Weber’s activities have been under scrutiny by federal prosecutors in New York investigating whether he and others complied with laws requiring those working for a foreign country or political party to register with the Justice Department.
An attorney for Weber declined to comment.
A Mercury partner, Michael McKeon, said Friday that the firm respected Weber’s decision.
“We respect Vin’s decision to resign from Mercury to focus his time and energy on protecting his reputation. He is an honorable man, a good friend and we wish him well,” he said.
Manafort had brought aboard several Washington lobbyists and lawyers — including Weber, Democratic lobbyist Tony Podesta and lawyer Gregory B. Craig — to participate in a broad effort to help improve Ukraine’s reputation in the United States.
Craig, who served as White House counsel for President Barack Obama, was charged with lying to federal officials who were examining whether he should have registered as a foreign lobbyist for the work. He is on trial in Washington and has denied wrongdoing.
Separately, prosecutors in the Southern District of New York have been examining Weber and Podesta’s lobbying work.
The Podesta Group and Mercury had reported in lobbying disclosures that they represented the European Center for a Modern Ukraine, a Brussels-based nonprofit organization that sought to help Ukraine improve its image in the West from 2012 to 2014.
Prosecutors alleged in the Manafort prosecution that the lobbying work was being directed by Manafort and Trump campaign official Rick Gates, and that the real client was the government of Ukraine. The two firms said they sought to comply with all disclosure requirements and that they were cooperating with investigators.
Earlier this month, in Craig’s ongoing trial, Gates testified that he told Weber and Podesta they were working for the government of Ukraine, despite paperwork indicating otherwise.
Weber, who served in Congress for 13 years, is a well-known figure in political circles and served as an adviser to GOP presidential hopefuls including Mitt Romney and Bob Dole.
Lawrence Jacobs, professor of political science at the Hubert H. Humphrey School of Public Affairs at the University of Minnesota, said he is surprised that Weber has been caught up in the controversy.
“Vin has an extraordinary reputation and bipartisan respect for his intelligence and integrity,” he said.
Ann E. Marimow and Spencer S. Hsu contributed to this report.