WASHINGTON (CNN) -- Donald Trump's campaign enters a new phase Monday as he heads to Washington to prove that he's more than a rabble-rousing outsider.

The billionaire businessman, who built his campaign eviscerating Washington, is meeting with a group of high-profile Republicans in the afternoon in an apparent attempt to improve his tense relations with the party. He will then address the American Israel Public Affairs Committee's annual meeting, an event that will test Trump's skills in working with Washington's foreign policy establishment.

The meeting comes as the Republican establishment intensifies its 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 will focus renewed attention both on the rift in the Republican Party and on his own policies and qualifications to lead. As he compiles a daunting lead 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

He enters the meeting with GOP officials from a position of strength, having received 7.5 million votes in Republican contests so far. A source familiar with the meetings told CNN that former House speaker and presidential candidate Newt Gingrich would attend the gathering, which was facilitated by Alabama Senator Jeff Sessions, who has endorsed the GOP front-runner.

Other lawmakers attending include Sen. Tom Cotton of Arkansas, Tom Marino of Pennsylvania, Duncan Hunter of California, Scott DesJarlais of Tennessee and Renee Elmers of North Carolina.

"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."

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.

Still, 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 Hillary 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.

That's one reason why the Republican front-runner's appearance before the AIPAC annual meeting will be so crucial. In addition to serving as a test of Trump's foreign policy knowledge, it will also call into question his own contention that his skills as a global businessmen would adapt seamlessly to leading U.S. foreign policy.

A sober, detailed performance -- which unusually will be a written address rather than one of his off-the-cuff monologues -- may also help the billionaire begin to address anxiety about the tone he often displays on the campaign trail.

Despite declaring himself the most pro-Israel candidate, Trump's views on stalled Israeli-Palestinian peace moves are at odds with much of the pro-Israel policy community in Washington. His 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 final status deal have raised 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."

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 President Barack 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.