The US’ chief hostage negotiator Robert C. O’Brien was tapped by President Donald Trump on Wednesday to replace John Bolton as the new national security adviser.

As a negotiator, he worked on a number of hostage cases, including the release of Pastor Andrew Brunson in Turkey and several others in Afghanistan, Yemen and Libya. President Trump has praised him for his “unparalleled success” in bringing home hostages.

John Bolton was fired on September 10, apparently over his handling of events in Venezuela, Iran, Afghanistan, and North Korea. If confirmed, he will be President Trump’s fourth national security advisor since he came to office in 2016.

There is little doubt about his experience. O’Brien has held different roles in the US State Department, serving under two secretaries of state, Condoleezza Rice in the Bush administration and Hillary Clinton in the Obama administration.

O’Brien has also worked as a foreign policy adviser for several Republican presidential campaigns, including Mitt Romney, Scott Walker, and Ted Cruz. In his 2016 book “While America Slept,” he criticized President Barack Obama of appeasement policy vis-à-vis Russia and China.

O’Brien is a founding partner in major legal firm (Larson O’Brien) in Los Angeles. His appointment was announced by President Trump via Twitter, whilst in the city for a campaign fundraiser.

Upon taking the job, O’Brien will become the highest-ranking Mormon in the U.S. government, which could also have electoral significance in the countdown to Presidential elections. O’Brien converted to Catholicism in his 20s.