The groups scout for military veterans, Sandersistas and others. But we found that what they generally do not scout for is competence at governing. In fact, many shy away from experience in government, on the theory that careerists are impure and inauthentic. As a representative of Justice Democrats, a group organized by former Sanders supporters, told us, “We don’t want career politicians, period.”

Although winning public office will never be easy, the proliferation of candidate pipelines is already making it easier for aspirants to run — and for groups to grow their own politicians. As a political consultant told us: “Access has been both demystified and democratized. People who used to think there’s no use running, because they would never win or it’s too big an unknown, are much less cowed by those factors.” Another consultant made the point more piquantly: “It’s become like a clown car. Everyone thinks they’re qualified and everyone jumps in.”

When we surveyed political consultants, they told us, by wide margins, that candidates in primary races are becoming more ideological and more inexperienced. Other research finds signs of a downward spiral. Extreme candidates heighten polarization in politics and paralysis in Congress, discouraging moderates and pragmatists (who reflect the preferences of most Americans) from entering politics.

Progressive activists we spoke with said they seek not just to influence policy but also to broaden the very concept of political viability. Though the goal is laudable, inexperience can compound the chaos that is already giving government a bad name. Even a handful of political renegades can have a crippling effect on a legislative body, which is one reason Mitch McConnell, the Senate majority leader, was desperate to keep Roy Moore of Alabama out of the Senate.

To be effective at their jobs, politicians need know-how, connections and I.O.U.s, which take years to accumulate. President Trump lacked all of those assets, so it is no surprise he has had trouble governing. In Congress and state legislatures, frustrated leaders find themselves saddled with anyone and everyone who prevails in low-turnout primaries, no matter how nutty or disruptive.

If governing ability and candidate quality matter, and if special interests and purist ideologues are not to become the leading suppliers of our politicians, it is essential that parties and professionals maintain a prominent role in recruiting and screening candidates — alongside, not instead of, voters. By virtue of their insider status, they have a long-term interest in having parties govern responsibly.