WASHINGTON • In an extraordinary rebuke of his party's presumed nominee, House Speaker Paul Ryan, the nation's highest-ranking elected Republican, said on Thursday that he was "not ready" to endorse Mr Donald Trump for president.

"To be perfectly candid... I'm just not ready to do that at this point," Mr Ryan said.

"And I hope to, though, and I want to. But I think what is required is that we unify the party, and I think the bulk of the burden on unifying the party will have to come from our presumptive nominee."

Mr Ryan's announcement represented a split among Republicans not seen in at least a half-century, and it came only two days after Mr Trump said he would unify the party after essentially clinching the nomination with his victory in the Indiana primary.

As the chairman of the Republican National Convention, Mr Ryan has repeatedly said he would support his party's nominee as Republicans seek to regain the White House and solidify control of Congress.

But the combination of Mr Trump's at times outrageous remarks and his broad rejection of many core Republican policies proved too toxic a brew for Mr Ryan as he defended his majority in the House, the reputation of his party and his own viability.

Within an hour, Mr Trump offered a biting rejoinder, saying in a statement that he was "not ready to support Speaker Ryan's agenda".

"Perhaps in the future we can work together and come to an agreement about what is best for the American people," he said. "They have been treated so badly for so long that it is about time for politicians to put them first!"

A party nominee has never failed to gain the support of a House Speaker or majority leader from his party in modern times.

In 1896, Speaker Thomas Brackett Reed ran against William McKinley and made it known he would not serve as vice-president, but ended up backing the nominee.

In 1964, Senator Barry Goldwater had to wait a bit uncomfortably for the endorsement of Senator Everett Dirksen of Illinois, but Mr Dirksen gave it and thus ended the stop-Goldwater movement.

While Mr Ryan's remarks caught Republicans off-guard, it also gave them essentially a permission slip to go their own way on Mr Trump's candidacy.

Had Mr Ryan issued a forceful endorsement, it would have put pressure on fellow House Republicans to follow his lead, a step many have been unwilling to take.

Mr Trump, who once said Mr Ryan would "pay a big price" if he did not support him, knows that the Speaker and other opponents did not have great sway over primary voters.

But Mr Ryan, who has helped raise hundreds of thousands of dollars for Republicans, may well be a more notable voice in a general election.

Even before Mr Ryan spoke out, Mr Trump was confronting the prospect of crippling defections within the Republican elite. Both Mr George Bush Sr and Mr George W. Bush, the only two living former Republican presidents, announced on Wednesday that they would not back his candidacy.

Mr Mike Shields, president of the American Action Network, a conservative outside spending group, said Mr Ryan had effectively cleared the way for others in the party to decide for themselves how to handle Mr Trump.

"This is something that every Republican can say at the local level," Mr Shields said.

"It's great leadership on Paul Ryan's part because it puts them in a place where all these other members can turn around and say the same thing, which is, 'Look, there are some things that we've got to stand for.'"

NEW YORK TIMES, AGENCE FRANCE-PRESSE