This weekend was an inflection point in the Republican presidential race — a moment in which some significant part of the GOP establishment came out of denial and realized Donald Trump might well become their party's nominee.

"The Republican establishment, for the first time, is saying, off the record, this guy can win," noted Joe Scarborough on MSNBC Monday morning. "I've heard that from everybody. I don't hear anybody saying he can't win the nomination anymore."

That doesn't mean Republicans have made their peace with a Trump victory. On the contrary — some are preparing to do whatever it takes to bring him down. Which could lead to an extraordinary scenario in which GOP stalwarts go to war to destroy their own party's likely nominee.

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Over the weekend I talked to a leading conservative who opposes Trump. I asked what would happen if January comes and Trump is still dominating the race. Would he and other conservatives make their peace with Trump's candidacy, or would there be massive resistance?

"Massive resistance," was the answer. "He's not a conservative."

Insiders have watched as Trump defied what many believed were immutable laws of the political universe. First they thought Trump wouldn't run. Then they thought voters wouldn't take a reality-TV star seriously. Then they thought gaffes would kill Trump as they had other candidates. None of that turned out as expected.

But there is one belief Trump has not yet tested, and that is the political insiders' unshakeable faith that negative ads work.

"I don't think Trump can withstand 10,000 points of smart negative in Iowa and New Hampshire," says one veteran Republican strategist who is not affiliated with any campaign. "It would force him to spend money. That's when this starts to get real for him." ("Points" refers to gross ratings points, a way of measuring TV ad buys; 10,000 points would be a really big buy, meaning the average viewer would see an anti-Trump ad many, many times.)

There is no central anti-Trump conspiracy. But one group that would like to play a leading role in taking him down is the Club for Growth. In September, the Club ran two ads against Trump in Iowa — 2,000 points — with one arguing that Trump is not a true conservative and the other hitting Trump for his support of the Supreme Court's Kelo decision on eminent domain.

"We primed the pump with our ads in Iowa," says Club president David McIntosh. "We did some polling afterward. The ads flipped Trump from first to second place among caucus-goers and put a dent in his approval rating."

McIntosh is looking for donors to fund an anti-Trump campaign that would hit hard in the month before voting begins. It might be a Club for Growth production, or it might be a combination of efforts. "There is no other group that has decided to do it," says McIntosh. "There are a large number of donors and political activists who want to do it."

The triggers for the anti-Trump onslaught would likely be: 1) if next month arrives with Trump still in the lead, and 2) if Trump begins airing his own ads. "Once that starts, you'll see a lot of people saying we've waited long enough," notes McIntosh.

While that is going on, officials at the Republican National Committee vow to stay out of things. Asked what role the RNC might play in any movement against Trump, strategist and spokesman Sean Spicer said, "None. None. Zero. It is up to Republican voters to decide who our nominee is, not the RNC." Indeed, other sources inside the RNC say chairman Reince Priebus has stressed to staff that they must stay out of candidate fights.

The anti-Trump campaign will face several challenges. The biggest is the voters who support Trump. Conservative groups like the Club believe they can convince those voters that Trump is not a true conservative. Perhaps they can. But what if a large number of his voters are not wed to conservative orthodoxy as defined by Washington-based organizations?

The other problem is Trump himself. If he decides to spend serious money on his campaign — and some GOP veterans still aren't convinced he will — he can launch a serious counterpunch to any anti-Trump campaign.

And then there is the fact that Trump is improving as a candidate. Just look at Sunday's interview on "Fox News Sunday" in which he was sharp, focused, and forceful. A talented candidate who does something over and over again will get better at it. Trump is better than he was just a month ago, which is not good news for his opponents.

Some anti-Trump Republicans still harbor hope Trump will begin to fade all by himself. Yes, Trump, who has been atop the RealClearPolitics average of national polls for three months straight, has outlasted the various flavor-of-the-months from the 2012 GOP race. But opponents point out that Rudy Giuliani led the poll average for an incredibly long time four years earlier — from February 2007 to January 2008 — before sinking when voting actually began. Their hope is the same will happen to Trump.

It could. But a closer look at the 2007-2008 polls shows that Giuliani was almost always trailing in Iowa, New Hampshire, and South Carolina. And of course, ignoring the early states killed his candidacy. Trump, on the other hand, is on top in those three states, plus Nevada — all the states that will vote first in February. His organization is growing. He is hiring smart operatives. The Giuliani analogy doesn't apply.

Which makes it more likely that the anti-Trump forces will ultimately have to take it on themselves to go on the attack. Their core belief is that Trump cannot withstand a long and withering bombardment of negative ads. But core beliefs have been cast aside repeatedly in this race. That might happen again.