(CNN) Donald Trump , defying skeptics and overcoming an avalanche of mockery from establishment Republicans, has extended his streak as the party's presidential frontrunner for almost five months.

With the Iowa caucuses less than two months away, Republican leaders are growing deeply alarmed by Trump's resilience. Anxious that his lead may not fade before the voting starts, GOP strategists have begun to brace for a long and painful nomination fight, with Trump's opponents hoping to grind him down over a period of months.

In a new CNN/ORC poll released Friday, Trump leads the GOP field of White House hopefuls at 36% — his broadest support this cycle. In a distant second place is Ted Cruz at 16%, giving Trump his widest lead this year, followed by Ben Carson at 14% and Marco Rubio at 12%. Just two days before, a Quinnipiac University poll showed Trump on top at 27%, followed by Rubio at 17% and Carson and Cruz tied at 16%.

The back-to-back polls are yet another confirmation of Trump's remarkable ability to withstand an onslaught of criticism. Since launching his campaign in June, the billionaire has barreled from the center of one controversy to the next, repeatedly bypassing predictions of demise from all corners of the political world.

But even in the face of more evidence of Trump's endurance — and grudging recognition that a Trump victory is no longer outside the realm of possibility — many establishment Republicans still insist that ultimately, the businessman is unlikely to clinch the nomination. They are starting to anticipate a protracted primary race that extends far beyond the early states and possibly into the summer, in which the party slowly wears Trump down.

Former Republican Utah Gov. Mike Leavitt, who was a close adviser to Mitt Romney's presidential campaign, said while Trump has clearly locked in a solid bloc of supporters, the majority of GOP voters are still up for grabs.

"In order to get the nomination, you have to get the majority of the votes at the convention. The presidential election is not going to be won in Iowa, New Hampshire or South Carolina -- it has to be won by a composite of all 50 states," Leavitt, who did not rule out the possibility of a Trump nomination, said in an interview.

He predicted that in the end, "there will be a consolidation of those who don't want him to be nominee."

Senior GOP officials have been reticent to publicly scold Trump, particularly as the candidate has continued to dangle the threat of a third-party run.

"People are reluctant to take him on because he lashes back like ISIS," said former Nevada Republican Gov. Robert List, who believes there's only a "small chance" that Trump becomes the nominee. "He has a wicked tongue and no reluctance to use it."

List also warned about the long-term damage that a Trump victory would unleash on the GOP.

"He's the typical sort of person that historically folks thought of as a typical Republican — wealthy, country club guy who has a strong will and doesn't listen a lot to hard working folks," List said. "I don't think that's the kind of a symbol that we want to put forth."

Nevertheless, party leaders are beginning to lay out contingency plans for a Trump victory.

The executive director of the National Republican Senatorial Committee circulated an internal memo offering advice to 2016 GOP Senate candidates, acknowledging that Trump "could win." Ward Baker advised Republicans to avoid getting "drawn into every Trump statement and every Trump dust-up" and to steer clear of the businessman's "more extreme positioning."

The internal NRSC memo, first reported this week by The Washington Post, offered a striking and rare glimpse into how the party is making concrete preparations to handle a Trump nomination.

For months, Trump's inflammatory rhetoric aimed at ethnic groups has alarmed establishment Republicans, particularly in light of the GOP's painstaking efforts in recent years to broaden its base. The real estate mogul has offended numerous demographic groups — Hispanics, Asian-Americans and more recently, Muslim Americans.

In the aftermath of last month's Paris terrorist attacks perpetrated by ISIS, Trump has doubled down on widely disputed claims that he saw thousands celebrating in New Jersey in the aftermath of the 9/11 attacks in 2001.

And speaking before the Republican Jewish Coalition on Thursday, Trump caused a stir when his remarks appeared to play to offensive stereotypes about Jewish people.

"I'm a negotiator like you folks, we are negotiators," Trump told the crowd of influential Jewish donors. "Is there anybody that doesn't renegotiate deals in this room? This room negotiates them -- perhaps more than any other room I've ever spoken in."

Former Virginia Republican Rep. Tom Davis, who served as chairman of the National Republican Congressional Committee, said there is real concern among GOP political strategists working on House and Senate races about a Trump nomination.

With such a lightening rod presidential nominee, down-ballot candidates would face tremendous pressure to turn their backs on Trump in order to win their respective elections, Davis said.

"You run your own race, you target independents, and Republicans who are likely to defect — you try to hold them. It's the only thing you can do," said Davis, who personally believes it would be "very difficult" for Trump to become the party's nominee. "It's difficult to disown the top of your ticket ... but you need to put distance between yourself and the top of the ticket."