Donald Trump's bizarre, fascinating, and appalling presidential campaign is the biggest political story of 2015. But nearly all the smart analysts believed, from the moment that Trump started talking about running, that such a thing could never succeed. Even now most believe that the project must inevitably wither and die, that despite his persistent lead in the polls the chances of Trump winning the Republican nomination are tiny. How is it that everyone underestimated the power of this phenomenon?

Before we proceed, it's important to acknowledge that predictions, particularly about events with binary outcomes like campaigns, are usually the least useful thing that a political commentator has to offer his audience. "Here's what's going to happen" doesn't tell you very much, while "Here's what's happening now, here's why it's happening, and here's what it means" promises a great deal more. But those who follow politics ought to be able to provide some ideas about what's likely to happen in a campaign.

Very few people even thought Trump was serious about running (a reasonable assumption given his prior flirtations with a presidential bid). When he started, we were all surprised by his rapid rise in the polls, then further surprised by his ability to stay at the top, where he has been for almost six months now. We've learned along the way — few believe any longer that Trump can say something so outrageous that his followers will desert him — but nobody can look back and say, "This is all playing out just as I foresaw."

People who spend time deconstructing the theatrical aspects of politics to show how the public can be manipulated have resisted the notion that a professional entertainer could really fool the public like this. Even as we analyzed Trump's support, locating its roots in the Republican base's contempt for its congressional leaders and a broader feeling of alienation and dissatisfaction among white working-class conservatives, no one thought his success would last.

One way or another, most everyone says, this has to come to an end. Perhaps his relative lack of on-the-ground organization in key states will do him in. Or the Republican establishment will find a way to bring him down. Or voters will coalesce around an alternative to him. Or — and to be honest, this is the one many of us really believe — sooner or later, the voters supporting Trump will come to their senses and realize that the guy is a buffoon and having him run the country is a horrifying prospect. The trouble is that the basis for most of these beliefs isn't based on evidence.

Among the many unprecedented things about Trump's campaign, never has a candidate had such a clear lead for so long and yet had his chances of winning the nomination dismissed by so many. This is why the prediction markets rank Marco Rubio as most likely to get the nomination, with Ted Cruz in second place and Trump in third. That's strange given that national polls show Trump leading with an average of 38 percent, Cruz in second with 18 percent, and Rubio in third with 11 percent (that's the average; the RealClearPolitics average shows about the same).

I'm not predicting that Trump is going to be the nominee. Maybe he won't. But he certainly has a good shot and his supporters have demonstrated that they genuinely believe Trump would be a good president.

If nothing else, the Trump campaign should stand as an epistemological object lesson for pundits. When we decide that some outcomes are unlikely or even impossible, it never hurts to step back and ask why we think so, what specific information we're basing our opinion on, and whether we've come to our belief more out of hope or impulse. That won't show us the way to the right answer, but it might provide us with a healthy dose of modesty.

And we shouldn't be discouraged by being wrong. After all, the difficulty in predicting how campaigns are going to turn out is a big part of what makes them so interesting. If they always went the same way, there wouldn't be much to talk about. And whatever else you might say about Donald Trump, he's made this campaign plenty interesting to follow.