Sanders is merely the latest such challenger to make some early noise in a Democratic primary race. He’s not even the first one from Vermont: In 2004, former governor Howard Dean rode his opposition to the Iraq war to the top of the field before eventually finishing a distant third in Iowa. That, combined with his famous caucus-night scream, was effectively the end of his candidacy. Dean followed in the footsteps of others, such as then-Sen. Gary Hart (Colo.) in 1984 (and briefly in 1988), California Gov. Jerry Brown in 1976 and again in 1992, and former senator Bill Bradley (N.J.) in 2000. Obama played this role in 2008. The most famous and most successful post-McGovern, pre-Obama challenge came from Sen. Ted Kennedy (Mass.) in 1980, when he almost defeated Jimmy Carter, the incumbent president of his own party.

All of these challengers had their moments, rising in the polls, firing up the grass roots and going from unknown underdogs to legitimate contenders. But every one of them, except Obama, ultimately came up short. Their early successes all had some similar explanations. First, the most liberal voters tend to tune in sooner and engage more actively, giving an initial boost to progressive candidates. Second, the overriding bias in political press coverage is toward a competitive race, which means that challengers often receive media attention that exceeds their chances of winning. Finally and perhaps most importantly, skepticism of the establishment is woven into the fabric of the Democratic Party — if the party leadership, the donors and the pundits are all for one person, many in the rank and file start to explore other options.