LOS ANGELES — President Barack Obama said Thursday that the Justice Department will try to “root out” cases of fraud or manipulation in oil markets, even as Attorney General Eric Holder suggested a variety of legal reasons may be behind gasoline’s surge to $4 a gallon.

“We are going to make sure that no one is taking advantage of the American people for their own short-term gain,” Obama said at a town-hall style meeting at a renewable energy plant in Reno, Nev.

The town hall was sandwiched in between Obama’s four fundraising events in California on Thursday — one in San Francisco and three in Los Angeles. The president was holding six fundraisers over the course of his three-day West Coast trip, aimed at high-dollar donors and young people, both of whom will be integral to a campaign that could set fundraising records.

“This is going to just as hard, if not harder, than 2008,” he said of his re-election bid.

With the 2012 campaign in mind, the White House is anxious to show the public it’s taking action to address rising gasoline prices. The national average price for a gallon of regular gasoline was $3.84 on Thursday, about 30 cents higher than a month ago and almost a dollar higher than a year ago.

Obama, decrying such levels as yet another hardship “at a time when things were already pretty tough,” said Holder was forming the Financial Fraud Enforcement Working Group.

The task force will focus some of its investigation on “the role of traders and speculators” in the oil-price surge, Obama said, and will include several Cabinet department officials, federal regulators and the National Association of Attorneys General.