Donald Trump is "antithetical to every Christian value" and it's an "embarrassment" that he's received widespread support from Christians, Jim Wallis has said.

In an interview with Christian Today, Wallis – a leading evangelical figure and founder of Sojourners magazine – said Trump's campaign has been run by "stoking and fuelling racial, gender and religious bigotry".

Since announcing his candidacy for president last year, Trump has campaigned hard for the evangelical vote – and initially appeared to have been very successful. He won the support of a number of high-profile evangelicals including the president of Liberty University, Jerry Falwell Jr, Eric Metaxas, Wayne Grudem and Robert Jeffress.

But in the last few weeks, that support appears to be crumbling. The release of a video tape from 2005 in which Trump can be heard openly speaking about groping women and trying to seduce a married woman caused waves among his conservative supporters.

Although some have remained firmly in Trump's camp – Jeffress maintains that supporters aren't "endorsing necessarily his [Trump's] lifestyle" and Falwell said he would continue to support Trump even if the allegations of sexual assault proved to be true – others have since pulled their endorsements. Conservative radio host Hugh Hewitt urged the Republican candidate to withdraw from the race. Grudem announced that he had reversed his support but then said he would vote for Trump after all.

Indeed the evangelical voice against Trump seems to be growing. More than 2,000 Liberty students have signed a petition opposing his candidacy, condemning his "flagrant dishonesty, consistent misogyny and boastful unrepentance". Beth Moore spoke publicly of her own experience of sexual assault and said to evangelical leaders who still supported Trump: "Try to absorb how acceptable the disesteem and objectifying of women has been when some Christian leaders don't think it's that big a deal." Russell Moore, of the Southern Baptist Commission's Ethics and Religious Liberty Commission, reiterated his own opposition to Trump.

What's more, Wallis says that though the narrative has long been that Trump enjoys majority evangelical support, that's actually never been the case.

"When the media in the US talks about evangelical support for Trump, it's talking about white evangelicals," he explains. "When you bring into conversation evangelicals who are African Americans, Hispanic, Asian Americans, it changes the whole picture. It's like they've been erasing the voices of 35 million people."

The word evangelical, Wallis says, has been "hijacked... by the religious right". He wrote in USA Today this week that it has become "political shorthand for white political conservatives who profess to be evangelicals and vote overwhelmingly for Republican political candidates, based almost solely on their opposition to abortion and gay marriage".

This isn't the full story, he tells Christian Today. "When you add up all the evangelicals, including the young white evangelicals who clearly don't support Donald Trump... that's a whole generation we've been leaving out."

Wallis believes that in reality, support for and against Trump will be split about 50/50 among evangelicals in America, but though this is lower than some polls have predicted it's still too many.

"Donald Trump is literally antithetical to every Christian value. It's embarrassing that Christians would support Donald Trump, to be honest," he says. "He's run a campaign by stoking and fuelling racial, gender and religious bigotry. What is often implicit in American politics – racial bigotry – he's made explicit. What is often covert, he has made overt... It's incredible that he has won the nomination for Republicans, and even more incredible that any Christian would support him.

"But black evangelical Christians don't, Hispanics don't, many of us don't, younger white evangelicals don't. We're correcting the record here. We've suffered identity theft from the religious right, and we're trying to restore it. This isn't just about the election or defeating Donald Trump, it's about restoring the integrity of the word 'evangelical'."

Wallis is among nearly 80 evangelical leaders to sign a petition on change.org that condemns Trump. The letter warns that the "meaning and integrity" of the Christian faith in the US hangs in the balance.

"We believe that racism strikes at the heart of the gospel; we believe that racial justice and reconciliation is at the core of the message of Jesus," it reads. "We believe the candidacy of Donald J Trump has given voice to a movement that affirms racist elements in white culture."

The Christian leaders distance themselves from any definition of "evangelical" as associated with mostly white, politically conservative, older men.

"We are not those evangelicals," they say. "We are Americans of African and European descent, Latino/a, Asian American, and Native American. We are women and men, as well as younger and older evangelical Christians. We come from a wide range of denominations, churches, and political orientations."

Evangelicals who do support Trump often say that they don't support his lifestyle, but that they prefer his polices to rival Hillary Clinton's – whose stance on abortion is cause for particular alarm. But Trump, Wallis says, doesn't actually "have any policies to speak of".

"He's a sexual predator, he has bragged about sexual assault against women... he's violated his marriage vows time and time again with three wives. He exemplifies the worship of money. It's not policy, it's who he is. He doesn't have any policies to speak of – this is a campaign run on racial resentment and white nationalism... he's not even a true conservative, as many conservatives have pointed out."

"This is a moral moment," he continues. "In the end, Christian leader are going to be held to account for their support of Donald Trump."

Wallis has not officially endorsed Clinton however, or indeed any candidate in previous elections. He admits she is "a real mixed bag as a candidate" though some of her policies particularly in favour of women and children are "very consistent with Christian values". Many Christians will vote "to be most effective against Donald Trump", he adds.

What it comes down to, Wallis says, is that the "bigotry" at the heart of Trump's campaign is intolerable.

"If white Christians acted more Christian than white, then black parents would be less fearful for their children. That's the issue.... Racism cuts to the core of the gospel. Racial justice and racial reconciliation are at the heart of the message of Jesus," he says.

"Reporters often ask 'what does evangelical mean?' I take them back to how Jesus defined it in his opening statement at Nazareth, I call it the Nazareth Manifesto. He said: 'The Spirit of the Lord is on me, because he has anointed me to bring good news to the poor, release the captives, and let the oppressed go free'."

Wallis points to the origin of the word 'evangelical', which is drawn from the word for 'good news'.

"That means any gospel that isn't the good news to the poor simply isn't the gospel of Jesus Christ."