A top political adviser in Sen. John McCain's presidential campaign helped arrange an introduction in 2006 between McCain and a Russian billionaire whose suspected links to anti-democratic and organized-crime figures are so controversial that the U.S. government revoked his visa.

Rick Davis, who is now McCain's campaign manager, helped set up the encounter between McCain and Russian aluminum magnate Oleg Deripaska in Switzerland during an international economic conference. At the time, Davis was working for a lobbying firm and seeking to do business with the billionaire.

There is no evidence that McCain did anything for Deripaska after they met at a social gathering over drinks and dinner. Deripaska was grateful for the introduction, writing a thank-you note to Davis and his partner and offering to assist them in a subsequent business deal, according to a copy of the note obtained by The Washington Post.

With a net worth of more than $13 billion, Deripaska is one of the richest men in Russia and a close ally of President Vladimir Putin. McCain has been one of Putin's sharpest U.S. critics, calling for Russia to be kicked out of the Group of Eight industrialized nations because of Putin's anti-democratic activities. The Arizona Republican has also repeatedly complained about the negative impact of Kremlin-linked oligarchs such as Deripaska.

When Deripaska met McCain, Davis was part of Davis Manafort, a lobbying firm that was being paid to provide political advice to pro-Russian and oligarch-funded candidates in Ukraine, according to interviews and news accounts. At the same time, McCain was publicly supporting those candidates' Western-oriented democratic rivals.

The socializing with Deripaska provides a case study in the challenges faced by McCain, a longtime foe of Washington lobbyists who has a well-known lobbyist as his top political aide. Davis, who has been a political adviser to McCain on and off since 1999, was part of a lobbying firm that worked not only for Ukrainian politicians but also for telecommunications firms, a lottery services provider and freight companies.

Mark Salter, a spokesman for McCain, said that meetings with Deripaska took place during official trips abroad by senators and that McCain did nothing improper. "Any contact between Mr. Deripaska and the senator was social and incidental," he added.

Salter said the contact between McCain and Deripaska did not constitute a "private meeting"; both men were part of larger gatherings.

Davis declined to comment, saying by e-mail that his activities regarding Deripaska "all relate to my private business and have nothing to do with Senator John McCain."

Within the campaign, Davis's role has been controversial from the start, as some aides in late 2006 argued to McCain that the Davis firm's work overseas conflicted with the senator's record as a pro-democracy champion and an advocate of reducing the influence of lobbyists in Washington, according to two people familiar with the conversations. The sources spoke on the condition of anonymity because of the sensitivity of internal campaign conversations. The aides questioned whether Davis should be given an important title in the campaign because that would make him more vulnerable to criticism, the sources said.

At first, McCain agreed to give Davis a less conspicuous title, one of the sources said. But then, the source added, "a few days later he [McCain] came back and said he had changed his mind."

Salter strongly disagreed with that recounting. "No one suggested Rick shouldn't be in the campaign or discussed such a thing with the senator," he said in an e-mail.