Robert C. O'Brien, the Trump administration’s top hostage negotiator, monitored the trial of rapper A$AP Rocky in Stockholm this summer. (Erik Simander/AP)

President Trump entered his next phase of vetting for a new national security adviser, interviewing his top hostage negotiator Robert C. O’Brien at the White House on Friday and combing through a growing list of candidates with diplomatic, military and business experience.

The ouster of his hawkish national security adviser, John Bolton, has set off a turbulent contest among the Republican Party’s hawks and doves to install an adviser who will help guide the president as he seeks to broker a landmark nuclear deal with North Korea, a withdrawal from Afghanistan and an easing of tensions with Iran.

The president’s most trusted foreign policy adviser, Secretary of State Mike Pompeo, is quietly supporting three of his close colleagues, according to U.S. officials and congressional aides familiar with the matter. Those candidates include O’Brien; Brian Hook, special envoy for Iran; and Ricky Waddell, assistant to the chairman of the Joint Chiefs of Staff.

Vice President Pence’s national security adviser, Keith Kellogg, and the president’s ambassador to Germany, Richard Grenell, are also under consideration for the position, the officials said, speaking on the condition of anonymity to discuss personnel decisions.

Each candidate is coming under scrutiny from influential advisers who have a stake in who becomes the next coordinator of America’s vast foreign policy apparatus.

In the deliberations, U.S. officials have questioned the loyalty of Hook, who entered the State Department under former secretary of state Rex Tillerson and has come under attack by Fox News host Tucker Carlson, who has called him an “unapologetic neocon” with an “undisguised contempt for Donald Trump.” Those criticisms have been blunted by the president’s son-in-law, Jared Kushner, who has worked with Hook on Middle East issues and spoke up for him during a recent meeting with Trump, said a senior U.S. official who spoke on the condition of anonymity.

Hook declined to comment.

O’Brien, a founding partner of a Los Angeles law firm, served in previous U.S. government roles focusing on Afghanistan and the Middle East. Officials view him as a “safe choice” given his strong rapport with colleagues at the State Department and Pentagon. His “affable demeanor” contrasts with Bolton, who was known as a ruthless bureaucratic infighter, the official said. “There’s a desire in the White House to have a team player.”

O’Brien has praised Trump for having “unparalleled success” in bringing home hostages, though his appearance in Stockholm in July to monitor the trial of U.S. rapper A$AP Rocky drew guffaws as critics assailed Trump for what they viewed as an inappropriate intervention in an allied nation’s legal matters.

O’Brien declined to comment.

Grenell is expected to meet with the president to discuss the position on Saturday. Like O’Brien, he previously worked with Bolton. Grenell has clashed with Pompeo and other State Department officials in his role as top diplomat to Berlin, where he has interpreted the role broadly and not always coordinated with other U.S. ambassadors in Europe, officials said.

Grenell did not respond to requests for comment.

Trump’s top negotiator for North Korea, Steve Biegun, has also been shortlisted for the national security adviser position, but officials have said he may vie for the deputy secretary of state position, which could put him in a position to become the top U.S. diplomat if Pompeo decides to run for Senate in Kansas.

As Trump weighs the critical personnel decision, his expressed desire to withdraw from Afghanistan and de-escalate with Iran have unnerved the neoconservative wing of the Republican Party and prompted feuds with other conservatives who favor a more restrained foreign policy.

On Thursday, Sen. Rand Paul of Kentucky and Rep. Liz Cheney of Wyoming launched into a multiday Twitter feud indicative of the two poles of Republican foreign policy.

“Few people have been as wrong on foreign policy over the last few years as the neocons and @Liz_Cheney,” tweeted Paul. “While they might exist, I sure haven’t heard of a war that Liz _Cheney didn’t want us to get involved in. When @realdonaldtrump wanted to end the war in Syria — you guessed it, #NeverTrumper Liz Cheney came out against the president.”

Cheney called the exchange “enlightening” and attempted to claim the pro-Trump banner for herself.

“There are some issues at the heart of that disagreement,” she said during a news conference at the House GOP’s annual retreat on Friday. “There are issues surrounding whether you put America first, as Trump does, or if you blame America first, as Rand Paul does and has for years.”

The fight isn’t purely academic, analysts said, as the president seeks to transition from a foreign policy of maximum pressure against North Korea and Iran, to one of dealmaking and diplomacy.

“We are entering into a new phase,” Tom Wright, a senior fellow at the Brookings Institution, wrote in the Atlantic. “Trump has always had two images of himself on national-security issues — as a militarist and as a dealmaker. As he nears the election, he hopes to move from the former to the latter.”

On Thursday, Trump shot down the idea of tapping Pompeo to serve simultaneously as national security adviser and secretary of state. He also said many potential candidates have expressed interest in the position.

“Everybody wants it badly, as you can imagine,” he said, despite his administration’s historically high job turnover rate. “A lot of people want the job — it’s a great job. It’s great because it’s a lot of fun to work with Donald Trump. It’s very easy actually to work with me. You know why it’s easy? Because I make all the decisions. They don’t have to work.”

Anne Gearan, Josh Dawsey, Missy Ryan and Greg Jaffe contributed to this report.