File photo: CGTN

The billionaires vying to become the Democratic candidates for the US presidential election have spent nearly 78% of total ad buy, NPR reported on Thursday.

Former New York City Mayor Mike Bloomberg and activist business executive Tom Steyer outspent their rivals with a combined $320 million. 

Yet their huge investment has not yet successfully helped them break into the quartet of front-runners: former vice president Joe Biden, Vermont Senator Bernie Sanders, Massachusetts Senator Elizabeth Warren and former South Bend, Indiana, Mayor Pete Buttigieg,

NPR reports the data from Advertising Analytics “captures the massive spending disparities in the field of current candidates.”

The top four candidates are neck-and-neck in polls for the Iowa caucuses. Steyer has spent the most there, followed by Buttigieg's $8 million on advertising in Iowa. Sanders isn't far behind, with $7.2 million spent. Warren spent $3.9 million and Biden spent $2.9 million.

Steyer, a former hedge fund executive and environmentalist, put millions into ad blitzes in early states. It makes him the top spender in both Iowa (over $12.1 million) and New Hampshire (more than $15.9 million). 

That's been eclipsed by Bloomberg, who's spent about $196.2 million since he entered the race in late November. $163.8 million of that has been on broadcast TV, with a substantial $22.1 million investment in digital and $10.2 million on cable.

So far, their gamble however has paid few dividends.

(Compiled by Bai Yuanqi)