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Jacquelyn Martin, Associated Press
Senate Foreign Relations Committee member Sen. Marco Rubio, R-Fla., left, jokes with Sen. Joseph Lieberman, I-Conn., before Rubio spoke about foreign policy at the Brookings Institution, in Washington, Wednesday, April 25, 2012.

WASHINGTON — Sen. Marco Rubio has outlined his vision of a more muscular American foreign policy as Republican presidential candidate Mitt Romney begins the search for a running mate.

Rubio gave a half-hour speech at the centrist Brookings Institution Wednesday. He spoke days after he campaigned with Romney, and unveiled a new immigration proposal that breaks ranks with some in his own party.

Both in Washington and around the country, the 40-year-old Cuban-American is thrusting himself forward as a fresh conservative.

He has remained coy about whether he would join Romney's ticket. But his careful criticism of Obama's leadership as well as isolationist tendencies in Rubio's own party reinforced the image he has projected of himself as a tough conservative but one moderate enough for national election.