Federal prosecutors in Manhattan are investigating whether President Trump’s personal lawyer Rudolph W. Giuliani broke lobbying laws in his dealings in Ukraine, according to two people familiar with the inquiry.

The investigators are examining Mr. Giuliani’s efforts to undermine the American ambassador to Ukraine, Marie L. Yovanovitch, one of the people said. She was recalled in the spring as part of Mr. Trump’s broader campaign to pressure Ukraine into helping his political prospects.

The investigation into Mr. Giuliani is tied to the case against two of his associates who were arrested this week on campaign finance-related charges, the people familiar with the inquiry said. The associates were charged with funneling illegal contributions to a congressman whose help they sought in removing Ms. Yovanovitch…

Federal law requires American citizens to disclose to the Justice Department any contacts with the government or media in the United States at the direction or request of foreign politicians or government officials, regardless of whether they pay for the representation. Law enforcement officials have made clear in recent years that covert foreign influence is as great a threat to the country as spies trying to steal government secrets.