Sen. Marco Rubio (R-Fla.) prepares to answer questions after delivering a keynote speech titled “American Leadership and the future of the Transatlantic Alliance” at Chatham House in London. (TOBY MELVILLE/REUTERS)

Sen. Marco Rubio is making a push this week to burnish his foreign policy credentials and establish gravitas on the world stage ahead of a possible 2016 presidential run, traveling to London to deliver a major policy address and meet British leaders.

At a time of vigorous debate within the Republican Party about the United States’ global role, the first-term senator from Florida is articulating a worldview that places him neatly between the GOP’s tea-party-led isolationist wing and its more established interventionist wing.

Speaking at the Chatham House think tank in London on Tuesday, Rubio paid homage to the “special relationship” between the United States and the United Kingdom and advocated a muscular foreign policy based on engagement with the world, both military and diplomatic.

Rubio was asked in a question-and-answer session where he falls between the isolationist views espoused by Sen. Rand Paul (R-Ky.), a potential 2016 presidential candidate, and the interventionist approach of Sen. John McCain (Ariz.), the 2008 GOP nominee.

“I actually reject those two spectrums,” Rubio said. “That talk of hawks and doves is 20th century Cold War language that no longer applies. I believe in a strategic foreign policy. A strategic foreign policy has a toolbox that has at your disposal diplomacy, foreign aid, soft power, military power, all sorts of things.”

Rubio took the unusual step of directly criticizing President Obama’s foreign policy during the trip, saying he did not support a proposed military strike in Syria earlier this year and was “personally skeptical” of the interim agreement reached last month over Iran’s nuclear program.

It is customary for U.S. politicians to avoid criticizing the president while speaking overseas. During the 2012 campaign, GOP nominee Mitt Romney painstakingly avoided direct criticism of Obama during a trip abroad.

Rubio’s intensified focus on foreign affairs comes at the end of a difficult year for the Cuban American senator. He has been hailed as next-generation standard-bearer for a party struggling to win over more Hispanic and young voters, but has had strained relations with the conservative base by helping to craft an immigration reform package.

With his foreign policy speeches, as well as trips to such countries as Israel and Libya, Rubio is trying to show that he meets the credibility test to become commander in chief.

“He’s in the process of dealing with the fact that he’s 42 and could pass for 32,” said Steve Schmidt, a strategist on past Republican presidential campaigns. Schmidt added, “He is working to build a reputation for seriousness and thoroughness and thoughtfulness on foreign policy issues that go beyond the typical Republican talking points on Israel or the war on terror.”

Richard Williamson, a former assistant secretary of state who was one of Romney’s top foreign policy advisers during the 2012 campaign, said that Rubio “is being shrewd to build up this pillar of his résumé and stake out a position as a responsible adult voice.”

The trip to London followed a speech in Washington two weeks ago in which Rubio said he would chart “a new vision for America’s role abroad” combining diplomacy, foreign assistance and military intervention.

Charlie Black, a longtime GOP lobbyist and adviser to presidential candidates, said, “The more Republicans who can stand up and give a coherent vision and draw a contrast, the better.”

Earlier this year, Rubio angered many conservative activists when he championed a bipartisan bill that would provide a path to citizenship for illegal immigrants. In the early 2016 handicapping, he is being eclipsed by other GOP presidential hopefuls, including Sen. Ted Cruz (Tex.) and New Jersey Gov. Chris Christie.

But Rubio appears to be making an effort to stand out from the pack by paying closer attention to foreign affairs.

“There’s an obvious opportunity, as you look at the field in 2016 on the Republican side, for somebody who can forcefully speak out about America’s role in the world,” said Lanhee Chen, who was policy director on Romney’s campaign. “Rubio’s becoming a powerful spokesman for those who believe America must remain robustly engaged on the world stage.”

A member of the foreign relations and intelligence committees, Rubio has taken eight foreign trips since assuming office in 2011, including to Israel and Jordan in February.

In London, Rubio met with George Osborne, the chancellor of the exchequer; Philip Hammond, the secretary of state for defense; and three members of Parliament who chair the foreign affairs, intelligence and defense committees.

Rubio documented the visit, including a stop at the gates outside Big Ben, with photographs sent via Twitter and posted on his Web site. He plans to sit for interviews with British news organizations, aides said.

In his speech, Rubio said the United States and the United Kingdom should be “blunt” about their differences with Russia and called for the countries to work more closely on Egypt’s transition to democracy.

He addressed international concern about the Republican-led U.S. government shutdown in October.

“Our federal government is in the grip of gridlock,” he said, adding: “I understand why so many at home and around the world fear that America’s best days may lie in her past. And yet I am here to assure you that, while the road before us will be long and difficult, our finest hour as a nation — and as an alliance — is yet to come.”