The number of contenders will drop as the qualifications for participating in party debates tighten (especially in September) and some run out of money.

Some know they have no chance at winning, but hope that the publicity might land them a cabinet post, lucrative book deal, or large speaking fees.

Most experienced political observers have assumed that the Democrats can defeat Trump, unless they swing too far to the left, turning off those who had supported Barack Obama and who then supported Trump, including the blue-collar workers and suburbanites who decided the 2016 election.

The problem for the Democrats, especially in this election cycle, is that voters in primary contests (in both parties) tend to be more extreme than the parties as a whole.

The Democrats’ recent swing to the left began with the 2016 challenge by Bernie Sanders — who calls himself a “democratic socialist” and isn’t a Democratic Party member — to Hillary Clinton’s presumed nomination.

Sanders, with his appeal as an insurgent and his unrealistic promises (as well as Clinton’s weaknesses), nearly undid her nomination. Young people found him an exciting anti-establishment figure.

Senator Elizabeth Warren was rising in the polls even before the debates.

But her vast policy agenda represents a leap forward in government intervention in the economy and other domestic arrangements; adds up to trillions of dollars, without a clear explanation of how to pay for it; and is unlikely to be approved by Congress (even if the Democrats recapture control of the Senate).

As she becomes one of the top two or three candidates (drawing some support from Sanders), these vulnerabilities are likely to be exposed.

Kamala Harris, born of professional parents from Jamaica and India, is more cautious than Warren and in some cases her positions — for example, on whether private health insurance should be abolished — have been contradictory.

She built her reputation on having been a prosecutor and attorney general in California, and has won national attention by using her prosecutorial skill in hearings, though not always fairly.

As a California prosecutor, she was partly progressive, but also demanded harsh sentences and reportedly kept some innocent people locked up.

In the second of the first pair of Democratic debates, Harris stole the show by attacking former vice-president, Joe Biden, who was at that point the frontrunner.

Dredging up a controversy from the 1970s, Harris pointed out that Biden, the former senator from Delaware, had opposed federally mandated busing, even though it would have achieved greater racial integration of schools.

A Gallup poll in the early 1970s found that only 4% of whites and 9% of African-Americans supported the highly controversial programme.

Harris, who, as a child, participated in a busing programme (albeit a voluntary one in Berkeley, California), also said that she was hurt by Biden’s recent, unartful, nostalgic recollection of working with two arch-segregationists in the Senate to get some bills passed decades ago.

(Both segregationists occupied powerful Senate positions and pro-civil rights Democrats working with them wasn’t uncommon, though Biden could have chosen less noxious examples.)

Harris sought to cut into Biden’s strong African-American support, which has been aided by his eight years as vice-president to Barack Obama.

Biden stumbled in his reply. He emphasised his strong pro-civil rights record, and, a couple of weeks later, he apologized for appearing to condone the two segregationist senators.

And it turned out that Harris’s position on busing isn’t much different from Biden’s. But her attack catapulted her up the polls.

The press attention afforded afterward to Harris’ long-planned attack on Biden — confrontations and zingers (though obviously rehearsed) make for good TV — is an example of what’s wrong with these ‘debates’ as a vehicle for choosing a candidate.

Sanders’ considerable recent decline in opinion polls — coming in behind Harris and Warren in some polls, as well as behind Biden — can be attributed to the fact that his act is no longer fresh.

He’s still long on promises and short on details, and he’s still grumpy and a yeller. But, mainly, Sanders is no longer the lone insurgent challenging the establishment figure.

Biden, however, has the greatest problems. Before the first debates, he hovered 20 points above his rivals.

Though this may have been due to his greater name recognition and his evident closeness with Obama, he has come across as dwelling in nostalgia.

He seems not to recognise how much US politics has become hyper-partisan, since he was in the Senate, before the Republican Party turned to the right and became openly obstructionist.

Moreover, Biden was never a great campaigner for the presidency, failing twice. He’ll be 77 in November (three years older than Trump) and would turn 80 in his first term (which would make him the oldest US president ever).

Two or three others remain plausible Democratic candidates.

Right now, some of the charm might be wearing off Pete Buttigieg, the preternaturally wise 37-year-old mayor of South Bend, Indiana, a gay man who volunteered to serve in Afghanistan.

‘Mayor Pete’ had an excellent spring, and he remains a darling of many Democratic donors.

But just before the debate, he had to confront an issue vexing officials in towns and cities across the country: a white policeman in South Bend had recently shot and killed an unarmed black man.

In the debate, the issue seemed to weigh him down. And he’s in trouble if he can’t attract African-American support, a problem he’s had since, as a new mayor, he fired South Bend’s first black police chief.

By the conclusion of the first pair of debates, many of the Democratic candidates had backed controversial leftist proposals, such as Medicare for All, which could end private health insurance and raise taxes; decriminalising undocumented immigration; covering undocumented immigrants in government health-care plans; and school busing.

A major dilemma of the Democratic nomination contest is that it is a test of two important attributes: which candidate can beat Trump, and which is offering the most appealing platform to primary voters.

These are not the same thing.

Elizabeth Drew is a Washington-based journalist and the author, most recently, of Washington Journal: Reporting Watergate and Richard Nixon’s Downfall.

Copyright: Project Syndicate, 2019.