DONALD Trump has suffered major blows in his attempt to unite a fractured Republican Party behind him in the race to become president of the United States.

In a stunning intervention, US House Speaker Paul Ryan, the highest ranking leading Republican, said he could not support Mr Trump. He called on the outspoken Republican candidate to strive for higher aspirations, adding: “I think what a lot of Republicans want to see is that we have a standard bearer that bears our standards”.

Former Republican presidents George HW Bush and his son, George W Bush, have also decided that they cannot endorse Mr Trump’s bid to be president.

The snub marks a first for Mr Bush senior, 91. He had endorsed Republicans in the past five US general elections.

Mr Trump began to reach out to party heavyweights, including making efforts to repair his at-times strained relationships with the Republican National Committee (RNC) and party donors.

A spokesman for George W Bush said the former president did “not plan to participate in or comment on” the 2016 presidential race.

While Mr Bush junior has largely stayed out of politics since leaving the White House, his refusal to publicly support his party’s nominee is remarkable. It underscores the deep frustration within some corners of the Republican party over Mr Trump’s candidacy.

His father, former president George HW Bush, also has not been heard from since Mr Trump’s string of victories in the north-east primaries and Indiana.

Mr Ryan said he was not ready to back Mr Trump. “I’m not there right now,” he said. Mr Trump retorted: “I am not ready to support Speaker Ryan’s agenda. Perhaps in the future we can work together.”

Mr Ryan was Mitt Romney’s running mate for the Republicans in the 2012 election. Mr Romney has already objected to Mr Trump as Republican nominee.

Both Bushs, Mr Romney and Senator John McCain have said they will not go to the Republican party convention in July when Mr Trump will be formally confirmed as candidate against Democrat Hillary Clinton. The Trump campaign also named Steven Mnuchin as its national finance chairman yesterday, saying in a statement that “he brings unprecedented experience and expertise to a fundraising operation that will benefit the Republican Party and ultimately defeat Hillary Clinton”.

Mr Mnuchin is chairman and chief executive officer of Dune Capital management LLC, a private investment firm, and previously worked at the New York bank Goldman Sachs.

The process begins just days after Mr Trump’s final two intra-party foes, Ted Cruz and John Kasich, suddenly dropped out, clearing his path to the nomination, as members of the celebrity businessman’s campaign were set to meet the RNC and enter a joint fundraising agreement needed for both his bid and Republicans to maintain control of Capitol Hill, aides said.

“In order to really govern, we need majorities in the House and Senate. We’re going to work with the party to raise money for down-ballot races to be successful,” said Corey Lewandowski, Mr Trump’s campaign manager. “Mr Trump has agreed to be available to raise money.”

“We’re going to with work with the RNC hand-in-glove,” Mr Lewandowski said. He added that a fundraising plan had not yet been set for Mr Trump’s White House bid, a hugely expensive undertaking.

Mr Trump said last night that he backed Brexit as migration had been “horrible” for Europe.