With Tuesday’s convictions in the criminal trial of President Trump’s former campaign chairman Paul Manafort, the special counsel, Robert S. Mueller III, has struck another blow in his investigation: five guilty pleas, 32 indicted individuals, 187 charges revealing startling evidence of Russia’s 2016 attack on our democracy, and now the conviction of one of the top operators in the Trump campaign orbit. Mr. Manafort’s conviction on eight separate counts means he could spend the rest of his life in prison.

The conviction conclusively and publicly demonstrates what many of us have said since the start of the investigation: This is no “witch hunt.” It instead is one of the most successful special counsel investigations in history. Coming alongside the guilty plea by Michael Cohen, the president’s former lawyer, implicating the president in campaign finance violations, it was a very bad day for Mr. Trump.

Mr. Manafort’s conviction cannot be diminished by arguing, as Mr. Trump and his coterie are fond of doing, that the misconduct was unrelated to the Trump campaign or Russian “collusion.” On the contrary, the trial evidence included Mr. Manafort’s close ties to pro-Russia forces and his desperate financial straits as he “volunteered” his time for the next president. The trial revealed how willing Mr. Manafort was to corruptly leverage his position of influence over Mr. Trump during the campaign for his own personal benefit. He offered briefings to a pro-Russia Ukrainian oligarch and dangled a position in the Trump administration in front of a banker who provided him a loan for which he would not otherwise have qualified.