President-elect Trump’s expected decision to nominate Sen. Marco Rubio (R-Fla.) as his secretary of State and Rep. Mike Waltz (R-Fla.) as his national security adviser is offering some reassurance to Trump critics in Washington and U.S. allies worried about his “America First” agenda.  Rubio and Waltz are known quantities, and their picks are leaving figures in the foreign policy establishment cautiously optimistic that a new Trump agenda won't torpedo long-standing U.S. relationships and policies.  Trump has not yet officially confirmed Rubio as his nominee for secretary of State, although a source familiar with the matter confirmed to The Hill that he is a serious contender, following reporting from The New York Times late Monday night on the expected nomination.  But that hasn’t stopped current and former lawmakers from corners often critical of Trump to offer quick praise for a pick they see as bringing some stability to what is likely to be an unpredictable administration.  Sen. John Fetterman (D-Pa.), while saying he disagreed with many of Rubio's policies, said he'd vote to confirm his Senate colleague. Former Rep. Adam Kinzinger (R-Ill.), who backed Vice President Harris for president, in an interview Monday night with CNN said Rubio is "by no means" a neoconservative, "but he’s somebody that understands foreign policy. And that’s going to be important.”  That said, there are concerns that Trump's picks are less important than Trump's own intentions.  “Again, and I’m willing to give as much leeway as possible, but you know, Marco has shown his ability to kind of change for whatever Donald Trump demands, so it really comes down to, ‘What does Donald Trump demand?’” Kinzinger said in the interview. Rubio’s votes against aid packages for Ukraine in February and April are held up as primary examples of how the Florida senator is willing to put politics over policy, raising particular concern for Ukrainian officials and NATO allies that want U.S. support for Kyiv to continue.  “He’s not principled,” said one European official.  Likewise, Waltz voted against the Biden administration’s Ukraine aid package that passed the House in April, arguing that the U.S. should not write a “blank check” for Kyiv and that European partners should do more, even as he voted in favor of U.S. military aid for Israel and Taiwan.  Still, some advocates for U.S. aid to Ukraine are giving Waltz leeway. The vote against Ukraine aid in 2024 marked a departure for Waltz, who had supported nearly every measure of U.S. assistance to Kyiv in the initial aftermath of Russia’s full-scale invasion in February 2022.  For Ukraine’s advocates in Washington, Waltz’s shifting position is viewed as the necessity of politics in his push to join Trump’s administration, and there’s cautious optimism that he views Russian President Vladimir Putin as an adversary.  It’s not clear how Waltz and Rubio might interact on Trump’s national security and foreign policy, assuming they get the posts.  Rubio, a three-term senator, is more familiar in foreign policy circles and described even by Democrats as balancing national security interests with efforts to advance respect for human rights and supporting democratic freedoms worldwide.  “I think he's a very good nominee,” Sen. Jeff Merkley (D-Ore.) told CNN on Tuesday.  “I have worked very closely with Marco Rubio on issues from the Uyghur Forced Labor Prevention Act, we paired up on that, to the Taiwan Relations Act. We've worked together on issues related to Hong Kong and the blocking the provision of tools that the police use in Hong Kong to suppress dissent. So there's a human rights element to his work that has been significant,” Merkley said.  The son of immigrants from Cuba, Rubio is a leading voice on Latin American policy, most recently serving as the ranking member of the Senate Foreign Relations subcommittee focused on democracy, human rights, women's issues and combating transnational crime in the Western hemisphere.  “He’s really thoughtful on policy matters and also elevated women’s voices in foreign policy,” said Algene Sajery, who served as senior foreign policy adviser for Sen. Ben Cardin (D-Md.) and worked with Rubio and his staff on the Senate Foreign Relations Committee. “I think he can sometimes let politics supersede good policy, but overall — in most cases — he has good intentions and is principled in his approach.”  That opinion is echoed by other foreign officials from Europe, who spoke to The Hill on the condition of anonymity to speak candidly.  “It’s a balanced choice, it could have been worse,” said one diplomat based in Washington, D.C.  This group of foreign policy experts and foreign officials were bracing for Trump to nominate Richard Grenell for America’s top diplomatic post.  Grenell, who served as acting director of national intelligence in the final days of Trump’s first administration, is a polarizing figure because of his confrontational and combative persona. “Rubio as Secretary of State and Waltz as National Security Advisor. When it comes to Ukraine, this might not be the dreadful news that many expected. The real question is where does Grenell end up,” Doug Klain, policy analyst at Razom For Ukraine, a nonprofit supporting Ukraine, posted on the social platform X.  Grenell is viewed as an extremely loyal foot soldier for Trump and, in a podcast interview in March, said, “If you want to avoid war you better have a son of a bitch as the secretary of State.” Rubio is viewed as having more tact and skill operating in diplomatic circles and with foreign leaders, and as a palatable envoy to Latin America in the face of Trump’s promises to seal the southern border, deport millions of illegal migrants and threaten military force against cartels in Mexico.  "In terms of the team President Trump is building, it’s obviously heavy on loyalists but many of these players bring their own credibility to the jobs,” said Stewart Verdery, who served in former President George W. Bush’s administration and is a founder of Monument Advocacy.  “The national security team in particular sends a message of seriousness and ‘strong’ power instead of approaches like ‘soft’ power or ‘leading from behind.'"  And as ranking member of the Senate Intelligence Committee, Rubio has been outspoken about confronting security threats from China and providing political and military support for Taiwan,  an area where Trump has raised doubt about American military commitments.  “The first point you can draw, is the focus is on China,” said the diplomat based in Washington. “If you couple this with [Waltz’s] national security adviser appointment, they are likable, they know how to communicate.” Likewise, Waltz has garnered a reputation as a China hawk. A former Green Beret, Waltz serves on the House Armed Services Committee, on the House Foreign Affairs subcommittee on Asia and nonproliferation, and is a member of the House Select Intelligence Committee.  “In terms of foreign policy with Rubio and Waltz, you have two known quantities, people with a long record of engagement in the foreign policy arena,” said former Rep. Carlos Curbelo (R-Fla.). “There’s an element of predictability there. Obviously they’re going to be executing the president’s policies but they’re going to do it in their respective styles in the context of their own experiences and expertise, and I think that’s given comfort to a lot of people.” Alex Gangitano contributed reporting