WASHINGTON, May 6 (Xinhua) -- Bombastic U.S. billionaire Donald Trump has surprised observers worldwide by securing the Republican Party (GOP) nomination for president, but his next challenge is to get the GOP to back him, experts said.

"Trump will have difficulty getting GOP support from mainstream party leaders who are upset with his failure to toe the line on key issues," Brookings Institution's senior fellow Darrell West told Xinhua, referring to the myriad changes to well-established policies that the GOP has supported for decades.

"He is more critical of trade agreements than many past nominees and he does not share their interventionist inclinations," West said.

Indeed, Trump has in recent foreign policy speeches advocated against getting involved in overseas military adventures, and has blasted U.S. involvement in the war in Iraq.

On the domestic front, he has said he does not want to cut any entitlement programs.

"These views make him anathema to conservative hardliners and those favoring hawkish military positions," West said.

Julian Zelizer, professor of history and public affairs at Princeton University, told Xinhua that Trump will have problems getting GOP backing in the race for the White House, although others will back him.

West said the party split will make it difficult for Trump to win in November.

"His only hope is if his unconventional party position allows him to win Democratic or Independent votes who otherwise would not support a Republican nominee. Trump hopes that he can put states such as Pennsylvania, Michigan, Ohio, and Minnesota in serious play and peel off enough disaffected Democrats to win those states," West said.

Zelizer said a split can hurt, "though as we saw in primaries he has been able to use these divisions and internal opposition to show himself as being an anti-establishment candidate."

"It has not hurt him with Republican voters thus far," Zelizer said.

For many political observers and analysts, Trump's nomination has been a shocker, as the controversial candidate was just a few months ago predicted to be a flash in the pan.

But those predictions proved false, as many had underestimated the sheer amount of frustration among white working class Americans -- especially those residing in rural areas far from urban media centers -- over the lackluster economy, which has not recovered in some areas since the 2008 crash.

Indeed, Trump is riding on a wave of anti-establishment sentiment in the United States, as approval for Congress and Washington insiders is at a low not seen any time in recent memory.

Trump has also gained loud applause from supporters who like his brash, in-your-face bravado and his willingness to go head-to-head with the Washington establishment.

But his detractors in the GOP are unwilling to see the Washington shake-up that Trump is proposing, as he said he plans to end a number of longstanding foreign alliances and change other policies that GOP has supported for decades.