WASHINGTON — Sen. Maria Cantwell has grilled oil executives on Capitol Hill.

The Washington state Democrat has demanded a federal investigation into rising gasoline prices.

And she has called for a crackdown on Wall Street speculators who she says are driving up gasoline costs — on Friday, the national average price of a gallon of regular gasoline stood at $3.72 ($3.90 in Washington state), up more than a dollar from a year ago.

As the higher costs put a renewed focus on energy issues, Cantwell, 52, once again finds herself in the thick of things in Washington, D.C., working on a hot-button issue that has dominated her agenda this year.

For Cantwell, chairwoman of the Senate Energy and Natural Resources subcommittee on energy, there's a glimmer of good news. As prices rise, she sees an opening for one of her pet causes: trying to get Congress to approve her plan for cutting emissions, which includes putting a price on carbon.

"I always thought when gas prices got closer to $5 a gallon ... that my colleagues would be all ears on a better solution," she said in an interview this week.

She's also hopeful that the public's displeasure with increased prices eventually will nudge the U.S. to stop relying so much on fossil fuels.

"If the Saudi Arabian government is saying it's time for them to diversify their economy, I think we should take heed that obviously we ought to be diversifying off of fossil fuels," she said.

Matt Barreto, associate professor of political science at the University of Washington, said Cantwell has seized upon "the low-hanging fruit" that can only help her as she seeks a third term in 2012.

"It sounds like she's running for election," Barreto said. "Those are good sound bites for anyone. Those are populist issues: People are typically upset about gas prices and energy prices."

Cantwell, who has been working on issues involving oil markets and gasoline prices since 2008, issued a study last week that found Washington state drivers will pay $672 million more for gasoline this summer, from Memorial Day to Labor Day 2011, compared to last year. Each driver in the state can expect to pay an additional $150, she said.

Cantwell said that her constituents are plenty angry about rising gasoline prices and that they let her know about it whenever she returns to the state.

"To say an earful all the time would be an understatement," Cantwell said.

In a speech on the Senate floor last month, Cantwell urged the Commodity Futures Trading Commission to use its emergency authority to rein in speculation that she said is shifting the oil futures market and driving up prices. She said that oil speculators on Wall Street are "profiting from Middle East turmoil" while the public pays the price.

Warren Aakervik, the owner of Ballard Oil Co. in Seattle, said it's good that Cantwell is spending so much time on the issue, adding that she has pinpointed a growing problem for his business. He said he's selling less fuel these days because his customers want less fuel.

Aakervik said that Wall Street speculators are being allowed to change energy markets too dramatically and too quickly. And he said that hurts his primary consumers in the commercial fishing industry.

"Just tell me what the hell happened in the last 24 hours that the guys today are paying seven cents a gallon more than they were yesterday," Aakervik said. "What happened? Was there an atom bomb? Did we start a war? No, it's some trader."

The last time Cantwell tried advancing her carbon-reducing legislation, it stalled. She said she never expected the bill — the Carbon Limits and Energy for America's Renewal, or CLEAR, Act — to pass immediately, calling it a good solution "whose time would eventually come."

And she's the first to admit that the conventional wisdom is that an emissions-reducing bill can't pass in the current session of Congress.

But Cantwell said that Congress should not shy from the task just because it's controversial. She noted that higher fuel standards were approved by lawmakers and are now accepted, despite the howls of protest that came when Congress acted.

Besides, she said, if Congress doesn't eventually put a price on carbon, consumers will continue to be on the losing end.

"Or to put it more bluntly, the coal-fired electricity user is getting a free ride at the expense of the kid on asthma, or those who depend on a water source not destroyed by a mountaintop removal, or the fisherman whose catch is filled with bio-accumulated mercury," she said in a speech in Washington, D.C., last month.

Cantwell said she plans to introduce a version of her carbon-reducing plan again later this year. The last time, it called for the federal government to auction off carbon shares to 2,000 or so fuel producers; the shares would have expired every two years, and over time the government would have offered fewer shares as a way to reduce carbon consumption. Seventy-five percent of the money raised would have been rebated directly to U.S. citizens, offering an average family of four a total of $1,100 a year in tax-free checks.

This year, Cantwell has issued repeated calls for the Federal Trade Commission to investigate any links between higher gas prices and an increase in wholesale oil markets. She said the price per barrel of oil over the past four years has varied drastically despite little change in the world's supply and demand.

Cantwell had a chance to press the point last month, when oil executives came before the Senate Finance Committee. She got Exxon Mobil's chairman to say that oil should cost between $60 and $70 per barrel if the price were based on supply and demand fundamentals. At the time, oil was trading at $98 per barrel. According to Cantwell's calculations, if oil markets were based on supply and demand, drivers would be paying about $1.50 less per gallon right now.

For Cantwell, it was an easy decision last month to vote to end subsidies to oil companies.

"Ending over $20 billion in taxpayer-funded giveaways to big oil shouldn't even be a question," she said. "Oil companies are some of the most profitable businesses on the planet."

It might all add up to smart politics, experts say.

Barreto said that Cantwell, who has yet to draw a big-name opponent for next year's race, is using the power of incumbency in a way that no challenger can.

"A challenger doesn't have the ability to garner the same headlines because they don't get a chance to interview experts and oil executives at Senate committee hearings," he said. "This is a huge incumbent advantage."