I don’t think that anyone has defined it that way. I think the populist idea is that good political ideas can come from below, from unsanctioned, unofficial, uncredentialed oppositional groups. The Democratic Party leadership does not believe this.
Populism involves some degree of breaking down the barriers between citizens and government. The Constitution was designed to suppress direct democracy in favor of representative democracy via checks and balances, the separation of powers, federalism, and the other stuff you learned about in eighth grade civics. The two-party system puts another intermediary in there, the party, and usually the parties and the candidates work through still another layer of vote-contracting intermediaries, the organized interest groups and the mass media. Populists want to get rid of some of those layers. (Populists are sometimes portrayed as Jeffersonian constitutionalists, but they generally advocated more simplified procedures and spent a lot of time fighting the Supreme Court, and that interpretation is incomplete at best.).
When elitist liberals or socialists get upset upon finding that their representatives are lying and unresponsive, they’re populists whether they know it or not. Elitism is institutional, not intellectual. To a political player, a Nobelist is one vote, the way a HS dropout is one vote, and a famous Nobelist is an opinion-leader, on a par with a comparably famous stoner celebrity (and far outranked by a really serious opinion-leader like Bill Kristol.)
The difference is that when they’re lied to, populists know what’s happening and get mad, whereas left intellectuals are baffled and mostly just whine. My mission in this world is to convince liberal intellectuals that they are People too, salt-of-the-earth folk scorned by the powers that be. But most intellectuals find this offensive — they think of themselves as unappreciated elite units, like princes switched in the cradle and raised by peasants. They’re sure that some day they will be recognized and restored to their rightful status.
Our political elite is well-educated but tough-minded. The Democratic Party’s pious renunciation of ideology, populism, and demagoguery has been accompanied by a rehabilitation of graft, corruption, and subservience to big money. (The post-WWII pluralists and consensus theorists were fairly open about this). What we have now is a spiffy, modern, Ivy-educated Tweed Ring*. And Boss Tweed and the others, when the chips were down, were reactionary servants of big money (“Bourbon Democrats“.) They used part of the graft to help out their voters, but they supported policies which hurt these same voters), and they made sure that whatever help the voters got was controlled by vote-contractors and received only by reliable supporters.
The suffering PhD masses have nothing to lose but their chains, but they’re mired in the toils of servility and ancient prejudice. An unpromising lot indeed, but be they ever so humble, we cannot afford to write off even the least of our brethren.
* The Moonies have Ivy-educated leadership now. The Mafia and the drug cartels send their kids to the best schools. Ahmed Chalibi, Ted Kaczynski, Jerome Corsi, Bill Kristol — all PhDs from the best schools. We humble folk just don’t know what to think.