Asked by prosecutors why she did so, Ms. Laporta said she was trying to protect her firm, KWC. “I could call Mr. Manafort and Mr. Gates liars,” she said, but “Mr. Manafort was a longtime client.” If she challenged him, she said, it was also possible her company could be sued.

Her testimony brought to life in dramatic fashion the financial machinations that prosecutors have been sketching out since Mr. Manafort’s trial on 18 charges of bank and tax fraud opened on Tuesday. And it underscored questions about how Mr. Manafort landed such a prime spot on Mr. Trump’s campaign, where he served as a delegate manager and then as campaign chairman for five months in 2016, when his political consultancy business was essentially bankrupt.

Prosecutors working for the special counsel, Robert S. Mueller III, spent much of the week on Mr. Manafort’s flush years, when a highly lucrative contract to promote pro-Russia political forces in Ukraine from 2010 to 2014 financed a life of opulence for the veteran political consultant. They showed how millions of dollars — wired from overseas bank accounts that were hidden from his accountants — paid for Mr. Manafort’s luxury clothing, rugs, home entertainment systems, home renovations, landscaping and three new homes.

But it was Mr. Manafort’s behavior once he fell on hard times after 2014 that produced the most compelling testimony. Prosecutors say that Mr. Manafort earned about $60 million working for pro-Russia oligarchs behind Viktor F. Yanukovych, who was elected Ukraine’s president in 2010. But after Mr. Yanukovych was ousted and fled to Russia in 2014, prosecutors say that Mr. Manafort’s income plummeted to zero. He then switched from hiding his income in foreign bank accounts to inflating it so American banks would extend millions in real estate loans to him.

Ms. Laporta testified that in August 2016, just a week before Mr. Manafort left Mr. Trump’s campaign amid controversy over his activities in Ukraine, she assured a potential lender that a $1.5 million loan to Mr. Manafort’s firm had been forgiven. In truth, she said, she was worried that the loan did not exist and was instead a ploy to lower Mr. Manafort’s tax bill. When she asked for documentation of the transaction, she testified, Mr. Gates gave her an unsigned letter, just over a page long, with almost no details. In an email, he wrote that he still needed to “chase down signatures.”