Mr. Manafort began working in Ukraine after the popular uprising in the winter of 2004-5 that became known as the Orange Revolution. Mr. Yanukovych, then prime minister, was declared the winner of a presidential election in 2004 that was marred by fraud and overturned by the country’s highest court after weeks of protests in favor of his pro-Western rival, Viktor A. Yushchenko.

Mr. Yanukovych had relied disastrously on Russian political advisers who underestimated voter frustration. After his defeat, he turned to American experts.

Mr. Manafort had begun working for one of Ukraine’s richest men, Rinat Akhmetov, to improve the image of his companies. Mr. Akhmetov was also a prominent sponsor of Mr. Yanukovych’s party, the Party of Regions, and he introduced the two men.

With Mr. Manafort’s advice, Mr. Yanukovych began a comeback, with the Party of Regions winning the biggest bloc in parliamentary elections in 2006 and again in 2007, returning him to the post of prime minister. At the time, Mr. Manafort called Mr. Yanukovych, a former coal trucking director who was twice convicted of assault as a young man, an outstanding leader who had been badly misunderstood in the West.

According to State Department cables at the time and later released by WikiLeaks, Mr. Manafort and his colleagues Phil Griffin and Catherine Barnes frequently pressed American diplomats in Ukraine to treat Mr. Yanukovych and his supporters equally so as not to risk being seen as favoring his opponents in the new elections. With Mr. Manafort’s help, the party was “working to change its image from that of a haven for mobsters into that of a legitimate political party,” the American ambassador at the time, John E. Herbst, wrote.

During this time, lucrative side deals opened for Mr. Manafort.

In 2008, he and the developer Arthur G. Cohen negotiated a deal to buy the site of the Drake Hotel on Park Avenue in Manhattan. One partner was Dmytro Firtash, an oligarch who made billions as a middleman for Gazprom, the Russian natural gas giant, and who was known for funneling the money into the campaigns of pro-Russian politicians in Ukraine, including Mr. Yanukovych. The three men intended to reopen the site as a mall and spa called Bulgari Tower, according to a lawsuit filed in Manhattan by Yulia V. Tymoshenko, a former prime minister of Ukraine. In the end, though, the project unraveled.