(CNN) — Even as Hillary Clinton's campaign attacked her rival, Barack Obama, for failing to "deliver on his promises," her husband, former President Bill Clinton said Saturday that a joint ticket pairing the two would be "almost unstoppable."

The former president referred to his wife's own comments that indicated a willingness to consider the prospect. "She said yesterday and she said the day after her big wins in Texas and Ohio and Rhode Island that she was very open to that and I think she answered explicitly 'Yes' yesterday," said Clinton during a Mississippi campaign appearance.

"I know that she has always been open to it, because she believes that if you can unite the energy and the new people that he's brought in and the people in these vast swaths of small town and rural America that she's carried overwhelmingly, if you had those two things together she thinks it'd be hard to beat."

He added that, in his view, Obama would win the "urban areas and the upscale voters" while Clinton claims "the traditional rural areas that we lost when President Reagan was president. If you put those two things together, you'd have an almost unstoppable force."

Hillary Clinton told a CBS interviewer earlier this week, shortly after she ended a string of 11 losses with wins in Texas, Ohio and Rhode Island, that a joint ticket "may be where this is headed. But of course we have to decide who is on the top of ticket. I think the people of Ohio very clearly said that it should be me."

The New York senator has made the suggestion in other interviews, as have her campaign surrogates. On Friday, Pennsylvania Gov. Ed Rendell told the National Journal that it was important for the winner of the Democratic nomination to make the offer to the runner-up this year.

The Obama team has largely avoided making similar statements.

UPDATE: In an interview with CNN affiliate KTVQ in Billings, Montana, Obama called the notion "premature," saying he has won twice as many states as Clinton and a greater share of the popular vote, and he believes he can maintain a delegate lead.

"You won't see me as a vice presidential candidate, you know, I'm running for president," said Obama.