The former New York City mayor Michael Bloomberg will endorse Hillary Clinton in a primetime speech at the Democratic national convention this week, according a senior adviser.

“This week in Philadelphia, he will make a strong case that the clear choice in this election is Hillary Clinton,” the former mayor’s longtime adviser Howard Wolfson told the Guardian.

Wolfson called Bloomberg “the nation’s leading independent and a pragmatic business leader” and said he had “supported candidates from both sides of the aisle”. The endorsement was first reported by the New York Times.

The speech will mark the former mayor’s first foray into presidential politics since he decided against his own presidential bid, reasoning it could bring the Republican presidential nominee, Donald Trump, closer to the White House.

Bloomberg left the Democratic party in 2000, won the mayoral election in New York City as a Republican in 2002 and brought the Republican national convention to the city in 2004. He became an independent in 2007, and served three terms as mayor – after he supported a change to city rules on term limits.

While some of Bloomberg’s positions are popular among Democrats – for example, his gun control advocacy and calls for immigration reform – others, such as his support for controversial police tactics and a deregulated financial sector, could ruffle some liberal Democrats.

In March, Bloomberg wrote in an editorial that some close allies had prodded him to run for president during an election cycle dominated by Democratic and Republican “partisan extremism”.

“As the race stands now, with Republicans in charge of both houses, there is a good chance that my candidacy could lead to the election of Donald Trump or Senator Ted Cruz,” he wrote. “That is not a risk I can take in good conscience.”

He went on to write that Trump “has run the most divisive and demagogic presidential campaign I can remember, preying on people’s prejudices and fears. Abraham Lincoln, the father of the Republican party, appealed to our ‘better angels’. Trump appeals to our worst impulses.”

Bloomberg’s endorsement could chip away at one of Trump’s major selling points – his alleged business acumen. Bloomberg is a fellow New Yorker and is one of the richest men in the world, with billions more than Trump, who claims to be worth $10bn but in 2012 appeared to have less than half that amount. Bloomberg may also attract a demographic that Clinton’s campaign has struggled with: less educated white men.

The Democratic national convention begins Monday, with a primetime speech by Bernie Sanders, whom Bloomberg has accused of “demagoguery” for the senator’s frequent attacks on Wall Street. The appearance of both men represents Clinton’s attempt to unify opposite wings of the party: she needs votes from progressives, who support advocates of financial hawks such as Sanders and the Massachusetts senator Elizabeth Warren, and she needs support from moderates, who consider Bloomberg a more appealing leader.

Sanders alluded to the divisions in the party on Sunday when asked by NBC whether he approved of Clinton’s choice for vice-president, the Virginia senator Tim Kaine.

“He is more conservative that I am,” he said. “Would I have preferred to see someone like [Massachusetts senator] Elizabeth Warren selected by Secretary Clinton? Yes, I would have.”

Clinton and Kaine, meanwhile, gave their first joint interview as a ticket, set to air on CBS’s 60 Minutes Sunday evening.

In a preview of the interview, both criticized Trump’s rhetoric, Clinton calling it “dangerous” and “demeaning” and Kaine calling his tenor “ridiculous”.

“When I see this ‘crooked Hillary’ or ‘lock her up’, it’s just ridiculous,” Kaine said. “It is beneath the character of the kind of dialogue we should have, because we have real serious problems, and look, most of us stopped the name-calling thing in fifth grade.”

Clinton added she would not respond directly to the calls of Trump’s supporters to “lock her up”.

“I’m not going to engage in that kind of insult-fest that he seems to thrive on,” said Clinton. “Whatever he says about me, he’s perfectly free to use up his own airtime and his own space to do. I’m going to talk about what he’s done. How he’s hurt people in business time after time after time.”