Political strategists often talk about campaigns in terms of “moments”.
They are points in time that irrevocably change the political landscape, forcing those involved to adapt their behaviour to a new baseline.
The attempt on Donald Trump’s life on Saturday night is the most significant moment in decades.
In the seconds after gunshots rang out across the showground where the Republican nominee was addressing his supporters, it became clear that everything had changed.
An instantly historic image of the scene, captured in time by an Associated Press photographer, shows Trump stumbling off the stage, dripping with blood and surrounded by Secret Service agents.
Moments after his brush with death, the former president raised a fist in defiance, his lips forming a single word: “Fight.”
This is the first credible assassination threat on an American president’s life since Ronald Reagan was shot in March 1981.
Other more feeble attempts have been made – not least against Trump, who has been the target of at least three previous plots. Several more would-be assailants have planned to kill Joe Biden.
The constant threat of violent death is something of an occupational hazard for American commanders-in-chief, and four sitting presidents have succumbed to it.
Nonetheless, there is no denying that this incident will have a significant impact on this year’s presidential race.
Trump has built his campaign on the idea that everyone is out to get him. Federal prosecutors, judges, election officials, rival politicians and journalists have all been accused of trying to bring down his campaign and prevent his return to the White House.
Many of those claims have rightly been contested. But after the incident in Pennsylvania, even Trump’s worst enemies cannot deny that there are some who would rather see him dead than re-elected.
The polls already suggest that Trump is likely to win back the presidency in November, after a torrid few months for his opponent and a criminal conviction that has had little impact on his popularity.
If history tells us anything, the events of Saturday will only increase his support. In the months after Mr Reagan was shot, the newly-elected Republican president saw a poll boost of eight points.
On Monday, Trump will greet supporters in Milwaukee, Wisconsin, to announce his running mate at the Republican Party’s national convention.
The security arrangements, which are already stringent, will likely be strengthened further in response to the major safety breach that came close to ending the nominee’s life.
But the rhetoric, too, will be different. Trump’s supporters are already claiming the assassination attempt as the latest battle in his war to rescue the US, and that framing will now dominate the campaign.
Minutes after it happened, the former president’s son posted a photo of his father being led away, with the caption: “He’ll never stop fighting to Save America”. After receiving medical treatment, Trump himself said it was “incredible that such an act can take place in our country”.
The subtext of that remark is obvious: that America is broken, and that he will fix it with the help of his supporters.
The attempt on Trump’s life is a stark reminder of how a single, groundbreaking, moment can upend an already tempestuous campaign.
Any remaining doubt that Trump will prevail in this year’s election is now receding, and Mr Biden’s gaffes earlier this week – while undoubtedly dramatic – now seem irrelevant.
Those alive in 1963 still ask each other what they were doing when they heard John F Kennedy had been shot.
In decades to come, they will ask: “Where were you when the bullet missed Trump?”