PORTLAND, Oregon -- Distancing himself from President Bush, Sen. John McCain pledged a new era of environmental stewardship Monday as he outlined his plan to address global warming, a cause he has embraced since environmental activists hounded him during his 2000 run for the presidency.

"I will not shirk the mantle of leadership that the United States bears," McCain said, alluding to Bush, who withdrew from the international agreement to curtail emissions known as the Kyoto Protocol. "I will not permit eight long years to pass without serious action on serious challenges."

Referring to melting glaciers in the Arctic Ocean and the vanishing habitats of polar bears and walruses, the Arizona senator and presumptive Republican presidential nominee said it was time to stop quibbling over the causes of global warming.

McCain pledged to "deal with the central facts of rising temperatures, rising waters and all the endless troubles that global warming will bring."

McCain is emphasizing the environment while he tours the Pacific Northwest this week, seeking the support of independent voters. Although environmental groups regard McCain more favorably over many Republicans, some view his record as disappointing.

The League of Conservation Voters has given McCain a lifetime grade of 24 percent on what it considers critical environmental votes, while his Democratic rivals, Illinois Sen. Barack Obama and New York Sen. Hillary Rodham Clinton, scored 86 percent and 87 percent.

Cathy Duvall, national political director for the Sierra Club, said McCain's emphasis on global warming glosses over "a mixed record on a whole range of other issues -- whether it's regulating polluters to protecting open space."

Even with environmental causes he has embraced, McCain sometimes is criticized for not going far enough. He has opposed a federal standard on renewable energy while his opponents have called for drawing as much as one-quarter of the nation's energy from such sources within two decades. He authored legislation raising fuel economy standards to 36 miles per gallon over 13 years in 2002, yet didn't support an alternate proposal increasing the requirement to 40 mpg.

McCain frequently touches on other environmental interests on the campaign trail that range from protecting Florida's Everglades to restoring jobs in Michigan by investing in "green" technologies.

On Monday, McCain said he would achieve his emissions targets through a "cap-and-trade system" similar to the one he introduced with Sen. Joe Lieberman, I-Conn., in the Senate in 2003. He said the approach, by allowing companies to buy or trade emissions credits, would allow the market to "reward any person or company that seeks to invent, improve or acquire alternatives to carbon-based energy."

Without providing specifics, McCain said he would "add to current federal efforts" to develop technologies such as plug-in, hybrid, flex-fuel and hydrogen-powered vehicles. He also called for applying better environmental standards for "every purchase government makes."

As he often does, McCain pushed expanding nuclear power, which some environmentalists deride as costly and risky.

"It doesn't take a leap in logic to conclude that if we want to arrest global warming, then nuclear energy is a powerful ally in that cause," McCain said, adding that innovations can reduce the potential hazards of nuclear power.

Both Clinton and Obama favor reducing greenhouse gas emissions by 80 percent by mid-century. On Monday, Clinton said McCain's proposal "does not go far enough." Obama chided McCain for "voting against virtually every recent effort to actually invest in clean energy."

Jim DiPeso, policy director of Republicans for Environmental Protection, said McCain's accomplishments are being overlooked.

"Yes, it's important to have a plan out there," he said, "but the real test of leadership is what do you do to make that bill law."