Grenell, a blunt and outspoken MAGA advocate, emerged from the Trump administration as one of the former president’s top diplomatic prizefighters. He served as U.S. ambassador to Germany, where he roiled the German diplomatic establishment, and then served as acting director of national intelligence — becoming the first openly gay Cabinet secretary in U.S. history. He has cemented himself as one of the most influential advisers on foreign policy issues in Trump’s orbit — Trump refers to him as “my envoy” — and many GOP experts consider him to be a top contender for either secretary of State or national security adviser. Right now, he’s a senior adviser to the Protecting America Initiative, a nonprofit focused on combating the influence of the Chinese government.

The Tennessee senator is one of the establishment Republicans who turned to the MAGA movement after Trump won in 2016. A George H.W. Bush administration veteran and early supporter of Jeb Bush, Hagerty went on to serve as Trump’s ambassador to Japan before running for Senate in 2020. Hagerty has used his position on the powerful Senate Foreign Relations Committee to stake out a reputation as a staunch China hawk. He was also among a small number of senators who voted against the national security aid package passed earlier this year with increased funding for Ukraine. He said at the time it was because the package failed to address border security.

O’Brien stands out as the only one of Trump’s four national security advisers who hasn’t had a high-profile falling out with the former president. O’Brien served as Trump’s special envoy for hostage affairs from 2018 to 2019, where he entered the MAGA spotlight in a bizarre chapter of Trump-era diplomacy: He helped free U.S. rapper A$AP Rocky from Swedish custody after an alleged brawl in Stockholm. (Trump intervened in the case, reportedly threatening a trade war with Sweden over the matter.) O’Brien would be welcomed in Brussels and Kyiv and is seen as a big supporter of NATO and Ukraine in the Republican Party.

Rubio is a former Trump opponent-turned-supporter with hardline views on many foreign policy issues, especially Cuba and Iran. Still, the Florida senator worked with Democrats on legislation that makes it harder for Trump to quit NATO. He has also tried to seem supportive of Ukraine without breaking entirely with Trump’s position that Kyiv must negotiate with Moscow to end the war. Rubio would sail through a GOP-controlled Senate and would likely pick up a few Democratic votes along the way. The big question is whether Trump would see him as too weak to be America’s chief diplomat.

Waltz, a U.S. Army veteran and former Green Beret, is one of the most strident China hawks in Congress and has supported U.S. aid to Ukraine while seeking to boost oversight of taxpayer funding for Kyiv’s war efforts. He was also at the vanguard of the Republican backlash over President Joe Biden’s botched Afghanistan withdrawal. Waltz, a Florida representative, has emerged in GOP circles as a potential contender for a top national security post in a second Trump term, including secretary of State, secretary of Defense or a senior intelligence agency posting.