For Joe Biden, it was his discomfiting touching of women. For Pete Buttigieg, his use of the phrase “all lives matter”. For Tulsi Gabbard, her comments about homosexuality. For Kirsten Gillibrand, her positions on immigration. For Kamala Harris, her record on criminal justice. For Beto O’Rourke, his jokes about his wife and children. For Bernie Sanders, his staff perpetrating sexual harassment in the last campaign. For Elizabeth Warren, her claim to Native American ancestry.

Democrats running for president in 2020 are on an “apology tour”, seeking to atone for past political sins. Some voters welcome it as an antidote to Donald Trump, an overdue attempt to set the social and political bar higher for the 21st century. Others are anxious that the candidates’ supporters will try to tear each other down with “wokeness” tests that could leave the party hopelessly divided.

Barack Obama said in Berlin on Saturday: “One of the things I do worry about sometimes among progressives in the United States … is a certain kind of rigidity where we say, ‘Uh, I’m sorry, this is how it’s going to be’ and then we start sometimes creating what’s called a ‘circular firing squad’, where you start shooting at your allies because one of them has strayed from purity on the issues.”

The default to contrition has alarmed some politicians and others who argue that the sole priority in 2020 must be defeating Trump, who never says sorry for anything.

Let’s not eat our own the way we nitpicked Hillary [Clinton] to death over her emails and other bullshit Bill Maher

Last month, after billionaire businessman and former New York mayor Michael Bloomberg announced he was not running, he cited his unwillingness to join the Democrats’ apology tour.

“Joe Biden went out and apologized for being male, over 50, white,” he said.

“He apologised for the one piece of legislation which is actually a pretty good anti-crime bill, which if the liberals ever read it, most of the things they like is in that bill. They should have loved that. But they didn’t even bother to read it. You’re anti-crime, you must be anti-populist.”

Biden, who has not yet declared his candidacy, has struggled in the past week to deal with complaints from women that his hugs, kisses and other tactile shows of affection were unwelcome. On Friday he made jokey references to the allegations, then offered a mixed apology: “I’m sorry I didn’t understand more. I’m not sorry for any of my intentions. I’m not sorry for anything that I have ever done. I’ve never been disrespectful intentionally to a man or a woman.”

The backlash against the 76-year-old former senator and vice-president, the architect of the 1994 Violence Against Women Act, has provoked a counter-attack from sympathisers and Trump critics.

Mika Brzezinski, co-host of Morning Joe, suggested that the #MeToo movement could be turned against Democrats and warned: “You’re eating your young. You’re eating those who can beat Trump. You’re killing the very people who have been pushing women ahead, who have been fighting for equal pay, who have been doing everything they can to respect women in their lives.”

Others have voiced concerns over lacerating criticisms of candidates on gender, race and sexuality issues that ricochet around social media every day. Frank Bruni, a columnist at the New York Times, expressed dismay over “a mini-debate” on the left over whether Buttigieg, a white upper-middle-class man, “is gay enough”.

Bruni argued: “It’s non-negotiable that Democrats hold their presidential aspirants to high standards on issues of racial justice, gender equality and more. It’s crucial that the party nominate someone who can credibly represent its proudly diverse ranks. But it’s also important that the party not demand a degree of purity that nobody attains.”

In February, the comedian Bill Maher urged viewers of his HBO show: “No more swiping left on presidential candidates. Let’s give them a chance. Let’s not eat our own the way we nitpicked Hillary [Clinton] to death over her emails and other bullshit.”

Maher added: “Kamala Harris has already had to play defence because it’s come out, when she was a prosecutor, she prosecuted people. Not very progressive. She should have found a way to apply more forgiveness, and the fact that she didn’t is unforgivable. Elizabeth Warren claimed to be Native American – so what? Trump claimed to be human.”

‘The overall mood is positive’

Republicans sense an opportunity to frame Democrats as consumed by identity politics and virtue signalling.

Kamala Harris takes the stage for a campaign rally at Morehouse College in Atlanta. Photograph: Curtis Compton/AP

Rich Lowry, editor of the National Review, wrote in February: “Democrats are about to ­embark on the first woke primary, a gantlet of political correctness that will routinely wring abject apologies out of candidates and find fault in even the most sure-footed. The passage of time will be no defense. Nor the best of intentions. Nor anything else.”

But many on the left contend that the candidates’ records are legitimate lines of inquiry and they should not be distracted by attacks from Trump, who is bound to paint them as socialists in any case. Instead, they posit, they should set a higher standard.

When I talk to most voters they’re actually mostly excited about the candidates Zephyr Teachout

Delvone Michael, a senior political strategist for the Working Families party who worked for Sanders in 2016, said: “It’s a direct response to the climate created by the president. People are distinguishing themselves by fessing up to past mistakes and moving on. Ultimately the election is going to be a referendum on Trump’s fitness for office, not on which candidate is the most ‘woke’.”

Zephyr Teachout, a former candidate for New York attorney general, said: “I think you’re seeing so many variations on the theme of apology because it’s becoming more and more clear that, as horrified as people are with Trump, most Americans, and certainly most Democrats, are not actually eager to simply just rush back to two and a half years ago.

“Since so many people were part of the mainstream Democratic party, they have to find a way of saying that they can bring something new.”

Teachout, an associate law professor at Fordham University, also rejected the notion of Democrats eating their own.

“When I talk to most voters they’re actually mostly excited about the candidates. If you’re not on Twitter all day, it’s a lot more positive than you might think from the perspective of the Twitter machine. I’ve just been visiting some friends and they sang the praises of about six of the candidates with a few concerns. So I think the overall mood is positive.

“It’s really important to share real differences between the candidates and to look into their histories and recognise that people can change but take history seriously. We are unbelievably lucky to have a real primary.”

Bill Galston, a senior fellow at the Brookings Institution thinktank in Washington and a former policy adviser to Bill Clinton, agreed that the Twittersphere bears little relation to what is happening on the ground.

“The metaphysics of the left never cease to amaze me,” he said. “This is hardly the first time we’ve seen purity competitions on the left. It’s almost definitional.

“The question is whether real voters are taking these ridiculous debates seriously. Mr Buttigieg managed to raise $7m out of thin air and elevate himself out of the 1% polling range, based partly on some high-profile appearances.

“Will the debate whether he is gay enough do anything but disgrace the participants? My answer is somewhere between no and hell no.”