Two days after making his inaugural campaign-trail appearance with presumptive GOP presidential nominee Mitt Romney, Sen. Marco Rubio (R-Fla.) on Wednesday took his latest step onto the national stage – and further stoked speculation about his vice-presidential aspirations – with a lengthy foreign-policy address at the Brookings Institution in Washington.

(J. Scott Applewhite/AP)

Rather than solidify his potential role as an attack dog, the speech appeared designed to bolster Rubio’s bipartisan credentials. Both the venue – Brookings, a leading think tank, is considered to have a Democratic tilt – as well as the tone of address suggested a far less partisan approach than the freshman Florida Republican has exhibited on the trail, where he has lambasted Obama as a president who “doesn’t know what he’s doing.”

“The easiest thing for me to do here today is give a speech on my disagreement with this administration on foreign policy,” Rubio told the Brookings crowd in opening his address. “I have many. But I wanted to begin by addressing another trend in our body politic. One that increasingly says it is time to focus less on the world and more on ourselves.”

He went on to deliver a speech defending U.S. involvement in crises abroad and focused on “how good a strong and engaged America has been for the world,” at one point even offering praise for former President Bill Clinton’s handling of the conflict in Kosovo.

Even when asked by a member of the audience how he differed with Obama on foreign policy, Rubio declined to harshly criticize the president.

Introducing Rubio was Sen. Joe Lieberman, the Connecticut Democrat-turned-Independent and foreign-policy hawk with whom the Florida Republican has developed a close working relationship since arriving in the Senate in November 2010.

Praising him as “a rising star in the next generation of America’s foreign policy leaders,” Lieberman argued that rather than focus inward as many in the Senate have done at a time when the country’s economy is struggling, Rubio has endeavored on the Foreign Relations Committee to look outward and take a role in shaping U.S. policy abroad.

“It grows from his own life’s journey, I believe, from tyranny to freedom,” Lieberman said of Rubio, whose parents immigrated to the United States from Cuba.

In his speech, Rubio focused extensively on China, Russia and Iran, each of which he mentioned more than a dozen times. By contrast, he mentioned Afghanistan only four times and said nothing about Iraq.

Rubio also told the crowd that he has “begun to rely heavily” on a new book by Brookings senior fellow Robert Kagan, whose name the Florida senator invoked half-a-dozen times in his address.

The neoconservative thinker, who also writes a monthly column for The Washington Post, served as speechwriter for former secretary of State George P. Shultz and was an adviser to the 2008 presidential campaign of Sen. John McCain (R-Ariz.), another foreign-policy hawk with whom Rubio is close in the Senate.

Rubio’s overarching message to the crowd was: “On the most difficult transnational challenges of our time, who will lead if we do not? The answer, at least today, is that no other nation or organization is willing or able to do so.”

On Iran, Rubio said that the U.S. should be “preparing our allies, and the world, for the reality that unfortunately, if all else fails, preventing a nuclear Iran may tragically require a military solution.”

“The goal of preventing a dominant Iran is so important that every regional policy we adopt should be crafted with that overriding goal in mind,” he said. “The current situation in Syria is an example of such an approach. The fall of Assad would be a significant blow to Iran’s ambitions. On those grounds alone, we should be seeking to help the people of Syria bring him down.”

When it comes to the Asia-Pacific region, Rubio argued that “the question of whether China’s rise will be peaceful and respectful of their neighbors is our biggest long-term challenge.”

“I still have hope that behind the curtain of secrecy that veils the Chinese state, there are voices who advocate for the peaceful and responsible rise of that nation. ... [But] at least for now, it would be foolish to be confident in the idea that China can be counted on to defend and support global economic and political freedom or take up the cause of human rights,” he said.

One area where Rubio did criticize Obama was on the issue of Russia, with which U.S. negotiations over nuclear disarmament in the wake of the 2010 START Treaty have hit a roadblock over Russian resistance to NATO-led missile defense plans in Europe.

“I know some here might disagree, and certainly the president would, but I feel like we have gotten precious little from Russia in exchange for concessions on nuclear weapons,” Rubio said.

He also took aim at Prime Minister Vladimir Putin by name, telling the crowd that the Russian leader “might talk tough, but he knows he is weak.”

“Everywhere he looks, he sees threats to his rule, real and imagined,” Rubio said. “And so he uses state-owned media to preach paranoia and anti-Western sentiments to Russians.”

He said of the United Nations Security Council that the multinational body “remains a very valuable forum, but not an indispensable one.”

“We can’t walk away from a problem because some members ... refuse to act,” he said.

Toward the end of his speech, Rubio realized he had forgotten to bring to the podium the final page of his prepared text.

“I left my last page of the speech. Does anybody have my last page?” he asked, before quickly being handed a copy.

At points, Rubio injected humor into a speech on an otherwise sober topic, drawing some laughs from the staid think-tank crowd.

He cited the 1970s as one era during which the U.S. “felt less than confident about the future.”

It was a period, he said, during which the country “experienced setbacks against communism in Asia, the collapse of trust in government, the oil shock, stagflation, high interest rates, Soviet expansion, the hostage crisis in Iran -- and disco music.”

And when asked about his thoughts on Afghanistan’s reconstruction, Rubio was blunt.

“I don’t think it is going to become Canada,” he said.