St. Louis ? Democratic presidential hopeful Barack Obama on Friday challenged hundreds of mostly young supporters to help him bring change to a nation demoralized by domestic woes and war.

The senator from Illinois, lagging behind rival Sen. Hillary Rodham Clinton in the polls, addressed a chilly outdoor rally and fundraiser at Union Station.

He was introduced by U.S. Reps. William “Lacy” Clay and Russ Carnahan, both Democrats of St. Louis. Clinton has won support from U.S. Rep. Emanuel Cleaver, of Kansas City, and former Gov. Bob Holden.

The event was billed as a “Missouri Countdown to Change,” an opportunity for grass-roots supporters to help.

“I can’t do it by myself,” Obama said. “Change happens from the bottom up. I’m asking you to trust in our ability to change.”