If not for Donald Trump, the biggest story of the current presidential race would be the surprising success of Sen. Bernie Sanders in the Democratic primary.
Polls show Sanders with a clear lead over Hillary Clinton in New Hampshire, and running quite close to Clinton in Iowa, leading many to speculate that he could win the first two states, and, in the process, significantly complicate Clinton's march to victory.
In response, Clinton has in recent days stepped up her attacks on Sanders, labeling him as, among other things, out of touch with political reality. This was not Clinton's plan: She was happy to have competitors, whose presence allowed her to avoid the appearance of a primary coronation, but at no point was she supposed to encounter anything like real resistance.
In a way, then, Clinton's plan was similar to Jeb Bush's plan for the GOP primary race. Bush hoped to come in early, sweep up supporters and donations in a shock-and-awe campaign, and thus render the race an already-done-with non-contest early. Bush raised the money, but the rest of the race hasn't gone according to plan, thanks in large part to the presence of Donald Trump.
In a way, then, the stories of Donald Trump and Bernie Sanders are the same stories—the outsiders whose campaigns turned out to have far more energy and staying power than anyone expected a year ago.
What the success of both Trump and Sanders signal is the breakdown of the party establishments, and the generalized dissatisfaction with the political status quo, especially when it comes to federal politics.
It's not an accident that their successes come as party identification continues to sink to all-time lows and as a Gallup survey finds that the most important problem facing the country is government. (Yes, most people who say they are independents still lean one way or another, but the increasing unwillingness to identify with either party nonetheless suggests the decline of both party brands, and their inability to serve as satisfy large swaths of the populace.)
Both Sanders and Trump, in their own very different ways, speak to those sorts of feelings. They are candidates of dissatisfaction and disaffection, of frustration and irritation; they are avatars of anger and alienation, embodying the sense that the system isn't working—or, at the very least, that it isn't working for enough people.
In the end, Sanders will probably not win the nomination. Trump has a better shot, turnout willing, although he's far from a lock. (Trump's closest competitor at the moment, Texas Sen. Ted Cruz, represents a related sort of disaffection in a somewhat more conventional way, though he is hardly an establishmentarian.) But even if both Trump and Sanders lose, their constituencies will remain, and continue to influence both how candidates run for office and what they do when they get there.
Which is to say that the story of Sanders and Trump is in some larger sense not the story of either candidate, but the story of a growing and general frustration with politics as it is typically practiced, and politicians as they typically present themselves—and it is a story that cuts across parties, and is not likely to go away when the election is over.