PRESIDENT OBAMA’S appearance at the Democratic National Convention was preceded by almost 10 minutes of introductory video. Such gauzy tributes have become standard fare at political conventions. But what was unusual about Wednesday night’s footage is that it didn’t just dwell on Mr. Obama’s accomplishments but also spotlighted a singular failure of his time in office: the inability to get Congress to enact gun control. Remarkably, gun control has emerged as a central element of Hillary Clinton’s presidential campaign — and it is clear from the careful way the convention choreographed the issue that Democrats think it will help them win in November.

The attention paid to gun violence and the need for gun restrictions over the four days in Philadelphia represents a sharp departure for Democrats and, of course, a contrast with the rigid Republican embrace of unconditional gun rights. Fearful of the clout of the National Rifle Association — particularly after the party lost the House in 1994 after pushing through a ban on assault rifles — many Democrats viewed gun control as a third rail of electoral politics, something to be avoided at all costs. At the past three Democratic conventions, The Post’s Philip Rucker reported, only one presidential nominee mentioned gun violence. In Denver eight years ago, Mr. Obama devoted a single sentence to it in his 5,000-word acceptance speech.

In Philadelphia, a succession of people whose lives have been horribly altered by guns took the stage along with gun-safety activists. The hall hushed as former congresswoman Gabrielle Giffords talked about her struggle to speak as the result of being shot near Tucson in 2011; and again when the mother whose son was killed in Orlando recounted how it took about five minutes for a church bell to toll for all 49 lives lost in last month’s massacre. The issue was framed not only with personal stories but also with explanations of how domestic terrorism and the safety of police officers are affected when the wrong people get guns. “We need more than grieving to protect our law enforcement officers,” said Charles Ramsey, former police chief in Philadelphia and the District. “Those who aim to do harm shouldn’t get a handgun, let alone an assault rifle.”

A ban on assault weapons was one of the common-sense reforms Mr. Obama sought unsuccessfully in the wake of the slaughter of elementary schoolchildren in Newtown, Conn., in December 2012. “That’s the closest I came to feeling disgusted,” Mr. Obama said in the video. We hope voters will agree with him and his party that such inaction is no longer tolerable.