President Donald Trump announced on Wednesday that he will appoint U.S. hostage negotiator Robert O’Brien to replace John Bolton as his national security adviser, after a “hard" lobbying campaign from the increasingly empowered secretary of State, Mike Pompeo.

“I have worked long & hard with Robert. He will do a great job!” Trump tweeted.

O’Brien represents a stylistic — but not necessarily an ideological — shift from the man he is replacing. People who have worked with and are close to O’Brien describe him as similarly aggressive as his predecessor on issues like Iran, but more of a congenial colleague than Bolton, who was known as a sharp bureaucratic infighter. And unlike Bolton, O’Brien, a career lawyer before working in government, is not a big name in the intelligence and national security world, indicating he will likely bring a much lower profile to the job.

“He worked for John Bolton at the United Nations and might be as hawkish,” said Andrew Exum, the deputy assistant secretary of defense for Middle East policy from 2015-2016 who knows O’Brien and has traveled with him. “But he’s certainly not as pugilistic.”

Another person close to O’Brien agreed that he is “definitely in the same school as Bolton on Iran,” describing him as “hawkish” but “more of a team player” — and more compliant with Trump and Pompeo’s demands. “Robert’s not going to push back too much,” this person said.

As the U.S. hostage negotiator, O’Brien got on the president’s radar via a very Trumpian combination — flattering comments and his involvement in an only-in-the-Trump era attempt to free an American rapper detained in Sweden.

When A$AP Rocky was jailed on assault charges, Trump made a show of dispatching O’Brien to Sweden to assist in the case, far different than the normal situations he dealt with in hostile countries and war-torn regions.

Trump tweeted frequently about the case from Washington, and claimed that O’Brien had called him “the greatest hostage negotiator that I know of in the history of the United States.” The State Department later confirmed to The New York Times that O’Brien had indeed described him that way. Rocky was eventually released to the U.S.

More broadly, O’Brien has been a part of what Trump sees as a major success of his administration — freeing American hostages.

Trump has frequently touted his administration’s record on the issue, sometimes referring to himself as “chief hostage negotiator.” During Trump’s time in office, his administration has secured the release of suspiciously detained Americans from places like North Korea and Turkey, although O’Brien was not yet in his role when Pyongyang handed over three Americans in May 2018.

Iranian Foreign Minister Javad Zarif also acknowledged earlier this year that he’d received a letter from O’Brien asking for the release of detained U.S. citizens, but described the letter as a one-sided demand rather than an effort to negotiate.

Trump on Wednesday focused on O'Brien's work as a hostage negotiator as he praised his new national security adviser.

"He did a tremendous job on hostage negotiation, really tremendous, like unparalleled. We've had tremendous success in that regard, brought home many people," Trump told reporters. "And through hostage negotiation, I got to know him very well myself. But also a lot of people that I respect rated him as the absolute number one choice."

Standing next to Trump, O'Brien returned the praise.

"We've got a number of challenges but there's a great team in place," he said. "I look forward to working with them, and the president, to keep America safe and continue to rebuild our military and really get us back to a peace through strength posture."

O’Brien isn’t a big name in national security circles. Fred Fleitz, Bolton’s former chief of staff who was also considered for the post, said he knows next to nothing about O’Brien except that he “seems to have pretty good credentials on paper.” Asked whether the Senate Intelligence Committee knew anything about O’Brien, a committee aide said, “nope, not really.”

But Pompeo, who has known O’Brien “for a long time,” urged Trump to appoint the hostage envoy, according to a senior administration official.

“Obviously Secretary Pompeo had a big say in who it would be,” agreed a senior White House official.

O’Brien did pop up during 2016 elections as a foreign policy adviser to Trump opponents Scott Walker and Ted Cruz, four years after he had advised Mitt Romney’s campaign on the same issues. In 2015, while he was still in the Cruz camp, O’Brien wrote that the Texas senator had an “opening” to differentiate himself on national security from Trump, “who has been playing up how chummy he will be with Vladimir Putin if he is elected.”

Prior to joining the Trump administration, O’Brien served as co-chairman of the State Department's public-private partnership for justice reform in Afghanistan under both President Barack Obama and George W. Bush. He also served as a U.S. Representative to the U.N. General Assembly in 2005, where he worked alongside Bolton.

Speaking to reporters aboard Air Force One Wednesday, Trump praised O’Brien as “fantastic.”

The former hostage envoy comes aboard as Trump faces a number of crises in the Middle East, including attempts to broker peace in Afghanistan with the Taliban as well as between the Israelis and Palestinians.

The administration is also grappling with how to confront an increasingly hostile Iranian regime, which Trump hit with a fresh package of sanctions just moments before revealing that he'd tapped O'Brien for the national security post.

Trump has also thrown out the possibility of a third nuclear summit with North Korean leader Kim Jong Un prior to next year’s election, in addition to the prospect of reaching a new arms deal with Russia and the ongoing push to beat back the Islamic State — all of which will require O’Brien’s involvement.

O’Brien, who has advocated for expanding the Navy and was reportedly considered in 2017 to serve as Trump’s Navy secretary, beat out Fleitz, a former CIA analyst who served as Bolton’s chief of staff; Lisa Gordon-Hagerty, the administrator of the National Nuclear Security Administration; Keith Kellogg, national security adviser to Vice President Mike Pence; and Ricky Waddell, a former deputy national security adviser under Trump.

Early on in the search, the president ruled out appointing Pompeo to take over the post in addition to his duties at the State Department, though White House counselor Kellyanne Conway signaled he would play a large role assisting Trump with selecting the administration’s fourth national security adviser.

Last week, Trump downplayed the record levels of turnover in his administration, asserting that a job in his White House was a hot commodity.

“Everybody wants it badly, as you can imagine,” he said. “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.”