OTTAWA - They don't want our "dirty" oil — and they want to kill the Canadian-built pipeline that would pump it across their fertile plains.

An outspoken coalition of American mayors is urging Canadian voters to grill federal election candidates on clean-energy alternatives to Alberta's oilsands. They oppose a plan that would see Alberta's "tar sands" oil pumped across their country to the Texas Gulf Coast through the $13-billion Keystone XL pipeline.

"As our friendly northern neighbours, we absolutely want Canada to be having this discussion," Mayor Jennifer Hosterman, of Pleasanton, Calif., said in an interview.

"I believe the pipeline is a multibillion-dollar investment to expand our reliance on the dirtiest source of transportation fuel currently available. It's a bad idea."

Hosterman is one of three U.S. mayors who told The Canadian Press they want to see Canadians using their election campaign to talk about North America's shared — and at times competing — energy and greenhouse-gas reduction challenges.

The issue surfaced briefly on the campaign trail Thursday, one day after U.S. President Barack Obama referred to northern Alberta's oil fields as "tar sands," telling a Pennsylvania town hall meeting "there are some environmental questions about how destructive they are."

Canada has approved the Keystone pipeline that TransCanada Corp. wants to build, and Prime Minister Stephen Harper discussed it with Obama at the White House in February. One month later, the U.S. State Department announced a further environmental review.

Last weekend, an editorial in the New York Times urged the State Department to reject the pipeline, saying it is "not only environmentally risky, it is unnecessary."

Some American mayors say the carbon footprint of Alberta oil is bad news for the environment, and they warn that a pipeline spill could contaminate a major source of drinking water for millions across several Midwestern states.

Hosterman was one of 25 U.S. mayors who recently registered their disapproval of the pipeline in a letter to Secretary of State Hillary Clinton. Hosterman serves on the water council for the United States Conference of Mayors, and is a member of the local government advisory commission to the U.S. Environmental Protection Agency.

"Its risks to the environment are a total unknown as the U.S. EPA has said in the draft environmental impact study, calling it inadequate and saying that it should be revised," Hosterman said.

"It doesn't fully look at potential oil spill response plans. It doesn't look at safety issues. It doesn't look at greenhouse gas concerns."

Hosterman and others say a pipeline spill would contaminate drinking water for millions of Americans. The Keystone pipeline would cut through the heart of the American Midwest, crossing the Ogahalla Aquifer — the naturally occurring underground water source that spans South Dakota, Nebraska, Kansas, Oklahoma and northern Texas.

Mayor Frank Cownie, of Des Moines, Iowa, said there has to be vigorous public discussion about protecting the aquifer that supplies fresh drinking water to eight states, and ending America's continent-wide dependency on oil.

"If someone was running in my election, I would be asking that question," Cownie told The Canadian Press.

"Tar sands certainly is an issue … we're looking for models for energy that are sustainable, renewable and have a much lower environmental impact."

Mayor John Dickert, of Racine, Wis., said politicians on both sides of the border should be looking for ways to reduce reliance on fossil fuels while developing alternative sources of clean, sustainable energy.

"While I understand that the president and the prime minister are always looking at alternatives and opportunities, I hope that they don't forget that our conversation really should be directed about cutting our reliance," Dickert said in an interview.

"I definitely hope it is part of the discussion in your campaigns and in the future of our two countries."

Mayor George Heartwell, of Grand Rapids, Mich., said it wasn't his place to tell Canadians how to govern themselves. But in a telephone interview, he quickly added: "I'm convinced that the tar sands mining project is ultimately a harmful one for the environment."

Citizens of Canada and the U.S. need to seriously devote themselves to developing alternative energy sources, said Heartwell. He said his city now gets more than one-fifth of its energy through renewable energy sources.

"I encourage citizens of all countries to be involved in that. We need to focus our energy and natural resources to developing alternatives to fossil fuel," said Heartwell.

Obama declined Wednesday to comment specifically on the Keystone project because it was under review.

"We've got to do some science there to make sure that the natural gas that we have in this country, we're extracting in a safe way," the president added.

"The same thing is true when it comes to oil that's being piped in from Canada, or Alaska, for that matter. We've just got to do these evaluations."

Liberal Leader Michael Ignatieff blasted the Harper government's environmental record when asked on a campaign stop Thursday about Obama's comments.

Ignatieff said he favoured ending subsidies for companies operating in the oilsands, and would invest the extra revenue in green technology development and reducing carbon emissions.

"I've said repeatedly this is an industry with an enormous and great future but it's got to clean up, and we need a federal presence here to work with the industry to get that right."