Democratic presidential candidate Elizabeth Warren has criticised billionaire rival Michael Bloomberg for launching his nascent White House bid with a $US37 million ($55 million) TV advertising blitz, accusing the former New York City mayor of trying to buy American democracy.

Key points: Mr Bloomberg will finance his campaign from his personal fortune

Mr Bloomberg will finance his campaign from his personal fortune Senator Warren has previously proposed a wealth tax on billionaires

Senator Warren has previously proposed a wealth tax on billionaires The Bloomberg campaign will skip early-voting states, including Iowa

Mr Bloomberg, 77, a media mogul who will use his personal fortune to spend freely on his campaign and has said he will not take donations, officially jumped into the White House race as a moderate Democrat on Sunday.

Senator Warren, 70, a liberal US senator from Massachusetts and one of the leading Democratic contenders according to polls, has proposed a wealth tax on billionaires and frequently rails against corporate America, a platform Mr Bloomberg has criticised.

At an event with voters in Ankeny, Iowa, Senator Warren opened her remarks by denouncing Mr Bloomberg's tactics.

"Michael Bloomberg is making a bet about democracy in 2020. He doesn't need people, he only needs bags and bags of money. I think Michael Bloomberg is wrong," she said.

"That's exactly what's now in play in 2020 — which vision, which version of our democracy is going to win. If Michael Bloomberg's version of democracy wins, then democracy changes. It's going to be 'which billionaire you can stomach?'"

Michael Bloomberg announced his presidential run last week.

Mr Bloomberg's campaign would not comment on Senator Warren's remarks, but at his first campaign event on Monday in Norfolk, Virginia he defended using his wealth to underwrite his candidacy.

"For years I've been using my resources for the things that matter to me," Mr Bloomberg said, according to a video posted by PBS. "I am going to make my case and let the voters who are plenty smart make their choice."

Bloomberg joins crowded field

Mr Bloomberg's late entry into the race, less than three months before the Democrats' nominating contests begin, reflects his concern that none of the 17 other candidates vying to take on Republican President Donald Trump in next November's election can beat him.

Despite Senator Warren's status among contenders in polls, moderates like Mr Bloomberg fear her planned costly expansion of government programs will alienate voters in battleground states.

At the same time, some Democrats have been unnerved by an uneven campaign performance from former vice-president Joe Biden, 77, while another leading candidate, South Bend Mayor Pete Buttigieg, 37, is seen in some quarters as too young and inexperienced.

At a campaign event in Creston, Iowa, Mr Buttigieg declined to comment on Mr Bloomberg's candidacy and his plan to skip early-voting states in favour of competing in larger nominating contests later in the calendar.

But Lori Hays, 59, a Buttigieg supporter from Creston, said Mr Bloomberg was making a mistake.

"As an Iowan, it makes me feel like they don't care about Iowa," Ms Hays said. "To me, Bloomberg thinks he can buy the White House with his billions of dollars and that we Iowans don't matter."

Reuters