Democratic presidential candidate Barack Obama on Saturday lashed out at rival Hillary Rodham Clinton, accusing her of dodging tough questions about Social Security.

Obama, campaigning at a senior center in Des Moines, said all the presidential candidates need to talk honestly about Social Security instead of sidestepping the issue, but he singled out Clinton — the front-runner for the nomination — for special criticism.

Obama's criticism of Clinton comes as she has been increasing her lead in national polls, leaving him and other Democratic candidates lagging far behind.

Obama's campaign arranged for him to be introduced at the event by Tod Bowman, a Democrat and a teacher from Maquoketa, Iowa, who tried to pin down Clinton during a debate in Iowa earlier this month on whether the government should tax workers' earnings above the present cap of $97,500 to help pay for Social Security benefits.

Clinton sidestepped the question in public, but told Bowman privately afterward that she didn't want to put an additional tax burden on the middle class but 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 an Associated Press reporter.

"A candidate for president owes it to the American people to tell us where they stand," Obama said. "... because you're not ready to lead if you can't tell us where you're going."

"She's not alone in avoiding answering this question directly — she's not alone in ducking the issue," Obama said. "Because conventional thinking in Washington says that Social Security is the third rail of American politics. It says you should hedge, and dodge, and spin, but at all costs, don't answer."

Bowman said he "got the feeling (Clinton) really didn't want to answer my question."

"After the forum ended, she came up and spoke to me, it was nice of her to do that, and I appreciated her taking the time to do that. But, when I asked my question again, hoping she would answer it, this time she gave me a conflicting response," he said. "It left me feeling disappointed, because I just didn't understand why she wouldn't tell me where she stood on an issue this important."

Bowman added: "It made me wonder, if a candidate won't answer a question on the campaign trail, how can we be sure she'll be honest with the American people when they're president?"

Obama spoke about his solutions for Social Security, and said President Bush's argument that the system is broken and needs and overhaul is an exaggeration.

"The underlying system is sound. The actual problem is a projected cash shortfall that can be readily solved. But the longer we wait to solve the problem, the bigger it grows," Obama said.

He said his plan for Social Security would strengthen the system by requiring the wealthiest Americans to pay their fair share. He would also keep Congress from borrowing from the Social Security trust fund, and oppose efforts to privatize the system, cut benefits or raise the retirement age, he said.

"There are a number of ways we can make Social Security solvent that don't involve forcing seniors to bear a heavier burden," he said.