Bloomberg told Reuters in an interview earlier this month he would contrast their experience to his success as billionaire chief executive of global financial information and media company Bloomberg LP and three-time mayor of New York City.

"None of them would know how to run a big organisation," he said of the seven other Democratic candidates seeking the nomination to challenge Republican President Donald Trump in November.

Some of his rivals have made clear they see the televised debate as their best opportunity to subject Bloomberg, who has been rising fast in the polls, to greater public scrutiny.

He will face tough questions over his past support for policies that have been widely criticised as racially discriminatory.

He will also have to defend himself against the charge he is trying to buy the election with his self-financed campaign. Public filings show he spent $US188 million of his own money through December.

"Bloomberg is spending an awful lot of money, but he has also not had his turn in the barrel," Representative Cedric Richmond, a co-chair of Biden's campaign, told reporters on a call on February 12.

Bloomberg qualified for Wednesday's debate in Las Vegas on Tuesday.

Asked by Reuters how he would differentiate himself from his rivals in a debate, Bloomberg said he would emphasise his experience as the former mayor of America's biggest city and as the head of a large multinational company.

As vice president under Barack Obama, Biden "reads speeches that somebody writes for him," he said during a campaign stop in Compton, California, on February 3.

"That's the job. His job was not to manage but to do some things the president needed done and for all I know he did a competent job."

Biden campaign spokesman Andrew Bates countered that the former senator oversaw Washington's stimulus program to counter the 2007-2008 financial market crisis, among other things.

Bates also pointed to 2008 remarks by Bloomberg in which he tied the collapse in the US housing market that led to the crisis to a ban on a discriminatory housing practice known as redlining.

"We're happy to talk about records," Bates said.

Bloomberg dismissed Buttigieg, the former mayor of South Bend, Indiana, as having limited experience, managing a city with roughly 100,000 people compared with New York's population of 8.4 million.

After winning the most delegates in Iowa and coming in second in New Hampshire, Buttigieg is considered a front-runner.

"He is the mayor of a town," Bloomberg said. "Let's not get too carried away."

Bloomberg, who only entered the race in November, is coming under increasing scrutiny over his past support of a policing strategy he employed in New York that ensnared blacks and Latinos disproportionately.

He apologised for that policy, known as "stop and frisk," just before announcing his candidacy.

He also faces questions over allegations of past sexist remarks. In an interview with ABC's "The View" on January 15, he said he regretted having told "bawdy" jokes.

Bloomberg, worth an estimated $US60 billion, is skipping the first four nominating contests in Iowa, New Hampshire, Nevada and South Carolina, and instead pouring hundreds of millions of dollars into a national effort to start winning delegates in the 14 states that vote on March 3.

The other Democrats expected on the Las Vegas stage - including Biden and Buttigieg as well as US Senators Bernie Sanders, Elizabeth Warren and Amy Klobuchar - are eager to confront him in a debate.

"He just can't hide behind the airwaves," Klobuchar told NBC's Meet the Press on Sunday, commenting on Bloomberg's TV advertising. "He has to answer questions and of course, I think he should be on that debate stage."

Biden said on Thursday he would challenge Bloomberg on stop and frisk and on redlining. Warren also attacked him on the redlining comments.

Warren and Sanders have accused Bloomberg of trying to buy the election and are almost certain to repeat that during the debate. Bloomberg said polling suggests attacks on his money would fall flat with voters.

"They don't care about the money and when you say, you know, Mike didn't inherit it, he earned it, (that's a) big plus," he told Reuters on February 3.

Sanders has bristled that Bloomberg is even allowed access to the debate stage. The former mayor didn't qualify for Democratic debates in December, January and early February.

But the party leadership changed the qualification rules, dropping fundraising requirements that disqualified Bloomberg because he is not accepting any contributions.

Australian Associated Press