BOWIE, Maryland -- President Barack Obama Thursday hit out at a wave of private and foreign cash swamping the US mid-term election campaign, urging backers to fight millions of dollars with millions of voices.

Obama stepped up his attack on the flood of outside money, often from unidentifiable sources and much of it financing opposition Republicans as they try to recapture Congress, pouring into races across the country.

"It could be the oil companies. It could be the insurance industry. It could be Wall Street. You don't know. Their lips are sealed. The floodgates are open, though," Obama said at a Democratic campaign rally in Maryland.

"Almost every one of these independent organizations is run by Republican operatives. They're posing as nonprofit, non-political groups," Obama said in his most outspoken attack yet on special interest cash.

"They've got names like 'Americans for Prosperity,' or the 'Committee for Truth in Politics,' or 'Moms for Motherhood' -- actually, the last one I made up," Obama quipped.

The torrent of cash has been swelled in this election by a Supreme Court decision in January that ruled campaign finance laws limiting the spending power of corporations, unions or advocacy groups infringed freedom of speech.

The top court's justices narrowly decided that big business firms could dip freely into their general funds to finance unlimited campaign advertising either in support or against a political candidate.

Obama mentioned a report by an offshoot of the liberal think-tank Center for American Progress this week, which said the US Chamber of Commerce may be using money from foreign corporations to attack Democratic candidates.

"We learned that one of the largest groups paying for these ads regularly takes in money from foreign corporations," Obama said, without mentioning the Chamber, often a fierce critic of his administration, by name.

"So groups that receive foreign money are spending huge sums to influence American elections, and they won't tell you where the money for their ads come from," Obama charged before a crowd estimated at 4,000 people, mostly students.

The president, who many analysts thought had rewritten campaign finance orthodoxy with his bumper fundraising in 2008, based on millions of small donors, complained conservative groups were outspending Democratic groups seven to one.

"This is a threat to our democracy. The American people deserve to know who's trying to sway their elections.

"So here's the bottom line. We're going to need to work even harder in this election. We're going to need to fight their millions of dollars with millions of voices."

Obama appeared at a historically black college in Bowie in his latest bid to convince the coalition of young and ethnically diverse voters who flocked to his campaign in 2008 to show up this year, even when he is not on the ballot.

His Democratic Party, reeling from high unemployment and the lingering economic crisis, fears heavy losses in mid-term polls in which the party of a first-term president often takes a beating.

Republicans, energized by the conservative Tea Party movement, need to win 39 seats to take the House and 10 seats to take the Senate.