The Club for Growth, a deep-pocketed conservative outfit opposed to Trump’s candidacy from its start, is leading the push against the billionaire frontrunner. Their plan, first previewed this week in a Washington Examiner column, is to blitz early states with ads highlighting what they consider Trump’s conservative apostasies. The group aims to draw support from major Republican donors increasingly alarmed at his staying power atop the polls as the first primary events swing into view, Club for Growth president David McIntosh tells Fortune.

But Trump, with his trademark bluster, says he welcomes the fight, vowing in an interview with Fortune to hit back “10 times harder.”

“This is what they do,” Trump said of the group. “They raise money for their own good in order to keep their salaries coming.”

The Club intends to open by elevating a relatively obscure topic: Trump’s support for eminent domain, the principle that allows the government to seize private property to make way for redevelopment. The group tested the attack last month in Iowa, spending more than $1 million on a pair of ads there. One of them highlighted Trump’s endorsement of the Supreme Court’s 2005 Kelo decision, which broadened the government’s ability to displace homeowners for commercial projects.

McIntosh says the ads, sponsored by the Club’s Super PAC, took a bite out of Trump’s Iowa support, pointing to his subsequent dip in the polls there. The latest Iowa survey — released Thursday by Quinnipiac — shows him trailing retired neurosurgeon Ben Carson by 8 points. Determining the ads’ precise impact is tricky, though. The spots coincided with Trump’s wobbly performance in the second Republican debate, which led to a steep drop in his national standing.

But Trump has rebuilt a commanding national lead over the last month. He now claims backing from nearly a third of registered Republicans, according to a Washington Post /ABC News poll released Wednesday and which Trump cited in the interview. In the survey, 42% of respondents said they expect him to win the nomination. That same sentiment is creeping through the ranks of the GOP’s establishment donors and operatives, some of whom dread that, without a well-funded intervention, Trump’s march will soon prove unstoppable.

“Political leaders and money-raisers and donors were thinking Trump will go away; he’s so ridiculous he’ll implode,” McIntosh said. “I think they’re starting to realize, especially as he begins talking about putting his own money into advertising and building a field operation, this guy now thinks he’s got a shot. We may need to spend resources to stop him.”

The eminent domain debate may seem like a hopelessly esoteric way to go about it. But McIntosh is convinced it marries two messages that resonate with small-government types—first, that Trump is in fact a liberal on key free-market questions, and second, that he’s leveraged government to enrich himself in real estate deals, making him as corrupt as the politicians he rails against.

Yet Trump doesn’t shy from his support for the government’s right to intercede to help move big real estate and infrastructure projects forward. “Nobody loves eminent domain, other than without it, you won’t have highways, you won’t have roads, you won’t have economic development, jobs,” Trump said, in a phone interview from his plane, as he prepared to fly to Iowa for a rally. Specifically, he pointed to the Keystone Pipeline, the proposal to link Canadian tar sands to Gulf Coast refineries that’s become a partisan flashpoint. “Without eminent domain, you wouldn’t build 20 feet of it, let alone 800 miles or whatever the length is,” Trump said.

Club spokesman Doug Sachtleben argued that the example muddies the issue, since government has long wielded the authority to make room for major public works projects. What enrages conservatives about Kelo is its expansion of that power for narrow commercial gain.

No matter. Trump, by his own admonition, is a counterpuncher. “They hit me, I’m going to hit each one of them 10 times harder,” he said. And he has a ready reply for any Club-issued attack. The group destroyed its credibility as a Trump critic, he says, when McIntosh sent him a letter two weeks before he announced his candidacy, asking for a $1 million contribution. “We informed them that we’re not interested and shortly thereafter they came out very viciously against me,” Trump said.

The Club presents a different version: That Trump asked them to put the request in writing, and while they accept donations without bias, they’ve been consistent on Trump’s White House ambitions, having written in a 2011 editorial that he is “a chameleon who’ll say anything to get attention, but not a serious candidate for the Presidency.” And it’s true that the Club — which its origins to a collection of Wall Street supply-siders looking to exert greater influence in Republican politics back in the 1980s — has tangled with Trump on taxes, trade and entitlements.

McIntosh figures it will cost up to $15 million to mount an anti-Trump campaign with a meaningful impact—roughly $5 million in Iowa and $10 million in New Hampshire. Of that amount, he ballparks the Club should raise about $2 million while other groups can supply the balance. “We don’t have a budget for it at this point,” he said.

Meanwhile, Trump says he’s ready to spend his own money to respond in kind. “I have a $605 million cash flow this year. Do you think this matters to me? I want to win, and I want to make America great again,” he said, adding that he’d planned to spend $25 million on ads to date but hasn’t needed to, thanks to blanket media coverage of his campaign.

Trump also said that anyone considering investing to take him down so a more traditional candidate can rise needs to consider their priorities in such a crowded GOP field. “The problem they have is this. Let’s say the other guy spends a couple of million bucks…. They gotta go through more than me. They’ve got other people in their way,” he said, “They’re much better off spending the money on themselves and trying to build up their image. Because a lot of the images are so bad. If they spend the money on me, they’re still not going to go anywhere, because they need recognition for themselves.”

That line of thinking by Republican moneymen could hamstring an effort like the one the Club aims to organize. Fred Malek, finance chair of the Republican Governors Association, said the field has to narrow and a clear Trump alternative needs to emerge for the current frontrunner to fade. “It’s going to be gradual, and it’s not going to be as the result of any concerted effort,” he said. And any campaign to force it could backfire. “A concerted effort across a group of establishment organizations or quasi-establishment organizations, I think, is going to breed resentment. I mean, ‘Who the hell are you rich guys to tell us what to do?’”

Fred Young, a conservative donor from Wisconsin who contributed $100,000 to the Club in August, describes himself as favoring “anybody but Trump.” He said he’s reserving judgment on the group’s campaign until he hears their pitch.