Obama and other Democratic presidential candidates previously have signaled support for this idea.
Meanwhile,said she would move more carefully than her rivals on dealing with looming shortfalls facing the Social Security system.
In an interview Sunday on NBC's "Meet the Press," Obama said increasing the income ceiling would allow relatively well-off taxpayers such as himself to pay a little more to rescue the system. He objected to benefit cuts or a higher retirement age.
Currently, only the first $97,500 of a person's annual income is taxed; the cap is scheduled to rise to $102,000 next year.
"I think the best way to approach this is to adjust the cap on the payroll tax so that people like myself are paying a little bit more and people who are in need are protected," the Illinois senator said.
"That is the option that I will be pushing forward."
But Clinton rejected that argument.
"I know it may sound good at first blush," said Clinton. "If you look at all the complexities of this, I think it's much smarter to say: Look, we're going to deal with the challenges by fiscal responsibility and we're going to use a bipartisan commission. And we're not going to do it by further burdening middle-class families."
"If you lift the cap completely that would be a $1 trillion tax increase," Clinton told reporters after a Veterans Day event.
Clinton said she would initially end the practice of borrowing from the Social Security trust fund. She said more responsible fiscal practices could bolster the economy, which would in turn make the fund more healthy.
Obama's proposal could include a gap or "doughnut hole" to shield middle-income earners from paying more in taxes, he said.
Obama said some tough decisions will be in order because Social Security is the most important social program in the country.
"It's not sufficient for us to just finesse the issue because we're worried that, well, we might be attacked for the various options we present," he said.
Obama also invoked his friend, billionaire Warren Buffett, who Obama said has expressed concern that he pays less in Social Security taxes than anyone else in his office.
"And he has said, and I think a lot of us who have been fortunate are willing to pay a little bit more to make sure that a senior citizen who is struggling to deal with rising property taxes or rising heating bills, that they've got the coverage that they need," Obama said.
Obama has tried to draw contrasts between himself and front-runner Hillary Rodham Clinton on Social Security, saying on the stump and in TV ads that she has dodged tough questions about its finances.
He made a veiled reference to that at a big fundraising dinner on Saturday.
"The same old textbook campaigns just won't do in this election," Obama said. "That's why not answering questions because we're afraid our answer won't be popular just won't do."
"I think primary voters do know where I am," said Clinton. "I am for solving the long-term challenges of the Social Security trust fund."
She rejected suggestions that there are a very limited number of potential solutions.
"I don't buy that," said Clinton. "I think there are a lot of solutions with many variations."
On the Republican side, candidateunveiled a last week that calls for reducing benefits to future retirees and creating voluntary personal retirement accounts. The plan is similar to one put forth by President Bush that ultimately stalled in Congress.