The golf writer James Dodson said last year that during a visit to a Trump golf course in 2013, Eric told him of his family company’s financing: “Well, we don’t rely on American banks. We have all the funding we need out of Russia.”

Around the time Donald Trump said goodbye to American banks, he said hello to Mr. Cohen, a lawyer whose résumé, one might have expected, would have screamed, “Stay away!” to a legitimate businessman.

From at least 1999, according to a recent Times report, Mr. Cohen had dealings with Russian mob figures and began finding business deals in, and with people from, Russia and the former Soviet Union. By 2007, Mr. Cohen was working for the Trump Organization as a fixer and deal maker.

Even during the 2016 campaign, Mr. Cohen pursued plans for a Trump Tower in Moscow, coordinating with Felix Sater, a felon with ties to Russian mobsters who had worked on other deals with Mr. Trump.

After Mr. Trump’s election, while the F.B.I. was investigating whether his campaign helped Russian efforts to put Mr. Trump in the Oval Office, Mr. Cohen visited his boss in the White House. During that February 2017 visit, Mr. Cohen left for the national security adviser, Michael Flynn, a plan to lift sanctions against Russia, which had been imposed for its attacks on Ukraine. These sanctions had squeezed the sorts of people Mr. Cohen dealt with. The plan was proposed by Mr. Sater and a Ukrainian politician with ties to Paul Manafort, a former Trump campaign chairman.

Mr. Flynn has since pleaded guilty to lying about his contacts with Russians, and Mr. Manafort is under indictment on charges of financial crimes that involved Russians.

At the moment, Americans are lucky to have Robert Mueller, the special counsel, examining all of this. Mr. Mueller was appointed after Mr. Trump fired the F.B.I. director James Comey because of his frustrations with the Russia investigation. Mr. Mueller has been looking at Mr. Cohen’s affairs and records from the Trump Organization. And one question that Mr. Trump’s lawyers say Mr. Mueller wants to ask the president is what communication did he have with Michael D. Cohen, Felix Sater and others, including foreign nationals, about Russian real estate developments during the campaign.

Russians and cash — they’ve been a part of Mr. Trump’s life for years, and now they’re elements of the investigation into whether his campaign conspired with Moscow to corrupt American democracy. Mr. Trump’s affection toward the Russian president has led many to ask, “What does Putin have on Trump?” Maybe the ledgers will tell.