Donald Trump's Republican presidential campaign has now collected victories in the vast majority of the first 15 states of the nomination contest. He's on pace to win more votes than any other GOP primary candidate before him, and his candidacy is helping fuel record turnout across the country.

And yet on Tuesday, as Trump continued to gather sweeping victories from New England to the Deep South, the urgent calls from establishment Republicans to stop him only grew louder and more apocalyptic.

“If we nominated Donald Trump,” Senator Marco Rubio, a presidential candidate from Florida, said Tuesday on CNN, “it will be the end of the modern Republican Party.”

In any other year, a candidate who amassed as many victories as Trump would be busy accepting stacks of endorsements and consolidating support of the party's power brokers. But Trump continues to be spurned not only as an outsider seeking political office for the first time but as a candidate who is stacking up wins without voting blocs crucial to winning the White House.

Four years after the party identified loosening immigration laws as a way to broaden its appeal and break its consecutive defeats in the presidential race, GOP leaders remain confounded by Trump. While his anti-immigration message is inspiring record numbers of white, conservative voters to the polls, it's also alienating Hispanics who could deliver either party the presidency.

“If this was Rick Perry or Scott Walker or Bobby Jindal or any other governor, the race would be over,” said Katon Dawson, a former South Carolina Republican Party chairman. “But people understand that we can't win a presidential race with just white people.”

Nearly three of every four Republican voters who didn't back Trump on Tuesday said they wouldn't be satisfied if he became the nominee, exit polls showed.

As voting was under way in 11 states on Tuesday, one of the most aggressive anti-Trump efforts from the party establishment—a group known as Our Principles PAC—received a boost from billionaires Todd Ricketts and Paul Singer and from Meg Whitman, the current chairwoman of Hewlett Packard.

They urged Republican donors to pump cash into a fresh effort to stop Trump, the New York Times reported. Hours later, Our Principles PAC released a new attack ad calling Trump a racist and announced it hired Tim Miller, who was most recently communications director for former Florida Governor Jeb Bush, whose pro-immigration campaign burned out after spending more than $130 million in the primary.

“The fight to stop Donald Trump from getting the nomination is intensifying regardless of tonight's outcome,” Miller said. “I'm pleased to be a part of it.”