Donald Trump is beginning his sixth month atop the polls in the Republican presidential race. Voting in Iowa is less than 30 days away. If the much-vaunted GOP establishment, dead set against a Trump nomination, wants to stop him, time is running out.

Yet beyond condemning Trump and fretting among themselves that he could destroy the party, top Republicans, including the donor class, have mostly chosen not to confront Trump. Why?

There are several reasons. First, creating an organization and spending millions of dollars to carpet-bomb Trump with negative ads in key states isn't easy; there aren't many people who could pull it off. Second, some donors think an anti-Trump offensive not only would not work but would backfire on an already unpopular GOP establishment. Third, some who do believe it could work think it should not be attempted until Trump's critics have agreed on an alternative candidate — which they haven't. Fourth, the anti-Trump opposition can't decide who should lead such an effort. And fifth, most GOP strategists and money movers continue to believe Trump will ultimately fail on his own, that in the end he will not be the Republican nominee.

"The problem with organizing an anti-Trump effort amongst big GOP donors is the immensity of it," said a former Romney fundraiser in an email exchange. "It would require real leadership, huge time commitment, huge oppo research effort, and massive $$s. This is not something one person with $10 million to $20 million to burn can accomplish. It needs the leadership of an organization already geared toward such an effort. Think Koch, Rove, etc."

Several of the sources interviewed mentioned the Koch brothers and Karl Rove, who have extensive political operations already up and running. If some big, established group took the lead against Trump, the GOP establishment figures suggested, others would follow. But that hasn't happened.

And even if such a task were undertaken, there's the question of whether it would be effective against a candidate who has captured the hearts of voters who detest the GOP establishment. "I have come to believe that the core group of people who are for Trump, you cannot pull them off with a crowbar," said GOP insider Henry Barbour in an interview Sunday. "They will be with him until they see he does not have a path to the nomination."

"Nobody I know has made a really serious effort in terms of trying to line the money up," Barbour added. "You'd be better off picking a horse and spending your money trying to support that candidate."

Then there is the backfire scenario. Trump often talks about the big moneymen — his fellow billionaires — who would love to stop him. Trump's followers support him in part because they believe he is strong enough to stand up against those forces. A billionaire-fueled attack on Trump would only confirm those beliefs -- and end up blowing back on the donors.

"Most of us feel the voters are smart enough to figure this out for themselves," Republican ultra-insider Fred Malek explained in an email exchange Sunday. "Any concerted, coordinated effort to take down Trump or anyone else would be resented and backfire, since the last thing real voters want or need is a pack of rich guys telling them what to do or stacking the deck. People generally feel, rightfully, that the wealthy already have a disproportionate influence, and that's not the way a democracy is supposed to work."

Serious talk about organizing a major anti-Trump offensive began last October, when it became clear the "Summer of Trump" was going to last into the fall. Some preliminary work, like research into Trump's business dealings in Atlantic City, was done. But nobody followed through on the talk. And Trump's popularity increased.

While Trump rose, the opposition splintered. "All the megadonors have somebody who they look to as their political guru," said one GOP insider in an interview Sunday. "Some of them listen to Ken Mehlman, maybe Karl Rove, maybe Wayne Berman, Steve Schmidt. And that group is divided on who they want." The size of the GOP field has led multiple establishment candidates to fight among themselves, and mostly not take aim at Trump. "All the establishment candidates and their establishment donors are focused on each other and who's going to be the premier establishment candidate," added another GOP insider.

Despite all that, there are some strategists who believe an anti-Trump campaign could still work. In a phone conversation Sunday, Stuart Stevens, chief strategist for the Romney 2012 campaign, said he is mystified that some other campaign — the still-rich Jeb Bush super PAC, for example — hasn't dropped a huge negative advertising bomb on Trump. "Look at Iowa," Stevens said. "The fact that someone who has been married three times, bankrupt four times, is in the gambling business and has speculated publicly about the possibility of dating his daughter is a leading candidate is absurd. I think 10,000 ratings points [a technical description for a major ad buy] would completely reorient voters to facts that are relevant."

So far, Stevens is in the minority. And perhaps the most fundamental reason the anti-Trump talk of October has not resulted in an anti-Trump campaign in January is that, despite Trump's present strength, most establishment insiders simply don't believe he can make it through the party nominating process. "Donors don't think he's going to be the nominee," said one of the anonymous insiders quoted above. "I don't."

Of course, conditions could change again, and the Republican establishment might unite in some new resolve to act. There could be renewed plans for an anti-Trump ad campaign or even talk, like that coming recently from Weekly Standard editor Bill Kristol, of a third party if Trump becomes the GOP nominee. (Yet another Republican insider called third-party speculation "more than chatter, but less than imminent.")

Trump has so far outrun the establishment's desire to bring him down. For a long time, many in the GOP thought Trump wasn't serious, and they therefore saw no need to take action against him. Now, for other reasons, they have still chosen not to act. And the longer they wait, the stronger Trump gets.