Donald Trump no longer has any rivals for the Republican presidential nomination, but he could struggle with the first order of business for any standard bearer: party unity.

Namely, about a quarter of GOP voters have said they would never vote for Trump in November, according to exit polls of recent primaries.

"We want to bring unity to the Republican Party," Trump said Tuesday night after his convincing Indiana victory made him the likely nominee. "We have to bring unity - it's so much easier."

The goal may prove elusive, however. As Trump's capture of the GOP was drawing to a close, many of its leaders, particularly conservative thinkers, were still resisting.

Instead of jubilation, conservative advocate Colin Hanna of Chester County described "a sick feeling in the pit of my stomach." Columnist George Will ripped Republican "collaborationists" who have shifted to Trump, saying they had forfeited the right to "participate in the party's reconstruction" after a possible electoral wipeout.

Others compared the Manhattan real estate developer to Sen. Joseph McCarthy or worse. Several conservative leaders tweeted pictures of themselves burning their GOP voter-registration cards, like Vietnam War draft resisters.

Republicans "are voting for suicide" if Trump is the nominee, said Erik Erickson, an influential conservative blogger.

"Trump cannot build a meaningful coalition outside of blue-collar white voters, white supremacists, and internet conspiracy theorists," Erickson said. "The rest of the voting public no more wants Trump than herpes."

Even as Trump soared to victory in Indiana, 42 percent of those who voted in Tuesday's primary there said they were "scared" or "concerned" by a possible Trump presidency, the exit poll found. And nearly one in four Indiana Republicans - 23 percent - said they would not vote for Trump in the general election if he is the party's nominee.

His performance on that score was an improvement. In the nine primaries held in March and April in which exit polls asked the general-election question, an average of 27 percent of GOP voters said they would not back Trump in November.

There is always a core of disgruntled partisans reluctant to coalesce around the national ticket after a tough intra-party fight, but Trump starts from a bigger deficit than usual among modern party nominees, said pollster Christopher Borick of Muhlenberg College in Allentown.

"The starting point is problematic for Trump. That's a pretty high number," Borick said. "Even if we see the normal return-to-the-fold pattern, there is still going to be a significant amount of Republican voters that might not be back."

When general elections are over, presidential candidates tend to have earned the support of nine out of 10 voters in their own party, he said. If Trump does not pull in that many, the pollster said, he will have to look elsewhere.

"Can you get replacement voters from among independents and disaffected Democrats?" Borick said. "That's the Trump gamble."

During the primaries, Trump discarded Republican orthodoxy on free trade - he opposes it - and also diverged from the party's prevailing philosophy on Social Security benefits, saying he sees no need to cut benefits to save costs. Many conservatives have said they don't think he shares their principles. And then there are the GOP leaders who worry that Trump's vows to deport millions of illegal immigrants and temporarily bar Muslims from entering the U.S. only make the party look intolerant and complicate efforts to expand its support.

But the likelihood that Hillary Clinton will be the Democratic nominee, and the desire to defeat her, is a powerful countervailing force for conservatives.

"I'm debating between voting for Trump - I think he's a fool, but Hillary is evil - and writing my dad's name on the ballot," said Joy Pullmann of Fort Wayne, Ind., managing editor of the Federalist and an education policy researcher at the Heartland Institute, a conservative think-tank.

Trump, she said, "doesn't have a guiding philosophy, so he does whatever advances his goal of being the first president in a gold-plated White House."

Hanna, the Chester County conservative who heads the national advocacy group Let Freedom Ring, said he worries about Trump at the head of the GOP ticket.

"I'm optimistic, and I don't go in for doom-and-gloom, but I have a sick feeling in the pit of my stomach," Hanna said. "I don't know what I'm going to do." As a committed conservative, he wants to stop Clinton, yet he is unclear on where Trump stands.

Phil English, a former GOP congressman from Erie, said he is more optimistic about Trump's ability to compete in Pennsylvania and similar states in the fall, as demonstrated last week by Trump's landslide win in the GOP primary here.

"If individual political leaders in the party don't accept him, that may be less important than where broader groups of voters land," said English, who had supported Wisconsin Gov. Scott Walker for the nomination.

Those broader groups made one thing clear - they like outsiders this year. A Monmouth University report issued Wednesday found that in the early primaries, 49 percent of Republican voters told exit polls they preferred outsider candidates to those with government experience. By last month that figure was 59 percent. As the numbers rose, so did those of the outsider who is now the presumed nominee.

"Mr. Trump is a remarkable reader of the public mood," English said. "I think he'll draw a very bright contrast with the Democratic opponent, and use his skills to appeal to those who haven't bought into his message yet."

215-854-2718 @tomfitzgerald