Donald Trump proved yet again Tuesday that he is, as he would say, “a winner,” even as individuals and groups have spent tens of millions of dollars to stop him.

With results still coming in, it looked like the Republican front-runner was going to take home a majority of the delegates from Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island on Tuesday. The lopsided wins came after Trump’s opponents, Texas Sen. Ted Cruz and Ohio Gov. John Kasich, formed a temporary alliance this week in an effort to slow his progress and as Republicans who do not like Trump have continued to spend against him.

As of late last month, various groups had spent nearly $67 million to run more than 53,000 attack ads against Trump, CNN reported. In contrast, Trump spent a mere $9.5 million in February, while Cruz dropped $17.5 million and Democratic front-runner Hillary Clinton had $31.6 million in campaign costs.

The Stop Trump movement has seen people from across the political spectrum in recent months open their wallets for campaign ads directed at blocking his path to the White House. The effort has seen some success. Outside groups spent almost $1.7 million on TV ads in Wisconsin to try to keep Trump from winning the state earlier this month. Cruz ultimately won the state by a wide margin.  

But Trump by far remains the favorite among Republican primary voters. Ahead of Tuesday’s primary contests, Trump held a substantial lead in the delegate count and was polling well — sometimes above 50 percent — in Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island.

That hasn't stopped conservative leaders from trying to stall Trump. The leading anti-Trump super PAC, Our Principles, bought nearly $300,000 of advertising in Maryland over the weekend, CNN reported. Overall, outside groups spent more than $2 million opposing Trump in the last week, according to the Center for Responsive Politics.

The Stop Trump movement started off slowly last fall when the huge Republican field diverted many candidates’ attention. But super PAC spending against Trump skyrocketed in February and March as the field narrowed and Republicans grew more alarmed about the prospect of a Trump nomination.

“People are starting to panic,” Johanna Dunaway, a political science professor at Texas A&M University, told Time magazine in March. “So now, you see the efforts to try to stop his path to nomination.”

Right to Rise USA, a super PAC supporting former Florida Gov. Jeb Bush, spent at least $9.6 million on anti-Trump ads, including a rare two-minute ad with a montage of some of the Republican front-runner's most controversial moments. Conservative Solutions PAC,  a group supporting Sen. Marco Rubio of Florida, dropped $16.7 million in anti-Trump ads earlier this year, including more than $7 million in Florida, where Trump went on to beat Rubio in his home state by 19 points on March 15.  

The attack ads are direct. They call Trump as a "reckless blowhard," "impulsive" and a "phony conservative." One showed women reading the insults he has aimed at female celebrities and politicians over the years. Others highlighted his failed business endeavors.

The problem is they just aren't working.

"Between Trump’s rise and Jeb Bush’s fail is a cruel irony: In an election season in which candidates from both parties have decried the influence of money in politics, the rich who are pouring their money into super PACs don’t seem to be getting much bang for their buck," Slate wrote in March. 

Many donors and operatives involved in anti-Trump PACs have come from Republican candidates who dropped out of the race. Bush's former spokesman Tim Miller is now a top adviser to the Our Principles PAC, and billionaire Marlene Ricketts, who supported Wisconsin Gov. Scott Walker and gave to Rubio and Cruz, has given millions to Our Principles.

The spending push underscores just how nervous some establishment Republicans are about Trump becoming the nominee. The New York business mogul’s bombastic personality and his tendency to make outrageous statements could scare off general election voters in November, the thinking goes — costing the Republican Party not just the White House but a significant number of down-ticket races as well. His most controversial remarks, such as calling Mexicans rapists, proposing a ban on all Muslims and his many off-color comments about women have led critics to call him racist and sexist — not adjectives the Republican Party wants associated with its choice for the White House.

There’s also the criticism that Trump is not a true conservative. Before Trump ran for president, he supported Clinton and other Democratic politicians, and has been more open on topics like funding for Planned Parenthood and transgender rights than many of his GOP competitors. He also has advocated a more neutral approach to the conflict between Israel and the Palestinians. Both Cruz and Rubio, who dropped out of the Republican race last month, have criticized Trump for not being a conservative, and Katie Packer, who heads the Our Principles super PAC, told International Business Times earlier month that she doesn’t believe Trump is even a Republican.

The head of the U.S. Chamber of Commerce, which often sides with the GOP on national issues, spoke out against Trump and his proposals while visiting Mexico in December. In addition to wanting to ban all Muslims from the United States, Trump has promised to build a wall on the U.S.-Mexico border and force Mexico to pay for it. He also says he will deport the nearly 11 million undocumented immigrants living in the U.S.

During his visit to Mexico, Thomas Donohue, the head of the chamber, said Trump would not be president and that he was “entertainment, but he’s not leadership for the American people,” Bloomberg News reported

Trump released a plan earlier this month that detailed how he would force Mexicans to pay for the infamous wall by reducing the funds sent to Mexico through money transfers, commonly known as remittances. Various economists said the plan was not feasible legally or politically. But like the money spent on attack ads, such criticisms have done little to slow Trump's march toward the GOP nomination.

After Tuesday, the next Republican races will be May 3 in Indiana and then Nebraska, West Virginia, Oregon and Washington later in the month. Cruz and Kasich are each hoping to win some of those races, but if 53,000 attack ads haven't stopped Trump so far, it's hard to imagine what will.