Republican operatives have told major donors it would require an eight-figure advertising campaign or campaigns to make any kind of dent in Trump’s surprisingly durable popularity. While many of the donors have privately voiced support for the cause, most have begged off writing big checks.

The reasons for their reluctance, according to interviews with donors and operatives, seem difficult to overcome at this stage of the race, particularly if Trump notches a few more primary and caucus victories.

The donors cite the lack of success of the few super PAC attacks that have already targeted Trump as evidence that such attacks have not ― and cannot ― halt his momentum. And they worry that, if they fund higher-profile attacks, they could come under attack from Trump, who this week fired a warning shot at one of the few major donors to the anti-Trump efforts, Marlene Ricketts, tweeting that her family “better be careful, they have a lot to hide!”

They’ve also concluded that big-money attacks could play right into Trump’s hands, reinforcing his claims that he’s independent from donors and moneyed interests who he claims control his opponents. And, if the attacks do get traction, they could ultimately help Democrats in the increasingly more likely event they find themselves pitted against Trump in the general election.

There’s a somewhat circular nature to donors’ trepidation about anti-Trump attacks, pointed out veteran GOP operative Katie Packer, who last month launched the anti-Trump Our Principles PAC.

“People weren’t sure he was a real threat ― they thought nobody was attacking him so he must not be,” said Packer, a former top adviser to Mitt Romney’s 2012 campaign. Now, though, “people are starting to change their tune,” she said.

Her super PAC in January received $3 million from Ricketts, the wife of billionaire TD Ameritrade founder Joe Ricketts, as well as a couple of other small donations, according to a report filed Saturday with the Federal Election Commission. It raised at least another $1 million this month, according to FEC expenditure reports. They show that the group has spent a total of nearly $4 million going after Trump in direct mail pieces, online ads, phone calls and TV and radio ads, most of which highlight the candidate’s earlier embrace of liberal positions on taxes, abortion rights and other issues that don’t square with the conservative positions he’s embracing now.

While Packer argued that those efforts helped increase Trump’s negatives in Iowa and South Carolina, she said the super PAC was limited by time and money. The group has experienced an uptick in interest from donors in the last few days as a result of Trump’s lopsided victory in Saturday’s South Carolina primary, she said. But she conceded that has yet to yield many new major donations, adding that conversations with donors alone “don’t buy TV ads.”

Another conservative group that has targeted Trump, the anti-tax Club for Growth, has had mixed results as it has stepped up its aggressive appeals to major donors for a new anti-Trump advertising campaign launching in states that vote in the March 1 Super Tuesday contests.

The group brought in a major new donor for its anti-Trump campaign this month, according to sources familiar with its effort. But they say it also has been turned away by donors skeptical of the impact of its $2.7 million in reported anti-Trump spending to date, including an ad campaign launched in September blasting Trump as a big-government liberal.

“There are a lot of donors who would like to do more, but they are going to have to regroup and find out what message works. Every time you go against him, it just makes him stronger,” said Frayda Levin, a Club for Growth donor and board member. She attended the group’s board meeting last week in Florida, where Club officials sought donations for the new anti-Trump effort and made the case that their earlier anti-Trump ads had an impact.

“The Club believes that their ads in Iowa were one reason why Trump did not win there, and they believe there are enough marginal voters that the ads will have impact” elsewhere, she said. But she acknowledged “we’re running against the clock.”

GOP strategist Liz Mair, whose anti-Trump Make America Awesome super PAC has raised all of $10,000 since it was created in December, said major donors are shying away from her group partly because they are scared of incurring Trump’s wrath. He has already threatened legal action against conservative groups that have advertised against him, including the Club for Growth (which, he alleged in a Tuesday tweet “came to my office seeking $1 million dollars. I told them no and now they are doing negative ads), and has called out conservative billionaires who he unsuccessfully courted (includingthe Koch brothers, Las Vegas casino mogul SheldonAdelson and New York hedge fund titan Paul Singer).

“We would totally donate to you if we could do it anonymously; we’re worried about Trump taking reprisals against us for donating to this,” Mair said, parroting reactions she’s heard from donors. “Suffice to say, there are a lot of people out there who want to stop Trump and are willing to donate to do it,” she said. “They’re just the rank and file of the base, not the establishment donors.”

The Ricketts family doesn’t seem at all intimidated by Trump’s swipe at them, said a Republican fundraiser who chatted with Ricketts family members at a Monday fundraiser in Chicago for Liz Cheney’s congressional campaign.

