Expanding on his weekend criticism and a new television ad, the first-term Illinois senator argued that Clinton, like others in Washington, is ducking the issue of fixing the retirement program that is expected to slip out of a surplus in about 2017.
"On issues as fundamental as how to protect Social Security a candidate for president owes it to the American people to tell us where they stand," Obama told about 100 people at a townhall-style meeting in Iowa.
Clinton answered back, unveiling a new ad in Iowa and New Hampshire that contends she challenged President Bush when he tried to introduce private accounts and has pushed legislation to help people care for adult family members.
"These days, it seems like every candidate on Earth is coming here for you. But which candidate has been there for you all along?" says the ad from the two-term New York senator.
Seniors play a critical role in the Iowa caucuses, set for Jan. 3. In 2000 and 2004, nearly 65 percent of those who showed up at the Democratic caucuses were older than 50.
Obama's proposal calls for changing the rules that now impose Social Security taxes on only the first $97,000 in income, a system that means higher earners don't pay taxes on all their income while many middle-class taxpayers do.
Clinton has sidestepped that question in public but told a man at one recent event in Iowa that she would consider a "gap," with no Social Security taxes on income from $97,500 to around $200,000. Anything above that could be taxed. Her answer was overhead by an Associated Press reporter.
Obama's criticism of Clinton touches on two of the core themes of his campaign, arguing that Clinton is too close to the Washington establishment to make real change and that he's willing to offer detailed solutions to the nation's problems.
"I don't think you're ready to lead if you can't tell us where you're going," Obama said.
Social Security was a distant idea later in the day as Obama appeared before about 150 college students at an event sponsored by MTV and streamed live on MySpace.
Obama was asked whether he would name a Republican in his administration and if he would join a ticket with Clinton.
"I would be willing to bring a Republican into my Cabinet because I don't think Democrats have a monopoly on wisdom," Obama said.
As far as becoming Clinton's running mate, he said, "It's way too early because we don't know yet who is going to be the nominee. I will say I'm not running for vice president."
During his appearance, Obama took questions from Coe College students and from those online through MySpace.
He was asked about his stepped-up criticism of Clinton.
"Senator Clinton and I have differences," Obama said. "If I didn't think I was the best person for the job I wouldn't be running. She and I may have distinctions, and it's important for voters, for caucus-goers, to know those differences. As long as I don't get personal about it ... we'll be fine."
After his MTV appearance, Obama spoke to a crowd of nearly 200 students at an outdoor rally.
"Throughout our history, change always happens because young people have somehow gotten activated. It's young people who have led change," said Obama. "It was young people who decided slavery was wrong, it was young people who decided segregation was wrong. It was young people who made sure women had the right to vote."
Asked during the forum who should play him in a movie of his life, Obama suggested Denzel Washington and Will Smith.
"Will Smith is a possibility because his ears match mine. In terms of my wife, there's nobody that good-looking. She would have to play herself," Obama said.