Republican insiders are sounding increasingly panicked about Donald Trump, but few of them seem willing to do anything to stop him.

That's the takeaway from a pair of great new reports by the Washington Post's Matea Gold and Robert Costa and the New York Times's Jonathan Martin.

According to these reports, some still expect Trump's campaign to collapse, and some are just hoping others will pick up the tab for an anti-Trump effort. But one of the main reasons elites are so hesitant to challenge Trump is a quite justified fear that Trump will viciously attack anyone who does so.

Indeed, Trump's explicit strategy of intemperately striking back against anyone who attacks him first — often with total disregard for factual accuracy — has been such an effective deterrent that even the Koch brothers are shying away from challenging him, according to Martin:

Two of the most potent financial networks in Republican politics, that of the hedge-fund billionaire Paul Singer and another led by the industrialists Charles G. and David H. Koch, have each had preliminary conversations about beginning an anti-Trump campaign, according to strategists involved. But Mr. Trump has already mocked Mr. Singer and the Kochs, and officials linked to them said they were reluctant to incur more ferocious counterattacks. "You have to deal with Trump berating you every day of the week," explained a strategist briefed on the thinking of both groups.

Consider that for a moment. Charles and David Koch are the sixth- and seventh-richest people in the world, according to Forbes. Each has a fortune far larger than Trump's own. And it's not like they have political careers to protect — they're basically untouchable.

But even the mighty billionaire Koch brothers are afraid to pick a fight with Trump. They don't want to deal with his attempts to assassinate their character — they don't need the headache.

Their fears are well-founded. Back when the Club for Growth began airing anti-Trump ads back in September, the billionaire repeatedly attacked the group as "pathetic," "phony," and "a scam operation." And he's been very clear that anyone who follows the club's lead will face the same fate.

There's still time, of course, for this to change. Republican operative Liz Mair has started a company called Trump Card LLC that is trying to raise money for an anti-Trump campaign. A pro–John Kasich Super PAC has also said it will spend $2.5 million on anti-Trump ads.

But with voting starting in Iowa a mere two months from today, the clock is ticking. Head over to the Post and the Times for the full reports.