Donald Trump is on course to be re-elected in 2020 because those in the current crop of potential Democratic presidential candidates are “too tame to deal with an untamed opponent,” one of the party’s key power-brokers has said.

Speaking to a group of Guardian journalists following a visit to 10 Downing Street and a session with black MPs, the Rev Al Sharpton warned that the likely challengers to Trump are failing to galvanise opposition to the president. “They’ve lost the ability to dramatise. He [Trump] understands spectacle and drama and they don’t.”

What was the civil rights movement if not drama? Martin Luther King was the master of street theatre Rev Al Sharpton

Citing the current controversy over migrant parents being separated from their children, the veteran civil rights leader said Democratic would-be candidates “should be right there, getting themselves arrested” by agents of Immigration and Customs Enforcement.

“What was the civil rights movement if not drama? Martin Luther King was the master of street theatre. No one would have listened if he just gave speeches.”

In a wide-ranging and candid conversation, Rev Sharpton expressed his fear that this lack of leadership could lead to disappointment in the midterm elections in November. Landslide wins for Democrats would take a mobilisation that he had not yet seen, he said. “You can’t just announce a wave, you have to organise a wave.”

As to who might take on Trump in 2020, he said that Oprah Winfrey “could beat Trump in a heartbeat”, speaking especially to poorer voters: “She’s been broke longer than she’s been rich,” he said. But he suspected the TV star and entrepreneur was not keen to run. Former vice president Joe Biden would enjoy strong black support, Sharpton said, not least because “he covered [Barack] Obama’s back for eight years”.

He noted that Bernie Sanders struggled to win African-American backing in 2016 because he “could only see class, not race and class.” He said Massachusetts senator Elizabeth Warren, tipped as the possible standard bearer of the Democratic left, might fare better as she was more readily attuned to “the racial dimension.” But overall, he believed the current Democratic field was lacklustre and too easily distracted by Trump’s “bizarre and theatrical shenanigans”.

‘Where are Black Lives Matter now? You gave Obama hell, but where are you all now with Trump,’ asked Rev Al Sharpton, right, pictured outside No 10 Downing Street on Wednesday. Photograph: Derek Peters/Rex/Shutterstock

Democrats needed to pay less attention to the president’s tweets or the latest twists in Robert Mueller’s probe into collusion with Russia, Sharpton said, and craft instead a message based on “rights, jobs and healthcare. You got to get the message right; then you’ll get the messenger.” He added that waiting for Trump to self-destruct “is not a political strategy.”

The longtime campaigner and baptist minister also had stern words for the activists of the Black Lives Matter movement. “Where are Black Lives Matter now? You gave Obama hell, but where are you all now with Trump?”

Warning that civil rights once thought safe were again under threat, he cited Monday’s ruling by the Supreme Court in favour of a Colorado baker who had refused to make a wedding cake for a gay couple on religious grounds. “This was one of the worst civil rights decisions I’ve seen in my lifetime,” Sharpton said, warning that once “homophobic” discrimination was allowed, sexist and racist discrimination would be next.

On Wednesday, following a meeting with black MPs, Sharpton visited 10 Downing Street to speak to Theresa May’s adviser on race issues, Nero Ughwujabo. Afterwards, Sharpton praised May’s establishment of a “race disparity audit”, measuring the effect of ethnicity on life chances in the UK, saying it was an approach he hoped to take back to the US.

But his admiration for the UK was not total. Referring to last month’s royal wedding, he said “the picture of inclusion created by the addition of a woman of a colour to the royal family” did not “reflect the reality” of a country that had also been rocked by the Windrush scandal.

As for Trump himself, Sharpton had concluded that the evidence of the president’s racism was now too overwhelming to dispute. “You don’t have to keep a white hood under the pillow,” he said, citing Trump’s baseless claim that Obama had not been born in the US and the president’s praise for neo-Nazi marchers in Charlottesville last year as “very fine people.”

But he urged opponents of racism and xenophobia in the US, Europe and beyond not to lose heart. “My message to the resistance movement globally is, don’t panic, but dig in.”