Warren’s problem is that a campaign that looked, at first blush, fresh and inspiring looks, at second blush, familiar and drearily predictable.

For a half-century, a certain type of Democratic voter has regularly fallen in love with a certain type of Democratic presidential candidate. Those candidates project an aura of reform; they inspire supporters with the intellectuality of their personas and the earnestness of their policies; they launch insurgencies against "politics as usual" and against the moneyed interests and establishment hacks of their own party.

And these candidates almost never become the nominee, and even less frequently become president.

The great challenge of Warren’s campaign has always been to prove that she does not belong in a parade of high-concept Democrats that includes such figures as Howard Dean (2004), Bill Bradley (2000), Paul Tsongas (1992), Gary Hart (1984), Jerry Brown (1976, 1980, 1992), and Eugene McCarthy in 1968.

The best way to separate from that parade is to wage a fight and win it, in a fashion similar to what Warren began doing this week. The point is not so much the fight as the opportunity to show another side of herself and give a pathway to voters who have kept their distance so far.

The enduring problem for progressive reformist candidates is that their campaigns come to be seen as precious. They convey that politics is an exercise in conceptual abstraction — may the smartest candidate with the most elegant message win — rather than an exercise in power and human connection. No one doubts these candidates are smart. They typically falter over doubts that they are tough — able to throw a punch and also take one. They comprehend policies but not necessarily the concrete human dimensions of those policies; they are articulate but labor to find language that resonates with voters who are not as well-educated or affluent as they are.

The counter-example in its own way proves the point. Barack Obama in 2008 fit squarely in that line of insurgent progressives challenging an establishment figure in Hillary Clinton. The big difference between Obama and Dean/Bradley/Hart/etc. was that once Obama demonstrated electability with early victories, he had strong support of African Americans. The reformist wing plus African Americans is a winner in Democratic politics; the reformist wing alone is typically a loser.

Sen. Bernie Sanders, who many expected early on would be Warren’s primary rival, does not fit neatly in the reformist tradition. His persona was always more warrior for the left than progressive policy reformer, and his support has skewed younger and more demographically diverse.

The same challenges confronting Warren — how to expand a narrow base — await a candidate whose surge, especially in Iowa, has come at her expense. South Bend, Ind., Mayor Pete Buttigieg is ideologically more centrist, but his appeal draws on the same insurgent-reformer-with-brains tradition that is powering her.

As for Warren, the mortal threat facing Buttigieg’s campaign is an inability so far to appeal to a diverse Democratic coalition, especially working-class and African American voters.