WASHINGTON - Although he's offered a preview of his second term, President Barack Obama's second inaugural address wasn't so much a list of legislative proposals as it was a plea for tackling challenges.

He said, "We must act, knowing that our work will be imperfect," and that today's victories "will only be partial." In a plea for unity to politicians and to the nation at large, he called for "collective action" -- even amid continuing disagreement about the role of government.

In his 18-minute speech, Obama didn't dwell on the most pressing challenges of the past four years. He barely mentioned the struggle to reduce the federal deficit.

But he echoed a theme from his campaign in saying that the nation's prosperity "must rest upon the broad shoulders of a rising middle class."

He defended Medicare, Medicaid and Social Security as programs that allow Americans to "take the risk that made this country great."

Parts of Obama's speech were more unexpected -- including his emphasis on the need to combat global climate change, and his firm new declaration of support for full gay rights.