It may prove to be the most-talked-about secret payment in American political history — the $130,000 that President Trump’s lawyer Michael D. Cohen paid to the pornographic film actress Stephanie Clifford to keep quiet about an alleged affair with Mr. Trump before he became president.

That payment to Ms. Clifford, known professionally as Stormy Daniels, was a mere 0.005 percent of the $2.4 billion spent on the 2016 election. But it could have an outsize impact on the presidency: The payment has helped spur a lawsuit by Ms. Clifford against Mr. Trump and a federal investigation into Mr. Cohen. Campaign finance watchdogs assert the transaction was the result of a secret, and illegal, effort to subvert election spending laws on behalf of the president.

Mr. Trump on Thursday rejected any notion that payments to Ms. Clifford had violated campaign finance laws, though in the course of his defense he contradicted earlier statements that he hadn’t known of payments to the actress.

The story behind the payment to Ms. Clifford — when Mr. Cohen paid it, how he paid it, whether he was paid back and by whom — will be critical to both the lawsuit and investigation, not to mention others that may come.