Washington (CNN) Donald Trump's campaign entered a new phase Monday as he went to Washington to begin to court the Republican political and foreign policy establishment and to convince doubters he's more than a rabble-rousing outsider.

The billionaire businessman, who built his campaign on eviscerating Washington, met with a dozen Republican officials and members of Congress and stressed the need to unify the GOP behind him.

"If people want to be smart, they should embrace this movement," said Trump at a news conference at Washington's Old Post Office Pavilion building that he is converting into a luxury hotel. He warned that if he is not embraced, the party would go down to a "massive defeat."

Later, Trump addressed the American Israel Public Affairs Committee's annual meeting, an event that tested his skills in working with Washington's foreign policy community.

In an unusual move, he spoke from prepared remarks during his speech at the Verizon Center. Trump said he didn't show up to "pander" to the audience about Israel, saying "that is what politicians do."

He went on to deliver a full-throated denunciation of Israel's enemies, including Iran, vowing to dismantle President Barack Obama's nuclear deal with Iran and what he said was Tehran's global terrorism network. He drew some chuckles from the audience when said he had studied the pact "more than anybody else."

Despite declaring himself the most pro-Israel candidate, Trump's views on stalled Israeli-Palestinian peace moves had previously been at odds with much of the pro-Israel policy community in Washington. His previous statement that he is "neutral" in the conflict and belief that a flurry of the kind of deal -making at which he excels can secure an elusive agreement have opened vulnerabilities for rivals to exploit.

"A deal is a deal," Trump said at the CNN debate in Miami earlier this month. "I think I may be able to do it, although I will say this, probably the toughest deal of any kind is that particular deal."

But the unabashedly pro-Israel tone of the speech at AIPAC is seemed designed to lay to rest fears of some in the Jewish community and also among Israel's supporters in the evangelical movement about what his presidency could hold. It was also one of the few occasions in his campaign so far that he has delivered a disciplined, planned speech and it differed from his habitual stream of consciousness style of delivery.

Before his speech, he released for the first time a list of his foreign policy advisers during a meeting with The Washington Post's editorial board . And in an interview with CNN's Wolf Blitzer , Trump said the U.S. should rethink its involvement in NATO because the defense alliance costs too much.

"Frankly, they have to put up more money," he said. "We are paying disproportionately. It's too much and frankly it's a different world than it was when we originally conceived of the idea."

The moves come as the Republican establishment and a group of influential conservatives intensify efforts to thwart Trump's march to the nomination, but with no unified strategy for how to achieve that goal.

Trump's high-profile day in Washington is focusing renewed attention both on the rift in the Republican Party and on his own policies and qualifications to lead. As he compiles a strong advantage in the delegate race, the businessman is starting to face examination not just as successful candidate, but also as the probable GOP standard-bearer -- and possible president and commander-in-chief.

Position of strength

The GOP meeting was facilitated by Alabama Sen. Jeff Sessions, who has endorsed the GOP front-runner. Former House Speaker Newt Gingrich and Bob Livingston, a former congressional leader, were at the gathering.

Livingston said outside the meeting that party unity is crucial and that Republicans should unite behind Trump to stop a Hillary Clinton presidency.

"It's a good reason for them to unify and get behind Donald Trump, and let's have a victory this time around," he said.

"I am really, really irritated by these people who think they are smarter than the American people. The American people are expressing themselves loudly in just about every state, most of the primaries and he's getting most of the votes," Livingston said. "And for me that's very, very important. I want to see the American people heard and I want to see Donald Trump president."

Other lawmakers at the meeting included Sen. Tom Cotton of Arkansas, Tom Marino of Pennsylvania, Duncan Hunter of California, Scott DesJarlais of Tennessee and Renee Elmers of North Carolina. But most of the lawmakers who were there had already backed Trump in the race, and top party leaders did not attend.

"He's clearly the front-runner," DesJarlais said as he entered the meeting. "In my district in Tennessee, he won almost 50% of the vote. I think he has the clearest path to the nomination and it only makes sense that he unify the party and get people behind him."

After the meeting, Cotton's office issued a statement saying that he remains neutral in the presidential race and is focused on beating the Democratic nominee, whether it is Hillary Clinton or Vermont. Sen. Bernie Sanders.

Trump's visit comes at a moment of rising concern among Republican leaders about the violent protests and unrest that have erupted at Trump's rallies, which he has been reluctant to condemn.

But Trump said at the news conference Monday that "I don't want violence" and said protests targeting his rallies had been the work of "professional agitators."

"These are not good people, the people who are supporters are unbelievably good people," Trump said.

Despite his visit to Washington, Trump is unlikely to cozy up to establishment leaders as he seeks to unify the Republican Party. The tone of his campaign, with its mockery of opponents and lambasting of Republican leaders, has elevated the billionaire to his current prominence in a year of anti-Washington fury -- so he is unlikely to suddenly tout endorsements by GOP grandees.

Trump's ascendancy, however, is starting to prompt Clinton and the Democratic Party to gear up for an all-out campaign to brand him as too unstable and unqualified to meet the grave challenges posed in the Oval Office.

In an attempt to show his seriousness on foreign policy, Trump released the names of at least five experts who work with Sessions to give him advice on global affairs. They include counter-terrorism expert Walid Phares, energy consultant George Papadopoulos, former Defense Department inspector general Joe Schmitz, managing partner of Global Energy Capital Carter Page and former Lt. Gen. Keith Kellogg.

Clinton, speaking at AIPAC before Trump, has had her own differences with the Israeli government of Prime Minister Benjamin Netanyahu, notably when she served as Obama's secretary of state. But she did not miss an opportunity to take aim at Trump on Monday morning.

"We need steady hands. Not a president who says he is neutral on Monday, pro-Israel on Tuesday and who knows what on Wednesday because everything is negotiable," Clinton said in a clear shot at the Republican front-runner.