While Mike Bloomberg's rivals battle it out in majority-white Iowa and New Hampshire, the billionaire presidential candidate is aggressively courting African American voters critical to any Democrat's chance of winning of the nomination.

The effort, backed by millions in ads, has taken him across Southern states that vote on March 3.

They're states where voters can decide a Democratic primary, from Alabama to North Carolina and Tennessee.

His pitch is one of electability and competence - hoping to capitalise on black Democrats' hunger to oust President Donald Trump.

But as he courts black voters he'll also have to reconcile his own record as mayor of New York City and past remarks on criminal justice.

Bloomberg's outreach aims squarely at former Vice President Joe Biden, who is banking on loyal black voters to resuscitate his bid after poor showings in Iowa and New Hampshire.

"Who can beat Donald Trump? That's what people care about," said former Philadelphia Mayor Michael Nutter, who is among the black leaders endorsing Bloomberg.

Nutter says Bloomberg's record of accomplishments outweighs the damage of flawed policing.

Bloomberg has no doubt been helped by his limitless financial resources and his strategy to focus on states conducting primaries on Super Tuesday.

He isn't competing in any states before the 14 Super Tuesday primaries.

One of the world's richest men thanks to a net worth of $US60 billion ($A89 billion), Bloomberg has spent more than $US300 million of his own money on ads.

They include spots on black radio stations, a Super Bowl ad featuring an African American mother who lost her son to gun violence and a national ad touting his work with Barack Obama on gun legislation and teen jobs.

He's also racked up endorsements from African American mayors and held events with key figures in the black community, including the meeting with black pastors in Detroit and speech at an Alabama Democratic luncheon.

Much of the outreach has been aimed at middle-age and older voters, who turn out more reliably, and appeals to a sense of pragmatism.

Bloomberg may not be the candidate you know best, the campaign argues, but he's the best poised to beat Trump.

And a recent poll shows signs of success.

The Quinnipiac University survey conducted after the Iowa caucuses found Bloomberg with 15 per cent support nationally, up from 8 per cent in a late-January poll.

That put him about even with Biden and Massachusetts Senator Elizabeth Warren and slightly ahead of Pete Buttigieg, who essentially tied with Sanders in last week's Iowa caucuses.

The poll showed Sanders leading with 25 per cent nationally.

Critics raise Bloomberg's criminal justice record in New York, specifically his embrace of a practice known as "stop-and-frisk" that disproportionately affected people of colour.

It could remain a mark on his civil rights record that cannot be erased by his recent apologies.

The issue flared again this week when comments he made in a 2015 Aspen Institute speech resurfaced.

In the audio, Bloomberg says the way to bring down murder rates is "put a lot of cops" in minority neighbourhoods because that's where "all the crime is".

He apologised for supporting the practice just days before launching his bid in November and repeated it on Tuesday.

He said the remarks "do not reflect my commitment to criminal justice reform and racial equity" but the timing of his apologies strikes some as political expediency.

Prominent activist Reverend Al Sharpton, who fought with him over stop-and-frisk, says he told Bloomberg in a phone call moments after his initial apology "it's going to take more than one speech but it's a step in the right direction".

Sharpton said Bloomberg had "an upside and a downside" as mayor but his biggest strength as a presidential candidate is his argument that he's the best one to take on Trump.

But Bloomberg also is viewed skeptically by some younger and progressive voters weary of putting another rich man in the White House.

And it's far from clear that his cool, business-like approach will connect more generally.

Bloomberg has so far been shut out of debates, allowing him to largely avoid tough questioning of his record.

That could change next week when Democrats hold their next debate. Bloomberg needs to meet a polling threshold in two more polls to make the stage for the first time.

Australian Associated Press