They're flooding the airwaves with campaign ads targeting Donald Trump. And they're paying for them with their own money.

Billionaires Mike Bloomberg and Tom Steyer have invested hundreds of millions of dollars in their efforts to win the Democratic Party's presidential nomination. And so far, it seems like that strategy is working. 

Bloomberg -- who skipped the Iowa caucuses and New Hampshire primary -- is mounting in nationwide polls, while Steyer is gaining ground in South Carolina, a key state that votes later this month.

But some of their rivals are crying foul, accusing them of corrupting the party's contest for the right to take on Trump in November.

"He's part of the problem," frontrunner Bernie Sanders said in an interview with SiriusXM radio on Sunday. 

"Look: Bloomberg -- anybody else in America -- has the right to run for president, but I think in a democracy, you do not have the right to buy the presidency."

- Flood of ads -

From when he entered the White House race in November until end of 2019, the 77-year-old Bloomberg, a media mogul and former New York mayor, spent about $200 million of his own money on ads, his campaign told AFP.

According to tracking firm Advertising Analytics, he spent more than $300 million through early February.

"It's completely unique. There's been nothing like this in the history of American politics," Bill Sweeney, an expert on politics at American University, told AFP.

Bloomberg -- the ninth richest person in the world, according to Forbes with a net worth of more than $55 billion -- has shaken up the Democratic presidential campaign.

While most other contenders have been on the trail for more than a year meeting voters, the ad blitz by the one-time Republican has already had a serious impact.

Bloomberg is now running third in nationwide polls behind Sanders and former vice president Joe Biden, according to an average compiled by RealClearPolitics.

Of course, polls at this stage of the game are to be taken with a grain of salt, because the race really is a state-by-state affair. But even on that level, Bloomberg is investing, building a vast grassroots staff and setting up campaign offices. Some of his rivals cannot afford to match such a presence.

Bloomberg is looking ahead to Super Tuesday on March 3, when voters in 14 states will cast their ballots. He will also formally skip the Nevada caucuses and the South Carolina primary.

- 'Absurd' -

Trump, himself a New York billionaire, has also hit out at Bloomberg, saying on Tuesday that the ex-mayor is "just buying his way in."

Sweeney says that the Republican president spent $60 million of his money when he ran for his party's nomination in 2016. 

But Bloomberg's rivals are leaning into the issue.

"It really is absurd that we have a guy who is prepared to spend, already, many hundreds of millions of dollars on TV ads," said Sanders, a US senator from Vermont.

Liberal Senator Elizabeth Warren chimed in, also accusing Bloomberg of "buying" his way into the next televised debate, after a controversial rule change will probably allow him to qualify.

Bloomberg's own team defends his big spending as an easy way to ensure his integrity.

"Unlike everyone else running for president, Mike Bloomberg has never taken a cent in campaign contributions from special interests or anyone else," campaign spokesman Stu Loeser told AFP.

"Mike also gives away most of his money through philanthropy to try and help people live longer and better lives."

Bloomberg is active in the fight against climate change, and the scourge of gun violence in America. He also gave $1.8 billion to his alma mater Johns Hopkins University in 2018 for student financial aid.

- What do voters think? -

Beyond Bloomberg, Steyer -- a onetime hedge fund manager -- is also in the Democratic race. 

The 62-year-old Californian is worth $1.6 billion, Forbes says. He's already spent tens of millions of dollars on ads in early voting states.

In South Carolina -- where he has shelled out $19 million, according to CNN -- he is running in second place according to a polling average. For now, Biden is still the frontrunner in the state.

Steyer has also backed up the ads with lots of time campaigning on the ground, sending teams of surrogates and his wife to rally voters in the state. "In the United States, a candidate can spend his or her own funds. And there's no restriction on that," Sweeney notes.

"The ultimate in all of this is the voters. There are plenty of examples of very wealthy people who spent millions of dollars and lose because the voters don't agree."

Steyer knows that all too well.  After spending $18.4 million in New Hampshire, according to Advertising Analytics -- to $5.3 million spent by Sanders -- he only ended up with 3.6 percent of the vote.

That translates to about 10,700 voters, or more than $1,600 spent on each one he won over.