Donald Trump has an ace in the hole for dealing with attacks from his Republican rivals

After Sen. Rand Paul (R-Kentucky) released a video criticizing real-estate magnate Donald Trump for his past allegiances with Democratic candidates, Trump had a familiar response at the ready.

"Recently, Rand Paul called me and asked me to play golf. I easily beat him on the golf course and will even more easily beat him now, in the world in the politics," Trump said in a statement.

Then he added: "Senator Paul does not mention that after trouncing him in golf I made a significant donation to the eye center with which he is affiliated."

Republican candidates are learning the hard way that Donald Trump has an ace-in-the-hole when they come after him. For one thing, they've probably dealt with him in the past. And there's a good chance they've asked him for money, too.

Since announcing his presidential campaign earlier this summer, Trump has traded barbs with many of his GOP rivals over immigration, foreign policy, and his conservative credentials.

But rather than engaging with them in detailed policy debates, the real-estate magnate has instead revealed embarrassing anecdotes about his personal relationships with the candidates, and in doing so, provided a somewhat unprecedented level of transparency into the interactions between politicians and the ultra-rich.

Before handing out Sen. Lindsey Graham's (R-South Carolina) personal cell-phone number last month, for example, Trump told a crowd that Graham sought a donation from Trump and was "begging" Trump to mention his name during an appearance on Fox News a few years ago.

"He said, 'Could you mention my name?' And I said, 'Yes, I'll mention,'" Trump told a crowd during a rally in Graham's home state.

Trump has also been blunt about how political donations have helped him gain power and credibility with various candidates from both parties over the years.

At the first Republican presidential debate last week, Trump was questioned about past donations to Democratic candidates, including current Democratic presidential front-runner Hillary Clinton when she ran for a seat in the US Senate.

But Trump maintained that the donations were essential to fostering good relationship with members of both parties. He bragged during the debate that Clinton had to attend his wedding because he gave so much to her campaign, and then he noted that he'd given money to many of his current Republican rivals in past elections.

"I give to many people. I give to everybody, when they call I give, and you know what? When I need something from them, two years, three years later, I call, they are there for me," Trump said.

Several Republican candidates have been forced to preface attacks on Trump with acknowledgments that they had once sought out the reality television star's company in order to get cash.

Trump has given money to Wisconsin Gov. Scott Walker (R), former Florida Gov. Jeb Bush (R), former New York Gov. George Pataki (R), Sen. Ted Cruz (R-Texas), and former Arkansas Gov. Mike Huckabee's (R) super PAC, according to a CNN review of federal campaign-finance records.

The real-estate magnate and his team have also been unafraid to comment on behind-the-scenes meetings and phone calls with influential political figures.

He disclosed an embarrassing meeting with a pollster who conducted a focus group that was critical of Trump's debate performance. He commented on a "congratulatory" phone call between himself and Republican National Committee Chairman Reince Priebus. And sources close to him appeared to reveal details of a conversation between Trump and former President Bill Clinton weeks before his campaign launch.

The real-estate magnate is revealing these to make a point: He won't be bought like the other politicians who will make or break relationships based on when its convenient for them. Even his critics think he has a point.

"He has clearly identified the problem," said Lawrence Lessig, the Harvard professor who is considering running for the Democratic nomination. He quickly added: "What he has not done is offer a solution."

Brett LoGiurato contributed reporting.