Nebraska Sen. Ben Sasse vowed never to support Trump because of his "relentless focus" on "dividing Americans." John Cornyn of Texas, the Senate's No. 2 Republican, predicted that Trump would be "an albatross around the down-ballot races." Ken Mehlman, who ran George W. Bush's winning campaign in 2004, penned a Facebook post castigating Trump by stating that leaders shouldn't need to research whether to reject Klan support. "They also don't mock people with disabilities, insult war heroes, divide people by religion and nationality, and insult women. #NeverTrump," Mehlman wrote.
Still time to defeat Trump
Allies close to Rubio and Cruz insist there is still time to defeat Trump -- a scenario that would involve winning their home states and pulling off strong results in other delegate-rich states in March. But in interviews, many GOP operatives acknowledged that scenario was highly unlikely, barring some huge upset on Tuesday night.
"When you look at operatives, donors, and the opinion elite, they are all scared to death that Trump is going to be the nominee," said Russ Schriefer, a senior adviser to Romney's 2012 campaign, as well as Christie's recent presidential bid. "But then you look at voters and they are quite comfortable with Trump as the nominee... What you are now seeing is this full court anti-Trump press, but it's late," Schriefer said. "Trump is no more dangerous than he was in August."
"Tomorrow night, if we believe the polls, he's going to win everything but Texas," Schriefer said. "If that were any other candidate, on Wednesday morning the party would be rallying around him or her."
Steve Schmidt, a GOP strategist who ran John McCain's campaign in 2008, said the current Republican reaction to Trump's electoral strength "depends where each individual Republican is in their stages of grief."
"Some people remain in the anger and denial phase while others are moving to acceptance," Schmidt said. He pointed out that while there is a lot of "loose talk" about the congressional wing of the Republican Party protesting Trump's nomination, congressional approval on "a good day" hovers around 15%.
"It's not as if there's a wave of good feeling around members of Congress or governors who all of a sudden are standing on principle in opposition to Trump's candidacy," Schmidt said. "You see a lot of trend lines converging at this moment that have been buffeting the Republican Party for a long time."
Schmidt added Trump's rise
reflects the divergence nationally within the GOP in recent years about the true meaning of conservatism.
"Stepping into that vacuum are the candidates who are now tearing at the fabric of the old conservative coalition," he said.
At the same time that there was a sense of resignation within some GOP quarters about Trump's glide path to the nomination, a number of GOP strategists pointed out that the party allowed him to succeed in large part by failing to coalesce around an alternative candidate last fall.
Despite all the rumors of an anti-Trump movement
last year in the form of super PAC campaign or other sustained attacks, nothing ever materialized.
"You have to help somebody win, not just hope somebody else loses," said Mike DuHaime, a GOP strategist who most recently advised Christie, but also worked as a strategist for McCain, former President George W. Bush and Rudy Giuliani, among others.
"Most people who sat on the sidelines last year, now they are wringing their hands because they can't believe they got to this point. They could have all gotten behind one candidate," he said.
Final day of campaigning
The final day of campaigning before Super Tuesday brought a preview of the fractious year ahead for Republicans.
There was an ugly confrontation
at Trump's rally at Radford, Virginia, after protesters interrupted him when a Secret Service employee appeared to put a photographer in a choke hold and throw him down on a table. (It was unclear what led to the altercation, which the Secret Service was investigating).
A collection of white supremacist groups placed a fresh round of robocalls in support of Trump in at least one Super Tuesday state -- Vermont -- a tactic they adopted before the New Hampshire primary.
In a final push to stop Trump, Cruz once again questioned the authenticity
of Trump's conservative credentials, citing an unsubstantiated report in BuzzFeed that Trump had taken a softer tone on immigration in an off-the-record interview with The New York Times editorial board.
But even the Texas Senator acknowledged to reporters that his chief rival would likely do well Tuesday.
"I believe after Super Tuesday, we will see this race become more and more a two-man race," Cruz said. "Donald is going to come out with a whole bunch of delegates. I believe we're going to come out with a whole bunch of delegates. And I think everyone else will be way, way behind. If that's the case, then it is time for the party to unify. Head to head, I beat Donald Trump and I beat him resoundingly," Cruz said.
Rubio, for his part, ridiculed Trump
for not distancing himself from Duke and other white supremacists: "He's unelectable now; he refused to criticize the KKK," the Florida Senator said in Tennessee.
"You say, 'David Duke' to me, I say, 'racist' immediately," Rubio said later at an Atlanta rally. "Why wouldn't he condemn the Ku Klux Klan? There is no room in the Republican Party for members of Ku Klux Klan or racists like David Duke."
'Not who we want'
Appearing with Rubio on stage, South Carolina Gov. Nikki Haley said she would not "stop until we fight a man who chooses not to disavow the KKK, that is not a part of our party, that is not who we want as president."
Trump insisted during a Monday interview on NBC's "Today" that he had not heard the question about Duke and alleged that CNN gave him a faulty earpiece for his Sunday interview on "State of the Union."
"I don't mind disavowing David Duke," he said. "I disavowed David Duke the day before at a press conference."
Trump continued slinging insults toward Rubio and Cruz, calling Rubio a "con man" and a "little guy," while charging that Cruz has "done nothing for Texas."
DuHaime noted that Trump, should he win, faces a difficult task in uniting the party, but he also predicted that the party would enter a new phase once faced with the reality of the choices in the election.
"Most likely," DuHaime said, "the great unifier will be Hillary Clinton."