The epidemic of 2020 Democratic presidential candidates that has already broken out, not to mention at least one Republican challenge to President Trump, assures a marathon slow dance to next year’s national party conventions.

By the time the Democratic National Committee holds its first presidential candidate debate this June, the would-be leaders of the country will have slogged their way through a feast, or a plague, of televised tests of talent and political proposals served up on the networks and especially on cable.

Almost nightly, one or another of them will be conducting a “presidential town hall” in which celebrity hosts or hostesses will preside for an hour or more, with candidates snapping up free air time in which to peddle their wares.

In a sense it’s a lesson well learned from Trump himself, who was a pioneer in capitalizing on the willingness of the nets, cable and the Internet to freely showcase the latest aspirants for political power and fame.

Trump has transformed campaigning via television from a noble exercise in public education to a carnival barker’s hucksterism, at little cost to the star of the show.

Callous or just cynical observers have suggested that Trump — in the wake of the popular success of “The Apprentice,” the show in which he famously intoned “You’re fired” to eager job applicants — saw a political application to the extravaganza, and played it all the way to the Oval Office.

The television networks and cable outlets have been happy to have this new array of free “talent” perform before their cameras, either on the traditional campaign stump or in their studios. And the sooner, the better to build their audiences without having to carry costly writers and other facilitators in the field of show biz.

There was a time at the dawn of TV, or at least in its adolescence, when presidential campaigning didn’t kick off until the beginning of the actual election year. In 1960, Democratic hopeful John F. Kennedy didn’t announce his candidacy until right after New Year’s Day.

The first actual votes for the Massachusetts senator were cast and won in his neighboring New Hampshire primary only a few weeks later. Then, in West Virginia, Kennedy erased the notion that a Catholic could not be elected to the presidency, and he went on to Wisconsin where he ousted Sen. Hubert Humphrey and was on his way to the nomination.

The one other Democrat with presidential hopes that year, Senate Majority Lyndon Johnson of Texas, never entered a single primary and had to settle for becoming JFK’s running mate. What a change from today, when White House aspirants practically live on free television to sell themselves, and this time around for nearly two years in the limelight of TV and the internet.

The long stretch obviously obliges candidates to spend much time and effort raising campaign funds and courting big donors. But the self-serving beneficence of the television networks and especially cable outlets that thrive on the availability of “free talent” also has created a whole new army of “talking heads” spouting their political views and dominating the nighttime airwaves.

Many of them are young and attractive women who now vie with the vanishing breed of male reporters as accepted or purported experts on the political game. Other seasoned political operatives and former officeholders committed to one party or the other have hung up their partisan hats and pass themselves off as reborn political virgins.

In any event, the result is unending public kibitzing over all the ambitious men and women who are drawn to the television cameras and the Internet discussion in numbers and intensity not imagined in the heyday of print journalism.

With that development has come the Trump-inspired assault on all aspects of professional newsgathering as “fake news” and on the press as “the enemy of the people,” challenging its credibility in a time when the public all the more needs fact and truth from verifiable honest brokers to guide its judgment of would-be elected leaders.

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Jules Witcover’s latest book is

“The American Vice Presidency: From Irrelevance to Power,” published by Smithsonian Books