Internal debates during President Donald Trump’s first two and a half years in office have been marked by acrimony, tension and high-stakes negotiations. So perhaps it was no surprise that Trump named as his fourth national security adviser the State Department’s lead hostage negotiator, Robert C. O’Brien.

No president has had so many national security advisers in his first term. However long O’Brien lasts in the job, his tenure will be defined less by his policy views and more by how he manages disagreements within Trump’s inner circle.

[Analysis: Bolton departure says much about Trump]

O’Brien is a lawyer and veteran diplomat. That is not a traditional career path for national security adviser, and his policy views are not as widely known as those of John Bolton, his predecessor in the post who left the job earlier this month. Bolton and Trump clashed in style and disagreed on numerous issues — including whether Bolton’s leaving was a firing or a resignation.

Interviews with a dozen senators from both parties and independent experts revealed a consensus that O’Brien’s record going forward will hinge on his ability to do a better job influencing Trump, who is sometimes unswayed by the consensus among his most knowledgeable advisers. O’Brien’s lack of a publicly known policy agenda, especially as compared to Bolton, may help him quietly do his job.

O’Brien’s success will also be driven by how well he can coordinate with military, intelligence and diplomatic officials, none of whom is more influential than Secretary of State Mike Pompeo, who often clashed with Bolton.

Trump sounds optimistic, though it is early days.

“I have worked long & hard with Robert. He will do a great job!” Trump tweeted Wednesday in announcing the appointment, which requires no Senate confirmation.

People skills

Senate Foreign Relations Chairman Jim Risch of Idaho said the personal relationship between a president and his national security adviser is critical.

“It’s really important that there be the right chemistry between the individual and the president,” Risch said. “I think it’s a good pick.”

Senate Appropriations Chairman Richard C. Shelby said O’Brien has a good relationship with Pompeo. “That’s very important,” the Alabama Republican added.

But Democratic Sen. Chris Coons of Delaware, a Foreign Relations Committee member, said O’Brien needs to coordinate with — and occasionally argue effectively against — a broader set of players than Trump and Pompeo.

“It is important for the national security adviser to strike a balance between being assertive — having a strong point of view and at times challenging the president’s assumptions — while also being able to work well across a number of significant departments and agencies,” Coons said.

O’Brien’s influence may ultimately be shaped simply by how long he perseveres in a position that Michael Flynn, H.R. McMaster and Bolton held relatively fleetingly. When Bolton left after 17 months on the job, it was the longest tenure of Trump’s three advisers.

Standard-issue conservatism

O’Brien, an attorney from California who specializes in international business, has held other diplomatic posts besides hostage negotiator, including helping to teach jurisprudence to Afghan judges and lawyers and serving as a delegate to the United Nations during the George W. Bush administration.

O’Brien also worked as foreign policy and national security adviser for several recent GOP presidential campaigns, including former Wisconsin Gov. Scott Walker and Sens. Mitt Romney of Utah and Ted Cruz of Texas.

His foreign policy views are not well known, though many of them were set forth in a 2016 book, “While America Slept: Restoring American Leadership to a World in Crisis.”

In that book and in published essays, O’Brien has advocated high defense spending. He has been particularly insistent on building a 350-ship Navy, which would be 27 percent percent bigger than the current fleet, but which many experts say is fiscally unrealistic. (The Navy’s goal is actually 355 ships.)

O’Brien also has made clear his opposition to the multinational agreement with Iran to limit its nuclear program, a pact from which Trump withdrew the U.S.

O’Brien, like Trump, is concerned about allies paying more for their security.

And O’Brien has said the United States should be assertive around the world but that its engagements are not limitless.

“Being the leader of the free world does not mean being the policeman of the entire world,” O’Brien wrote in his book.

As hostage negotiator, O’Brien succeeded in obtaining the release of rapper A$AP Rocky from a Swedish jail and, less conspicuously and so far unsuccessfully, has toiled to obtain the release of journalist Austin Tice, who has been held in Syria since 2012.

Margaux Ewen, executive director of the James W. Foley Legacy Foundation, which works to secure the release of hostages and improve journalists’ safety, expressed the hope that O’Brien’s ascension to the adviser position will ensure White House emphasis on those tasks.

Nonetheless, a national security adviser’s views should not matter terribly much, since the adviser is supposed to execute the president’s policies. When someone like Bolton comes in with a policy agenda to implement behind the scenes, it can do a president a disservice, said Loren DeJonge Schulman, who served at the National Security Council and Pentagon.

“O’Brien’s coming in as a bit of a blank slate, and that may end up helping him,” said Schulman, now an analyst at the Center for a New American Security.

If personalities are more important than policy positions for a national security adviser, then the relationship with Trump will be the most crucial of all. How that will go is ultimately unpredictable.

Trump “promised to be unconventional,” said Coons, “and he has overperformed in that category in foreign relations.”

Andrew Clevenger contributed to this report.