Republican presidential candidate Donald Trump speaks during a rally in Spokane, Wash., May 7, 2016.
Republican presidential candidate Donald Trump speaks during a rally in Spokane, Wash., May 7, 2016.

WASHINGTON - Donald Trump, the presumptive U.S. Republican presidential nominee, is not all that worried that key party leaders say they won't support him in the national election against the likely Democratic nominee, former U.S. Secretary of State Hillary Clinton.

As the billionaire real estate mogul all but clinched the party's presidential nomination last week, the last two Republican presidents, George H.W. Bush and his son, George W. Bush, and numerous other party officials declared they have no intention of supporting his maverick candidacy.

"Does it have to be unified?" Trump asked about the Republican Party on ABC's This Week news show Sunday. "I'm very different than everybody else, perhaps, that's ever run for office? I actually don't think so.

"I think it would be better if it were unified, I think it would be, there would be something good about it," he said. "But I don't think it actually has to be unified in the traditional sense."

Ryan 'not ready'

The party's top current elected official, House of Representatives Speaker Paul Ryan, said he is "just not ready" to endorse Trump and wants to make sure he would uphold the party's traditional conservative principles before agreeing to support him.

House Speaker Paul Ryan of Wis., gives a speech en
FILE - House Speaker Paul Ryan of Wis., gives a speech entitled "Building a Confident America" at Gaston Hall at Georgetown University in Washington, April 27, 2016.

Ryan has opposed Trump's call to temporarily block Muslims from entering the country and the two disagree on U.S. foreign policy and trade issues.

He and Trump have scheduled a meeting Thursday in Washington to air their differences, but the brash Trump, a one-time television reality show host who has never held elective office, said it's possible the two may just "go our separate ways."

Other conservative leaders have raised the possibility of fielding a third candidate against Trump and Clinton, but third-party candidacies have not fared well in U.S. presidential elections, almost always trailing far behind the Democratic and Republican nominees.

Numerous candidates

Some Republican lawmakers said they will support Trump even though they originally preferred other presidential candidates.

The 69-year-old Trump surged to the top of the Republicans' one-time field of 17 candidates with calls to build an impenetrable wall along the U.S.-Mexican border to halt the stream of illegal immigrants into the United States and to deport the 11 million already in the country.

He has won more than 10 million votes in state-by-state party nominating contests, with his last two challengers dropping out of the race for the party's presidential nomination after Trump scored an impressive win in last week's contest in the Midwestern state of Indiana.

Democratic presidential candidate Hillary Clinton,
FILE - Democratic presidential candidate Hillary Clinton, center, greets supporters as she visits her campaign field office in Oakland, Calif., May 6, 2016.

Neither Trump nor Clinton has officially clinched their parties' presidential nominations yet, but they are trading verbal taunts at each other.

Attacking the Clintons

At a rally Saturday, Trump, who is married to his third wife and through the years has bragged about sexual exploits, lampooned Clinton for the marital infidelities of her husband, former U.S. President Bill Clinton.

"She's married to a man who was the worst abuser of women in the history of politics," Trump said.

Clinton has disparaged Trump as unfit to be the country's commander in chief.

"We can't have a loose cannon in the Oval Office" at the White House, she told cheering supporters at a rally.

Numerous polls show Clinton ahead of Trump in the election to pick the successor to President Barack Obama, who leaves office next January.

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