How is a 74-year-old self-described Democratic socialist from one of the least populous states in the country turning the Democratic primary upside down and proving an adept challenger to one of the most established candidates in modern politics?

Easy, supporters of Senator Bernie Sanders say: He represents an unyielding crusader who will restore decency to American politics. Mr. Sanders is ideologically pure at a time when everything and everyone else in Democratic Washington seems to revolve around compromise. And as this primary is proving, many Democrats (and even some Republicans) are frustrated with compromise. In some way, Mr. Sanders’s appeal stems from his own un-electability.

But first, an exploration of the facts: Before Hillary Clinton announced her candidacy, political observers expected her to run away with the Democratic nomination. And that conventional wisdom appeared to pay off, at first. Mrs. Clinton still leads Mr. Sanders by roughly 25 points in national polls — the same spread that there is between Donald J. Trump, the Republican front-runner, and Mike Huckabee, the trailing former Arkansas governor. Still, the gentleman from Vermont has succeeded in neighboring New Hampshire, where he is virtually tied with Mrs. Clinton.

To understand how Mr. Sanders has outperformed expectations in the Democratic primary, you have to look at how he’s long been outperforming expectations in Washington, where he first arrived after winning a seat in Congress in 1990.