When Donald Trump announced his bid for president, most Republicans laughed off his candidacy. When he began topping national polls, these skeptics urged patience; after Labor Day, their thinking went, the businessman’s campaign would collapse under the weight of his ego and bombast.

Now, after 100 days with Trump atop national and state polls, and with no indication that he is going away, the rest of the GOP field and party officials face a new, stark reality: Trump might win, and Republicans have no plan to take him out.

In an email this week obtained by RealClearPolitics, Republican strategist Alex Castellanos laid bare the GOP’s new normal.

“I’ve resisted the idea that Donald Trump could and would become the Republican nominee,” he wrote. “Unhappily, I’ve changed my mind.”

“In my experience,” Castellanos added, “the odds of Trump’s success have increased and been validated in the past few weeks.”

That candid assessment, increasingly shared by many within the party, seems at odds with the apparent lack of urgency shown by the other top-tier Republican presidential campaigns. In an interview published this week by Bloomberg News, Jeb Bush’s longtime confidant Mike Murphy, who leads the pro-Bush Right to Rise super PAC, insisted that Trump is “dead politically.”

“He’ll never be president of the United States, ever,” Murphy said.

Many campaigns and candidates thought the same thing throughout the summer. But as the calendar turned to fall, any assuredness that Trump would self-destruct has mostly evaporated among Republicans. Donors are growing nervous, as are the campaigns. But there is no consensus about what action should be taken in response.

Bush has been the highest-profile GOP candidate to aggressively and consistently take on the real estate tycoon, which seems to signal that his campaign now takes Trump seriously as a threat and not just an irritant. Bush has sought to question his opponent’s conservative credentials, and has tried to cast doubt on his qualifications for the presidency.

During the second Republican debate last month, Bush caught Trump in a lie about having lobbied for casino gambling in Florida, which Trump denied. More recently, the two men have clashed over whether President George W. Bush bore responsibility for the 9/11 terrorist attacks, with Bush releasing a Web video accusing Trump of foreign policy naivete.

But Bush has not benefited from these attacks: While the frontrunner has remained atop the polls, the former Florida governor’s share of support has dropped. Meanwhile, Bush’s camp has not put money behind these attacks in the form of ads to try to amplify their message.

With Republican campaigns either failing to gain traction against Trump or not taking him on directly, it might seem reasonable to expect that outside forces in the party, particularly donors, would step up to fill the void. But, for the most part, that has not happened.

One exception has been Club for Growth, which spent $1 million on anti-Trump advertising last month in Iowa and has hopes to raise more. Separately, former South Carolina GOP chairman Katon Dawson has reached out to Republicans to gauge interest in an anti-Trump super PAC, although it is unclear if this effort ever got off the ground. Dawson did not respond to requests by RCP for comment.

These efforts, however, have done little to diminish Trump’s standing in the polls.

Meanwhile, Republican networks with more resources to take on Trump have not yet moved to do so. The Koch brothers’ network of donors, though mulling whether to throw support behind one candidate, does not appear poised to actively oppose anyone in particular. Despite the continuing evidence of Trump’s strength, many donors within the network still believe his campaign will eventually collapse.

And even if there were to be any organized effort to take on Trump from outside the official campaign framework, there is no model for what that would look like. The most likely first steps would be donors pooling money for focus groups and polling, to test what messages might work against Trump.

For now, little is happening, adding to the confusion and concern among establishment Republicans. And if attacks were to be employed, there is little certainty that voters would move away from supporting the “outsider” candidates who have surged in this election cycle.

The bottom line for now is that, without donors or outside groups lining up to take down Trump, the onus is on the candidates to do so themselves -- or not.

"Ultimately, this is not something that Republican candidates can completely outsource to another entity. It's really something the candidate himself has to undertake,” said Eric Fehrnstrom, a former senior adviser to Mitt Romney’s 2012 campaign.

But it sometimes seems as though the rest of the GOP field is operating in an alternate election universe where Trump isn’t the frontrunner leading every poll in every state as well as nationally; where Marco Rubio needs to fell Bush, and vice versa; where Ted Cruz is the favored conservative alternative to the would-be establishment choice. And few of the lower-tier candidates appear poised to drop out soon, which means that the non-outsider-candidate vote remains splintered.

In his interview with Bloomberg, Murphy imagined a primary wherein the Republican nominee is someone who does not win any of the first four voting states. But history does not favor such an outcome.

“It is very difficult to imagine a scenario where someone who does not win one of the first four races is still a serious candidate after the first four races,” said Stuart Stevens, the former chief strategist for Romney. He pointed to the momentum a candidate earns in the media once they have won, which carries them on to the next primary.

In his email, Castellanos predicted that the “most likely” scenario today would be for Trump or Ben Carson to win in Iowa, followed by Trump winning in New Hampshire. In either scenario, Trump would be poised to sweep up the nomination: No Republican has become the standard-bearer without winning Iowa or New Hampshire.

“Time is running out,” Castellanos warned. “This race is solidifying and there isn’t much time left for it to change.”

Fehrnstrom echoed that assessment: “We’ve got 90 days for any one of these middle-of-the-pack candidates to make a strong run for the top. If they can’t do that and there’s no breakout, I think the establishment is going to have to prepare for a Trump nomination.”