Deputy Attorney General Rod J. Rosensteinspeaks at a news conference at the Justice Department in Washington on Friday. (Andrew Harrer/Bloomberg)

FRIDAY’S FEDERAL grand jury indictment of 13 Russians for conspiracy to interfere illegally in the 2016 presidential election presents powerful evidence that Moscow staged an attack on the United States’ democratic political process. The facts, doggedly accumulated by special counsel Robert S. Mueller III despite much hostility from President Trump, show that the Russians’ goal was to foment “distrust towards the candidates and the political system in general,” as the indictment puts it. And the chosen means was “information warfare,” reportedly waged via provocations on social media and the occasional in-person grass-roots activity. It began in 2014 and involved Russians engaging in political activities under false, sometimes stolen, identities; no Americans wittingly cooperated with this particular plot, though some did so unwittingly, according to the indictment.

The indictment thus undercuts any lingering suggestion that Russian interference is a myth or a hoax, and Mr. Trump, who has often suggested as much, should have acknowledged the new evidence Friday. Instead, his first reaction was to claim vindication on Twitter. “The Trump campaign did nothing wrong,” he wrote, adding, “no collusion!” This was inappropriate on two levels.

First, though the indictment did say that there was no knowing American collusion with the Russian social media campaign, and though it did not say that it affected the results, it also showed that the vast majority of Russian propaganda supported Mr. Trump’s campaign and attacked that of his Democratic opponent, Hillary Clinton. You would think Mr. Trump would take a moment to repudiate that support, even in hindsight, and to declare that no foreign power has a right to campaign secretly against an American candidate.

Second, Mr. Mueller has not finished his investigation and has not ruled out the possibility of collusion. We don’t yet know whether Donald Trump Jr.’s eagerness to meet with Russians offering “dirt” on Ms. Clinton’s campaign was an isolated incident. Nor has the special counsel yet weighed in on the question of possible obstruction of his investigation by President Trump.

Meanwhile, the evidence of a Russian assault on the U.S. election is a serious development in and of itself that any responsible president would respond to in a serious way. Such an attempt to delegitimize the American system could only have gone forward with the knowledge and approval of Russian President Vladi­mir Putin. It reflected the Kremlin’s all-too-accurate judgment that a divided and polarized U.S. electorate would be vulnerable to the same sort of dirty tricks Russia has pulled in Europe. In a statement, Mr. Trump declared that “we cannot allow those seeking to sow confusion, discord, and rancor to be successful,” though he strangely blamed not Russia, but rather “outlandish partisan attacks” by his opponents, which, he said, “further the agendas of bad actors, like Russia.” The only message he should be sending now, both to the American people and to Moscow, is that Mr. Putin is responsible and that the U.S. government will respond to his covert attacks with appropriate retaliation.

President Trump continues to insist the Democrats are responsible for any story relating to Russian interference in the 2016 election despite a year's worth of evidence to the contrary. (Meg Kelly/The Washington Post)

The grand jury’s indictment shows how far Russia is willing to go to manipulate and discredit our democracy. Mr. Trump’s own intelligence chiefs warned this week that the 2018 election is under threat. Given the baffling and inexcusable absence of presidential leadership, Congress must step up to defend the nation.