VALDOSTA, Ga. — On the eve of Super Tuesday's crucial primaries, a sharp new divide erupted between Republicans who pledge to fall in line behind Donald Trump if he wins their party's nomination and others who insist they can never back the bombastic billionaire.
The fissure could have major implications beyond the primaries, exposing the looming challenges in uniting the party after the election, no matter who wins.
Nebraska's Ben Sasse, a rising star among conservatives, became the first current senator to publicly raise the prospect of backing a third party option if Trump clinches the nomination. In a letter posted on Facebook late Sunday, Sasse urged Republicans to consider whether a party led by Trump would still represent their interests.
"If our party is no longer working for the things we believe in — like defending the sanctity of life, stopping Obamacare, protecting the Second Amendment, etc. — then people of good conscience should stop supporting that party until it is reformed," he wrote.
Other Republican leaders were less explicit, but sent similar messages on Monday, particularly in light of Trump's refusal to immediately disavow former Ku Klux Klan leader David Duke's support.
Mitt Romney, the party's 2012 nominee, called that "disqualifying." And South Carolina Gov. Nikki Haley, campaigning in Atlanta alongside Florida Sen. Marco Rubio, said she would "not stop fighting a man who refuses to disavow the KKK."
Trump said he had not understood the interviewer who first raised the question about Duke, and he did later repudiate him. "How many times do I have to continue to disavow people?" he said.
Several high-profile Republicans and conservative writers have embraced an anti-Trump social media campaign, using the Twitter hashtag "NeverTrump."
Trump has won three of four early primary contests, roiling a party that had assumed his populist appeal with voters would fizzle. Instead, he's only grown stronger and appears to be in commanding position heading into Super Tuesday, the biggest single-day delegate haul of the year.
The Associated Press asked Republican senators and governors across the country if they would back Trump if he secured the nomination. Just under half of those who responded would not commit to backing him, foreshadowing a potentially extraordinary break this fall.
"I am increasingly concerned by Donald Trump's statements and behavior, and I have serious concerns about his ability to win the general election and provide presidential leadership," Indiana Sen. Dan Coats said in a statement to AP.
Tensions boiled over during Trump's rally Monday in Radford, Virginia, where he was repeatedly disrupted by demonstrators, including 20 or more chanting "Black lives matter." At another point, he asked a protester, "Are you from Mexico?" after he was interrupted during remarks about immigration. He ordered several people to be removed, then cast himself as a unifying political force.
"Believe it or not, we're going to unify this country," he said.
If Trump sweeps most of the states up for grabs Tuesday, he could amass a delegate lead that would be difficult for any rival to overcome. Texas Sen. Ted Cruz is banking on a win in his home state to keep him in the race, while Rubio wants to stay close in the delegate count until the primary hits his home state of Florida on March 15.
Meanwhile, Democratic front-runner Hillary Clinton is solidifying her lead. Like Trump, Clinton could begin putting her party's nomination out of reach for rival Bernie Sanders with a strong showing on Super Tuesday.
As Trump has rolled through the early voting states, he's received enthusiastic endorsements from New Jersey Gov. Chris Christie and from Alabama Sen. Jeff Sessions, one of the most vocal opponents of immigration -law changes on Capitol Hill. Several other party officials have said they would back the real estate mogul if he does become the nominee, though some say their support would be reluctant.
"I'm a Republican and I will support the nominee," Alabama Gov. Robert Bentley said Monday. "I do not think he is our best nominee, but I will support the nominee."
Party leaders are particularly worried about the ripple effect of a Trump nomination on other races. Arkansas Gov. Asa Hutchinson urged Republicans to remember that their White House pick also impacts "the Senate and whether we're going to continue to elect Republican governors across the country."
A new commercial released Monday by Arizona Rep. Ann Kirkpatrick, who is challenging John McCain for his Senate seat, served as a likely preview of how Democrats would seek to link GOP candidates to Trump. The 60-second spot highlighted several of Trump's more incendiary statements and questioned how McCain could say he would support him in the general election.
Some Republicans said Senate candidates and others would have to forgo party unity in order to save themselves.
"It's not going to be a team effort. It's going to be every man for himself," said Matt Mackowiak, a GOP strategist who is unaffiliated in the 2016 race.
Pace reported from Washington. AP writers Thomas Beaumont in Atlanta, Kimberly Chandler in Montgomery, Alabama, and Andrew Taylor, Donna Cassata and Steve Peoples in Washington contributed to this report.