Al Jazeera's Rosiland Jordan reports from Maryland where Obama has campaigned for the Democratic candidate
In the run-up to mid-term elections set for November 2, Barack Obama is back on the campaign trail to win back popular support for the Democratic Party and himself.
Al Jazeera's Rosiland Jordan reported from a rally on Thursday in Maryland where the US president criticised Republicans aiming to recapture control of congress.
Obama voiced concerns that the Republicans had received a flood of funding from unidentifiable sources supporting their campaign for senate seats 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 in front of signs that read "I am change".
"Almost every one of these independent organisations is run by Republican operatives. They're posing as nonprofit, non-political groups."
The surge in cash flowing into campaigns for this election is owed mainly to a supreme court court decision in January that narrowly decided big business firms could spend freely to finance campaign advertising in support of or against a political candidate.
Obama also mentioned a report by an offshoot of the think-tank, Centre 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, which is often a vocal critic of him.
"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.
"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."
Republicans need to win 39 seats to take control over the House of Representatives and 10 seats to take the senate.