Well-heeled political donors are learning the hard way that their wealth is proving little match for Donald Trump.

Super-political action committees and other outside groups have spent about $40 million so far on ads, mailings and other efforts to derail the real-estate mogul's bid for the Republican presidential nomination, according to Federal Election Commission records. Nevertheless, Trump remains the clear front-runner.

Efforts to oppose Trump escalated last month after a string of victories across the south gave him a formidable lead. One of the groups, Our Principles PAC, raised $8.4 million in March—more than it had in the two previous months combined—to go after the front-runner. The money, which has been largely spent attacking Trump, wasn't enough to keep him from a victory in his home state of New York this week. Now he's the only candidate who could still win enough delegates to lock up the nomination before the party's convention in July.

Some of the Republican Party's most prolific donors are behind the effort, which also includes Club for Growth Action, a super-PAC. Joe Ricketts, founder of TD Ameritrade, and his wife, Marlene, together have given $5.5 million. Investment banker Warren Stephens and his brother, Jackson Stephens, have combined to give $7.1 million. Richard Uihlein, chairman and CEO of packaging supply firm Uline, contributed $2 million. Paul Singer, the hedge fund manager and moderate Republican, has put in $1.5 million. The two super-PACs have spent a combined $21.9 million attacking Trump, FEC records show.

Attacking Trump

“The anti-Trump efforts have been quite effective,” William Oberndorf, a San Francisco investor, said in an e-mail, pointing to the group's work in helping John Kasich win Ohio and preventing Trump from locking up the nomination early. Oberndorf, who's given $750,000 to Our Principles through March, said he's now hoping for Kasich to prevail at the convention with Florida Senator Marco Rubio as his running mate. He called Ted Cruz “as dangerous and unelectable as Trump.”

Clifford Asness, the libertarian founder of hedge fund AQR Capital Management in Greenwich, Connecticut, gave $1 million to Our Principles in early March. Later in the month, he wrote in an op-ed online that he's long opposed Trump for his positions on immigration and free trade, but that he was rethinking his opposition after his father, a Trump supporter, died of a brain tumor.

“I can’t dismiss Trump as easily as before,” Asness wrote on March 14.

Our Principles and Club for Growth are just two of a number of big-money groups ganging up on Trump. American Future Fund, a conservative nonprofit that doesn't disclose its donors, spent about $4.2 million on anti-Trump ads in Florida, and about $6.6 million overall opposing Trump.

Super-PACs backing other candidates have also attacked Trump. The groups supporting Cruz and Kasich have spent more than $3.1 million against Trump. The super-PAC backing Rubio spent $4.8 million on the effort before he dropped out of the race last month.

The groups have seen some signs of success. Our Principles spent $1.3 million in Wisconsin before its April primary, helping lift Cruz to victory.