"I think that the Republican super PACs and the Romney campaign combined will outspend the Democrats probably 2 1/2 [or] 3 to 1 [from] here on in," the former Democratic president and Obama supporter told CBS News program "Face the Nation."
"And I think this is the only time I can remember when a lot of the national polls are closer than the polls in the so-called swing states," Clinton said. "That's because the Obama campaign doesn't have as much money, so they have to concentrate that in the swing states."
Clinton said he thought Obama would win, "assuming the debates are even a draw."
"But I think you can't know because of the enormous financial advantage that Citizens United gave to these Republican super PACs and because of the work they have done and will do on Election Day to try to reduce the number of young people, first-generation immigrants and minorities voting," Clinton said.
Citizens United was a 2010 U.S. Supreme Court case in which the court said in a 5-4 ruling the government may not ban political spending by corporations in candidate elections. Dissenters said allowing corporate money to flood the political marketplace would corrupt democracy.
Clinton told CBS Republicans are counting on lower turnout that will match 2010, when Republicans scored major gains in so-called midterm elections, retaking control of the House and nearly retaking the Senate.
"They have a theory that if ... the people who vote in 2012 look more like the 2010 electorate than the folks that elected the president in the first place in 2008 -- that if they get enough of those folks to stay home -- they can still win," Clinton said.
The Republican National Committee had no immediate comment.
Clinton predicted Republicans and Democrats would work together to avert a major budget crisis if Obama wins re-election.
"If he wins, then he can't run again -- the calculus of the Congress will change," Clinton said. "They will be facing this fiscal cliff. It is doing exactly what it was intended to do. It will force them to concentrate."
The fiscal cliff refers to tax hikes and major cuts in federal spending set to automatically go into effect Jan. 2. They would reduce the federal budget deficit. But economists say they would also dramatically slow down the economy.
"As soon as this election is over, the incentives for gridlock will go way down and the incentives for action will go way up," Clinton said.
"But that won't be enough unless we also reach a 10-year budget agreement that will hold interest rates down when the economy starts to grow," he said. "See, when the economy starts to grow and people start borrowing money again, and banks start loaning money to small businesses and not just big ones, interest rates will go up, because there will be more competition for money.
"If interest rates were the same today as they were when I was president, the payment on the debt -- that is, what the taxpayers have to pay every year -- the financial debt would go from $250 billion to $650 billion a year. They can't let that happen," Clinton said.
Looking to the presidential election after this one, Clinton said he had "no earthly idea" if his wife, Secretary of State Hillary Clinton, would run in 2016.
She has said she plans to leave the State Department if Obama wins a second term.
"She's tired. She has really worked hard," Clinton said, adding Hillary Clinton planned to "take some time off, kind of regroup, write a book."
When asked if he thought his wife was the most qualified person to run, he said: "I have never met anybody I thought was any better than her at this. But, again, we have got a lot of able people in our party who want to be president. There is never a shortage of people who want to be president."
He added: "But whatever she does, I am for her, first, last and always."