The jury has heard how Manafort made tens of millions of dollars for work with pro-Russian politicians in Ukraine.
Gates, who also served in a senior role in Donald Trump's presidential campaign, is expected to face aggressive cross-examination once prosecutors are finished questioning him.
Downing will probably focus on Laporta's interactions with Gates, who pleaded guilty and is cooperating with Mueller. Mr Gates was Mr Manafort's partner in the consultancy.
Manafort's attorneys have signalled they will seek to blame Gates and have accused him of embezzling millions of dollars from Manafort.
HuffPost Illustration/Getty Images Rick Gates, the star witness in the case against Trump campaign manager Paul Manafort.
"At Mr. Manafort's request. we did not disclose the foreign bank accounts".
Manafort is on trial in Virginia for various financial crimes he was charged with as part of special counsel Robert MuellerRobert Swan MuellerSasse: US should applaud choice of Mueller to lead Russian Federation probe MORE's ongoing investigation.
Gates has already admitted to lying to investigators once.
Prosecutors on Monday went through a list of overseas corporations and Gates testified that all of them were controlled by Manafort and contained income earned by his political consulting work.
Gates also testified he and Manafort knew it was a crime because they had been notified by Manafort's accountants in emails. Gates will follow the testimony of tax preparer Cindy Laporta, who says she came to suspect Gates was misleading her about Manafort's finances. Rick Gates became the first member of the Trump campaign to publicly admit to crimes.
In a court filing on Monday, prosecutors said they may call Federal Bureau of Investigation forensic accountants Morgan Magionos and Renee Michael to read from Manafort's emails.
But Ms Laporta's testimony raised the stakes for Mr Manafort, legal experts said.
After that, Gates sent her a document dated March 6, 2014, that purported to represent a $900,000 loan from a Cyprus entity, Telmar Limited, to Manafort's company. During that process, Gates admitted to two felony charges, but his testimony would be the first time he details those crimes face-to-face with Manafort, his former boss and mentor. When Robert Mueller began investigating Russian interference past year, Manafort's questionable lobbying work - the sort of activity that has historically rarely been investigated in Washington - was put under a microscope, exposing him and his associates.
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