Adelson, one of the most coveted GOP donors, has rejected entreaties from Packer’s super PAC and other groups seeking cash to oppose Trump, according to a source close to the casino billionaire. Adelson would seem a natural funder for such efforts, since he distrusts Trump’s commitment to Israel’s defenses, which is the mogul’s animating issue.

But the source close to Adelson explained “right now everything is negative, and he doesn’t want to be involved in that.” In fact, as POLITICO reported Monday, Adelson isn’t planning to write big checks to any groups playing in the presidential race until after the GOP primary is settled. “That’s the lesson of 2012, when his money was spent attacking Romney. He’s not interested in attacking any Republicans, no matter who they are.”

Adelson’s feelings are common among GOP donors and operatives, according to a POLITICO analysis of FEC filings. It found that only 4 percent of the $238 million in advertising aired by big-money groups so far has targeted Trump.

It did find that the attacks on Trump by conservative groups had increased somewhat since he began cementing his status as the GOP frontrunner in November. Before then, the only groups reporting spending money primarily to oppose Trump were the Club for Growth and a pair of liberal super PACs (both of which support Democratic presidential frontrunner Hillary Clinton).

Overall, super PACs and other outside groups have spent $9.5 million opposing Trump, according to the filings. That makes him the most-targeted presidential candidate, ahead of his GOP rival Marco Rubio (who was targeted by $8.1 million in spending) and Democratic frontrunner Hillary Clinton ($7.6 million), according to POLITICO’s analysis of FEC filings. They detail individual advertisements and other so-called independent expenditures by super PACs and other outside groups supporting or opposing candidates.

Some major donors have expressed frustration at the relatively paltry spending on Trump attacks by the super PACs created to boost Trump’s Republican rivals. They had raised a combined $320 million through the end of January, but have spent only $2.1 million on ads that primarily target Trump, according to the analysis.

A particular source of exasperation was Right to Rise USA, the super PAC created to support the since-suspended campaign of Jeb Bush. While Bush repeatedly assailed Trump during debates and on the trail, Right to Rise, which raised an astounding $119 million, reported spending only $25,163 on independent expenditures in which Trump was the primary target. That was the lowest amount Right to Rise spent targeting any candidate, and it paled in comparison to the $1.9 million in attack ads the group unleashed against Marco Rubio, Bush’s protege-turned-presidential-rival.

Some of the few anti-Trump efforts that Right to Rise did launch were mocked as silly and ineffective, including a billboard calling him “unhinged.”

The super PAC ― which spent the overwhelming majority of its money, $78 million, supporting Bush in vain ― is believed to have more than $10 million left in the bank. And while some donors have urged the group to spend its remaining cash trying to try to take down Trump, sources in GOP finance circles said the group is more likely to refund the cash to donors.

Part of the reluctance among GOP candidate-allied super PACs to attack Trump stems from uncertainty about who would benefit if the attacks were effective. None of the remaining candidates has been able to successfully position themselves as the leading alternative to Trump, despite jockeying by Rubio, Ted Cruz and John Kasich to claim that mantle.

“You have to spend money going after your real competition,” said Houston private equity investor Fred Zeidman, a major Bush supporter who donated to Right to Rise. The debate among donors was “do we go after the front runner or do we go after the front runner in our lane,” Zeidman said. He explained that his preference, generally, would be to avoid spending super PAC money in GOP primaries. “I’m just not supportive of using them to bash each other, and to save the other side money on oppo research.”

The reluctance to wade into a competitive primary is keeping both the Koch network and the Crossroads groups from dipping into their coffers to try to take down Trump.

At its winter gathering of major donors a few weeks ago in Indian Wells, Calif., Koch network operatives discussed the possibility of launching an anti-Trump campaign and presented focus group results on different possible approaches, according to sources who attended. The effort would have been a major departure for the network, which intends to spend $889 million on policy and political advocacy in the run-up to the 2016 election. While it has increased its political spending over the years, it’s largely avoided primaries, instead focusing its efforts on issue-based attacks on Democrats.

Even though many Koch network donors support Rubio (whose campaign this week landed a top Koch operative) and were mostly supportive of the prospect of taking on Trump, sources familiar with the network’s planning told POLITICO it has concluded its resources would be better spent elsewhere.

Asked Tuesday whether there was a chance the network might still go after Trump, James Davis, an official at the central group in the network, Freedom Partners Chamber of Commerce, said “We do not have any plans to engage in the presidential primary.”

Likewise, the Crossroads groups, which have so far reported spending $113,000 attacking Clinton, intend to sit out the increasingly bitter GOP primary fight, according to spokesman Ian Prior.

“American Crossroads’ focus will continue to be on Hillary,” he said.