Republicans usually demonize Barack Obama in their political advertising, but GOP Sen. Gordon Smith aligns himself with the Democratic presidential candidate in a new ad, citing legislation they worked on to improve fuel efficiency standards.

Observers in both parties said it appeared to be the first time in this election cycle that a GOP Senate candidate had aligned with Obama.

The ad, responding to an independent Democratic ad linking Smith to high gas prices, credits Obama with saying that "Gordon Smith led the fight for better gas mileage and a cleaner environment."

Democrats say Obama said nothing of the kind.

The two senators worked together on the 2006 fuel efficiency bill, but Obama never called Smith a leader in the fight and never praised him by mail, said Matthew Miller, a spokesman for the Democratic Senatorial Campaign Committee,

In a more measured statement, Obama's campaign said the Illinois senator appreciated that his "record of bipartisan achievement is respected by his Democratic and Republican colleagues."

But Obama is backing Democrat Jeff Merkley in the Oregon Senate race, said Obama campaign spokesman Bill Burton.

Smith, the lone Senate Republican on the Pacific Coast, is trying to keep his seat in a state that's been tilting toward Democrats. Polls show him running ahead of Merkley, the Oregon House speaker, but with less than a formidable lead.

Smith has studiously avoided any mention of his GOP credentials in his ads and campaign literature. A recent ad featured two Democrats, including former Oregon Rep. Elizabeth Furse, praising Smith for speaking out against the Iraq war in December 2006, after Republicans lost control of Congress.

Tim Hibbitts, an independent pollster in Portland, said the ad shows Smith is "legitimately, seriously concerned about this year." He said Smith runs the risk of alienating GOP voters, but that he apparently has concluded the greater risk is to be seen as a steadfast conservative.

Smith's spokeswoman, Lindsay Gilbride, said the ad highlights Smith's oft-repeated pledge to work with the next president, whether it is Republican Sen. John McCain, whom Smith has endorsed, or Obama.

During the primaries, the North Carolina Republican Party used video of Obama and his controversial former pastor, the Rev. Jeremiah Wright, in an ad. Wright's comments about race and criticism of the U.S. threatened Obama's campaign earlier in the year.

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Matthew Daly reported from Washington.