at trial in Alexandria, Virginia, on charges that pre-dated his stint on the Trump campaign and involved his work with pro-Russian politicians in Ukraine.
In a statement, the White House distanced Trump from the man who helped get him elected in November 2016 against the odds in a bitterly contested campaign in which he defeated Democrat Hillary Clinton.
"This had absolutely nothing to do with the president or his victorious 2016 presidential campaign," White House spokeswoman Sarah Sanders said. "It is totally unrelated."
The probe, which Trump often describes as "witch hunt," has cast a shadow over the presidency.
Manafort had steadfastly refused to cooperate with Mueller even as the Virginia jury convicted him on bank and tax fraud charges.
Prosecutors had accused him of hiding from U.S. tax authorities $16 million he earned as a political consultant in Ukraine to fund an opulent lifestyle and then lying to banks to secure $20 million in loans.
Trump last month praised his former campaign aide for not entering into an agreement with prosecutors, as the president's former personal lawyer Michael Cohen had.
On Aug. 22, Trump wrote on Twitter: "Unlike Michael Cohen, he refused to 'break' - make up stories in order to get a 'deal. Such respect for a brave man!"
Jury selection was due to begin on Monday in a second Manafort trial on charges including conspiring to launder money, conspiring to defraud the United States, failing to register as a foreign agent and witness tampering.
Instead Manafort entered the plea.
Manafort made millions of dollars working in Ukraine before taking an unpaid position with Trump's campaign for five months.
He led the campaign when Trump was selected as the Republican presidential nominee at the party convention.
Moscow has denied interfering in the 2016 election and Trump has said there was no collusion.
Former federal prosecutor Harry Sandick said it was too soon to say how much Manafort's cooperation could affect the investigation into possible collusion between Trump's campaign and the Russians.
"It's hard to see how this is good for Trump," Sandick added, "but we don't know yet if it is bad for Trump."
"The big question is what information does he have that the special counsel is interested in," said Sandick. "Cooperation is supposed to be complete. So if he has information about the president or the president's family and advisers that would have to be shared," Sandick said.
(Reporting by Nathan Layne and Karen Freifeld Additional reporting by Jonathan Landay and Susan Heavey; Writing by Alistair Bell; Editing by Howard Goller and Grant McCool)
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