Potential Republican vice presidential candidate Marco Rubio said Wednesday that a unilateral "military solution" from the United States may be needed to stop Iran from acquiring a nuclear bomb.

Rubio, a freshman senator from Florida, said in a sweeping foreign policy speech that it was imperative that the United States not "stand on the sidelines" of a simmering Middle East, but instead provide leadership to resolve global crises.

"Our preferred option since the US became a global leader has been to work with others to achieve our goals," Rubio said in an address at The Brookings Institution think-tank in Washington.

But "America has acted unilaterally in the past -- and I believe it should continue to do so in the future -- when necessity requires," he added, alluding to Iran.

Rubio, a member of the Senate Foreign Relations Committee, campaigned this week with presidential candidate Mitt Romney, and has been touted as a possible running mate as Republicans seek to defeat President Barack Obama in November.

With Romney winning all five primaries Tuesday and the Republican National Committee now hailing him as "our party's presumptive nominee," the general election battle has begun, and Rubio's speech could be seen as the opening salvo on the foreign policy front.

In an address highlighting the need for muscular US leadership, the son of Cuban refugees said multinational cooperation must be a priority in dealing with the Syrian regime and a rising China.

But he focused on Iran, noting that "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."

US officials should operate on a "dual track," with the United Nations and other international groups to rein in the Islamic Republic, he said.

"We should also 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."

Rubio, 40, noted how Washington should be prepared to bypass the United Nations when "bad actors" prevent the global body from taking meaningful steps, such as on Syria.

"The Security Council remains a valuable forum, but not an indispensable one," he said. "We can't walk away from a problem because some members of the Security Council refuse to act."

While agreeing with Obama that "global problems do require international coalitions," Rubio reminded the president that "effective international coalitions don't form themselves," but need to be instigated and led by the US.

"And that is what this administration doesn't understand."

The White House was prepared.

The Democratic National Committee swiftly called out Rubio's "dishonest attacks" against Obama's record and said that under the president, "we have successfully confronted our enemies and strengthened our alliances to effectively meet the challenges we face overseas."

The DNC also said Rubio's "revisionist history" should be viewed "even more skeptically coming from a man who's using the opportunity of this speech to audition for a another job."

Rubio had a message for his own Republicans, saying they should not embrace a more inward vision "that increasingly says it is time to focus less on the world and more on ourselves."

"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," he said.

Rubio lambasted Russian president-elect Vladimir Putin's preaching of "paranoia and anti-Western sentiments" and said the "curtain of secrecy that veils the Chinese state" makes it unwise to trust them to lead on global economic and political freedom.

He pointed to unrest-plagued Syria as an example of the need for US engagement.

"The region is waiting for American leadership," Rubio said.

"You need the center of gravity to instigate this coalition (supporting opposition groups in Syria) and move forward with a defined plan. In the absence of American power and American influence and American leadership, it's hard to do that."