Michael Bloomberg at the DNC. Alex Wong/Getty Images Over the last week, Democratic presidential nominee Hillary Clinton has campaigned with three billionaires: Michael Bloomberg last Wednesday at the Democratic National Convention, Mark Cuban on Saturday during a campaign event, and Warren Buffett on Monday.

Clinton is campaigning with the ultrarich for two reasons:

These three men can make the case, more credibly than almost anyone else, that Donald Trump is a bad businessman. It drives Donald Trump crazy to be criticized by people with more money than him.

Unlike Trump, Clinton's billionaires are self-made. Cuban has particular authority when he mocks Trump for having to "ask daddy for a small loan of a million dollars" when he started his career — and when he calls Trump a "jagoff."

Bloomberg, who has four times as much wealth as Trump claims to have, was especially cutting last Wednesday at the convention:

"Most of us who have our names on the door know that we're only as good as our word. But not Donald Trump. Trump has left behind a well-documented record of bankruptcies, thousands of lawsuits, angry shareholders, and contractors who feel cheated, and disillusioned customers who feel ripped off. Trump says he wants to run the nation like he's run his business. God help us."

The speech got under Trump's skin. The following day, Trump complained about a pro-Clinton speech from a "little guy" whom he wanted to hit "so hard his head would spin."

Buffett, the wealthiest of these three men, dragged Trump on Monday by focusing on the 1995 public offering of Trump's Hotels & Casino Resorts — ticker symbol "DJT," of course.

He described what happened after the IPO:

"The next ten years, the company loses money every year. Every single year. He takes out $44 million in compensation during that period. If you, in 1995, when he offered this company, if a monkey had thrown a dart at the stock page, the monkey on average would have made 150%. But the people that believed in him, that listened to his siren song, came away losing well over 90 cents on the dollar. They got back less than a dime."

"I don't know another businessman that brags about his bankruptcies," Buffett added, in reference to Trump's past self-praise for his own "tremendous" use of bankruptcy. "You know, to tell you the truth, why not? I mean, it's his claim to stardom."

Clinton, like Trump's opponents in the Republican primary, has sought to use Trump's business record against him, painting him as a fraud whose success is more reality television than business reality. It's been a hard case to make because Trump seems so rich. With that big jet and all of those marble-covered homes, how unsuccessful could he be?

It's a complex argument to make, but the best people to make it are those who are unambiguously more successful than Trump.

I expect to see Hillary's billionaires on the campaign trail a lot over the next three months, arguing that Trump isn't even good at business.