WASHINGTON - George W. Bush said Tuesday that he's backing presumptive Republican White House nominee Mitt Romney.

The former president offered a four-word endorsement of Romney as the doors of his elevator were closing after a speech in Washington.

Bush said: "I'm for Mitt Romney."

ABC News caught Bush after the speech, prompting his unscripted - but not surprising - endorsement.

Bush's parents, former President George H.W. Bush and Barbara Bush, endorsed Romney in March during an appearance in Texas. Former Florida Gov. Jeb Bush also publicly backed Romney.

In his speech Tuesday, Bush praised the Arab Spring movement and said the United States shouldn't fear the spread of freedom, even if it doesn't know what policies the countries will pursue.

"America does not get to choose if a freedom revolution should begin or end in the Middle East or elsewhere," Bush said. "It only gets to choose what side it is on."

And the United States, Bush said, should always be on the side of freedom.

The ex-president's remarks came at an event marking the launch of his presidential institute's "Freedom Collection." The event also featured brief remarks by his wife, Laura Bush, and a question-and-answer session by video with Myanmar democracy leader Aung San Suu Kyi.

Bush joked that he had found his own freedom "by leaving Washington."

Since leaving office in January 2009, Bush has tried to avoid politics. He was deeply unpopular with voters who blamed him for the economic crisis that unfolded on his watch.