Hillary Clinton said on Sunday she was convinced that energy companies were manipulating oil prices and vowed to launch an investigation if elected president.

The New York senator is vying with Barack Obama, her rival for the Democratic nomination, to strike the most hostile stance towards “big oil” as they exploit public anger at soaring energy prices ahead of Tuesday’s primary contests in Indiana and North Carolina.

“I am absolutely convinced that these record profits we’ve been seeing from the oil companies are the result of factors beyond supply and demand,” she said on ABC’s This Week. “I think there has been market manipulation.”

She called for probes by the justice department and the Federal Trade Commission and promised to challenge Opec, the oil producers’ cartel, on its role in the alleged manipulation.

The average price of petrol has reached a record $3.60 in the US, up 65 per cent from 18 months ago, and opinion polls show the issue emerging as one of voters’ top concerns ahead of November’s election.

Both Democratic candidates have promised to impose a multi-billion dollar windfall tax on oil company profits, with Mrs Clinton planning to invest the proceeds in renewable energy. Mr Obama would use the cash to fund tax breaks for working families.

But the rivals are sharply at odds over a populist proposal to suspend federal petrol and diesel taxes this summer to provide temporary relief to consumers.

The idea was first floated by John McCain, the Republican presidential candidate, and was later embraced by Mrs Clinton with the proviso that oil companies should pay for the lost tax revenues. Mr Obama has rejected the proposal as a “Washington gimmick” that would save the average family only $30 (€19, £15) and do nothing to curb the underlying ­problem.

“This defines the difference between myself and Senator Clinton,” he told NBC’s Meet the Press. “It is a political response to a serious problem that we have neglected for decades.”

Mr Obama’s position has won support from many economists and senior politicians who say a so-called “gas tax holiday” would backfire by creating an incentive for people to use more fuel, putting more upward pressure on prices.

Michael Bloomberg, the mayor of New York, said it was the “dumbest” idea he had heard for years, and Nancy Pelosi, the House speaker, has also voiced opposition. But the policy has proved popular among voters, helping Mrs Clinton eliminate Mr Obama’s longtime poll lead in Indiana and narrow his advantage in North Carolina.

Ray Connolly of the American Petroleum Institute, an industry lobby group, said increased taxation of oil companies would only worsen the supply crunch by discouraging exploration for fresh reserves.

He said investigations had shown that the industry had little control over pricing, adding that the biggest factor behind recent increases was surging demand from China and India.

Oil companies are braced for a more hostile White House next year regardless of who is elected because all three main candidates have a chillier relationship with the industry than does President George W. Bush.

Mrs Clinton warned there was “something seriously wrong with our economy” when ExxonMobil’s record $11bn first-quarter profits were judged a disappointment by Wall Street last week.

“This is truly Dick Cheney’s wonderland,” she said, referring to the US vice-president and his close ties to the energy sector.

● Mr Obama beat Mrs Clinton by just seven votes in the Guam Democratic caucuses on Saturday, splitting the Pacific island’s four nominating delegates evenly between them. A record 4,521 people took part in the contest – three times more than in 2004.