A fresh string of attacks by Donald Trump this week on rivals in the GOP establishment — including one delivered against a prominent Latina governor in her home state — raised new doubts about his ability or desire to unite the party’s badly fractured leadership.

Now the presumptive Republican presidential nominee, Trump had been expected by many political strategists and party leaders to extend olive branches to his foes and vanquished opponents, many of whom could be crucial allies in the general election against the Democratic nominee, most likely Hillary Clinton.

Yet the real estate mogul does not always appear to be interested in doing so. The revived feuding this week has only added to the concerns of holdouts such as House Speaker Paul D. Ryan (R-Wis.), who reiterated Wednesday that he was not ready to endorse Trump and remained opposed to some of his core policies.

The intraparty skirmishing began with an attack on New Mexico Gov. Susana Martinez (R) during a campaign rally in Albuquerque, where Trump blamed her for mismanaging the state’s economy and suggested that she was shirking her responsibilities to her constituents.

“She’s got to do a better job. Okay? Your governor has got to do a better job,” Trump told a cheering audience Tuesday night. “She’s not doing the job. Hey! Maybe I’ll run for governor of New Mexico. I’ll get this place going. She’s not doing the job. We’ve got to get her moving. Come on: Let’s go, Governor.”

Speaking at a rally in Albuquerque May 24, Republican presidential candidate Donald Trump criticized Gov. Susana Martinez (R-N.M.), saying, "Hey! Maybe I’ll run for governor of New Mexico. I’ll get this place going." (Reuters)

Next, at a campaign event Wednesday in Anaheim, Calif., Trump rattled off a string of attacks that played like a greatest-hits collection from the raucous GOP primary contest. He knocked South Carolina Gov. Nikki Haley’s decision to endorse Sen. Marco Rubio (R-Fla.), mocked former Florida governor Jeb Bush for his energy level and blasted 2012 ­Republican presidential nominee Mitt Romney as a “choker.” None of the three have endorsed him.

“Poor Mitt Romney. Poor Mitt. . . . I mean, I have a store that’s worth more money than he is,” Trump said, adding later: “He choked like a dog. . . . Once a choker, always a choker.” He also called Romney “stupid” and joked that he walked “like a penguin.”

The attacks — delivered in Trump’s distinctive and belittling style — would no longer appear to be in his best interest now that the primary campaign is over and he faces a tough and well-funded opponent in Clinton. They could also further undercut his standing among women and minorities, who are strongly opposed to him in public-opinion polls.

Martinez — the nation’s first Latina governor and New Mexico’s first female governor — is the chair of the Republican Governors Association, which has deep coffers and can play a vital role in driving GOP turnout during an election year. Before Trump, she was also widely considered to be a leading pick as a potential 2016 vice-presidential candidate.

Martinez has criticized the way Trump describes illegal immigrants and decided not to attend his Albuquerque rally.

“The Governor will not be bullied into supporting a candidate until she is convinced that candidate will fight for New Mexicans,” a Martinez spokesperson said in a statement after the event.

At a rally in Anaheim, Calif., on May 25, Republican presidential candidate Donald Trump said he has "a sore that's worth more money than" Mitt Romney. He also said the 2012 Republican candidate was "stupid," a "choker," and that he walks like a penguin. (Gillian Brockell/The Washington Post)

Rubio, who dropped out of the presidential race in March, defended Martinez on Twitter as “one of the hardest working and most effective Governors in America.”

In a Wednesday news conference on Capitol Hill, Ryan declined to specifically address Trump’s attacks on Martinez but defended her record. “Look, I’ll just leave it at this: Susana Martinez is a great governor,” Ryan said. “She turned deficits into surpluses. She cut taxes. She’s a friend of mine, and I think she’s a good governor. I will leave it at that.”

He also said he remained undecided on endorsing Trump: “Look, I don’t have a timeline in my mind, and I have not made a decision. “Nothing has changed from that perspective, and we’re still having productive conversations.”

But it is not just Trump’s penchant for picking fights that continues to give some Republican leaders pause. Many remain skeptical that his ideas align with the core beliefs of the conservative movement itself.

Trump and Ryan, for example, remain divided on one of the central pillars of the mogul’s campaign: immigration reform, specifically Trump’s call for mass deportation of an estimated 11 million undocumented immigrants in the country. Ryan, who supports a path for legal status for such immigrants, said the House will not put such measures on its 2017 agenda.

“Obviously, securing the border is part of national security, we believe,” he said. “But I’ve made my view on [mass deportation] pretty darn clear, and I’ll just leave it at that.”

Ryan’s six-part House agenda for next year — which he previewed during the news conference — also does not mention trade, another area where Trump holds positions dramatically different from the party’s pro-free-trade leadership. Ryan also suggested that he remained skeptical of Trump’s understanding of the limits of executive authority.

“We want to make darn sure that our standard-bearer understands, appreciates and respects and supports the Constitution and the kinds of principles that come with it, and those are some of the conversations we have been having,” he said.

Trump’s allies have rejected the idea that he will be unable to unite the party’s establishment. They note that many members of Congress and other elected Republicans have backed Trump since he emerged as the presumptive nominee.

“I understand that Paul Ryan is trying to have it both ways,” Mark Burns, an evangelical pastor and television evangelist from South Carolina who frequently speaks at Trump’s rallies, said at the Anaheim rally. “Donald Trump is going to unite this party and, come November, we’re going to elect a new president by the name of President Donald Trump.”

Nearly 6 in 10 voters say they have a negative opinion of Trump, according to a Washington Post-ABC News poll released earlier this week, though they have improved since March. Even so, Trump’s negatives are nearly matched with Clinton’s, and the poll showed that the two are caught in a statistical dead heat, with Trump at 46 percent and Clinton at 44 percent.

The poll also showed that Republican voters overall are warming to the mogul, with 85 percent now backing Trump. Some of that support could be tenuous, how­ever: Nearly half of Republican-leaning registered voters do not believe that Trump’s views reflect the core tenets of the party.

Johnson reported from Albuquerque and Anaheim. Mike DeBonis and Emily Guskin in Washington contributed to this report.