Kenneth P. Vogel writes in Politico:

Some of the biggest Republican donors, who collectively have contributed tens of millions of dollars to shape the presidential race, are tightening their purse strings out of frustration with their inability to boost their favored candidates, or to slow Donald Trump.

Rather than continuing to write huge checks to support the cluster of establishment candidates jockeying to emerge as the leading alternative to Trump, a billionaire real estate showman roundly despised by the GOP elite, these donors have mostly retreated to the sidelines. They’re watching anxiously, hoping that the field sorts itself out, according to interviews with a half dozen major donors or their representatives.

Many of the donors are urging the deep-pocketed groups they’ve already funded to begin spending against Trump, even as some recognize the potential for such spending to backfire, and are increasingly questioning the efficacy of big-money advertising campaigns more generally.

“It’s unbelievable. I mean Jeb Bush spent $42 million in New Hampshire ― what did he get for it?” said billionaire GOP mega-donor Stan Hubbard, a Minnesota media mogul. “It’s frustrating.”

[…]

Several donors expressed frustration that the establishment candidates and the super PACs supporting them spent considerable time and money attacking one another ― and not Trump ― in the run-up to the Iowa caucuses and New Hampshire primary.

Perhaps as a result, many donors who supported candidates who have since dropped out appear inclined to wait until after the Feb. 20 South Carolina primary to pick a new horse.

Wisconsin roofing billionaire Diane Hendricks, who gave $5 million to a pro-Walker super PAC and is being hotly courted by supporters of both Rubio and Ted Cruz, intends to wait until the cluster of primaries and caucuses on March 1 ― Super Tuesday ― before backing another candidate, said sources familiar with her plans.

[…]

GOP mega-donor John Jordan, who decided to support Rubio after Walker ― his first choice ― dropped out, predicted that one of the remaining establishment candidates would emerge from the pack on March 15, when a handful of delegate-rich states hold primaries.

“One of them will do better than the other, and it will be impossible for the relative loser to make the case to donors that he should continue,” he told the Associated Press. Pointing specifically to Bush and Rubio ― the former governor and current senator, respectively, from Florida, which is among the March 15 states ― he said “donors will simply move to whoever wins that state, and it will happen nearly instantly.”

But some donors worry that, by then, Trump might be unbeatable.

They’re looking for one of the existing deep-pocketed outside groups to take action sooner to halt Trump’s rise, with several donors specifically citing the political operation helmed by the billionaire industrialists Charles and David Koch.