Among the many things Donald Trump has accomplished in his seemingly unstoppable quest to become president of the United States has been to unite an otherwise disparate group of wealthy conservative donors. One of the only things these millionaires have in common is how much they want to prevent Trump from becoming their party's nominee.

A small group of extremely rich Republicans have given millions of dollars in recent weeks to various super PACs with the sole purpose of stopping Trump, according to the most recent campaign finance filings that were released last week.

The individuals that have spent the most money trying to stop Trump are members of the incredibly wealthy Ricketts family from Nebraska. Since the beginning of this year, Marlene Ricketts, who is married to TD Ameritrade founder Joe Ricketts, and their son Todd, have given $5 million to Our Principles PAC, the largest group that is solely dedicated to thwarting Trump.

The Ricketts family, who also own the Chicago Cubs baseball team, have been longtime Republican donors. During the 2008 election, they were behind the effort to link Barack Obama to controversial comments made by his former spiritual adviser, the Rev. Jeremiah A. Wright Jr. Another of the Ricketts' sons, Pete Ricketts, is the Republican governor of Nebraska.

Our Principles PAC began running ads against Trump in January ahead of the Iowa caucuses. As Trump continued to decimate his rivals one by one in recent weeks, additional mega-donors have steadily flocked to the group.

Among its million-dollar donors are Warren Stephens, a billionaire Republican investment banker from Arkansas, and Paul Singer, a billionaire philanthropist and hedge fund manager from New York.

Singer is averse to the spotlight but has quietly become one of the biggest donors in the Republican political world. His politics span the ideological spectrum — he is a vocal supporter of gay and lesbian rights, a donor to Senator Marco Rubio's now-ended campaign, a close ally of Mitt Romney and has been described as a "passionate defender of the 1 percent" by Forbes magazine.

'If all of us join forces in a concerted effort to expose his record and his rhetoric, it is possible to stop him.'

Another super PAC dedicated to Trump's demise, Club for Growth Action, saw a spike in donations last month. The group has spent years raising and spending money for conservative congressional candidates, but has focused heavily on the "Stop Trump" movement in recent months. The Club for Growth's super PAC made its first presidential endorsement ever this week by formally backing Senator Ted Cruz.

The super PAC raised a record $4 million dollars in February, according to the group's FEC filings, up from just $1 million the previous month. Stephens, along with his brother Jackson, gave $2.5 million to the group. Richard Uihlein, a CEO from Wisconsin and longtime Republican donor, gave another $500,000 in February.

Club for Growth, which was described by Politico as the "pre-eminent institution promoting Republican adherence to a free-market, free-trade, anti-regulation agenda," has not wasted any time spending that money. Club for Growth Action unveiled a $1 million ad buy in Wisconsin yesterday attacking Trump and urging people to vote for Cruz instead in the state's primary on April 5.

"Donald Trump is the worst candidate in the Republican field, and he can't win in November," Club for Growth president David McIntosh said, according to the Milwaukee Journal-Sentinel.

It is a testament to Trump's candidacy that he has been able to bring together so many rich donors who, until now, had been supporting every other candidate in the Republican race besides him. Singer, the hedge fund manager, was backing Florida Senator Marco Rubio before the senator suspended his campaign earlier this month. The Ricketts clan was funding Wisconsin Governor Scott Walker's failed presidential bid last year. Stephens had also donated hundreds of thousands of dollars to super PACs backing Jeb Bush, Chris Christie and Rubio, before all of them dropped out. Uihlein has given $1 million to the super PAC supporting Ted Cruz, in addition to another $1 million to Club for Growth this year.

In a memo outlining Our Principles' plan to stop Trump, the director Katie Packer, harangued donors and other political operatives for not getting on the stop-Trump train sooner.

"In my nearly 30 years of political involvement, I have never seen a campaign where the frontrunner had no aggressive, strategic campaign launched against him," Packer wrote in late February. "In this election, Republicans have spent over $215 million in ads and voter contact. Just 4 percent ($9 million) of that [has been] spent to challenge Trump."

Packer's memo also stressed that such an effort could only work if there was cohesion from Republicans.

"If all of us join forces in a concerted effort to expose his record and his rhetoric, it is possible to stop him," the memo read. "If not, we will have the nominee we deserve."

Our Principles has been blanketing the airwaves with anti-Trump radio and television ads in upcoming primary states for the past month. They were responsible for one spot that ran before the March 15 contests, featuring comments that Trump has made about women over the years, including his sage advice that "you have to treat them like shit."

So far, these expensive efforts to attack Trump haven't made much of a dent. He now has nearly twice as many delegates as his closest rival, Cruz, and only needs to win about 55 percent of the delegates that remain up for grabs to clinch the nomination.

Trump's response to the concerted plan to take him down has been, predictably, to issue threats via Twitter. After he found out that the Ricketts family were spending money against him, Trump tweeted in February, "they better be careful, they have a lot to hide!"