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Not that the Udall camp hasn’t had its moments. In May, for example, the Schaffer campaign released a televised ad showing a mountain range and identifying it as Pike’s Peak near Colorado Springs. It turned out to be the Denali in Alaska.

What makes Mr. Schaffer unique is the setup, the way he lures his unwitting opponents into making assertions that he knows he can refute immediately. Asked to comment, Mr. Schaffer demurred.

“You make it sound like something remarkable, but I’m just stating the facts,” he said. “For most politicians, these wouldn’t be gotcha moments, but they are for Mark Udall.”

The Udall campaign accused Mr. Schaffer of trying to dodge the issues. “I think Bob’s trying to distract people from the fact that he’s so out of touch that the best he can do is play games,” Ms. Trujillo said.

Indeed, there are risks involved with Mr. Schaffer’s strategy. While he has succeeded in catching Mr. Udall off-guard on a number of occasions, the Republican has put himself at risk of alienating voters by appearing unduly strident or combative next to the affable Democrat.

“Udall, while obviously liberal, is a pleasant person. He comes across as pleasant, and you can go a long way in life with a good smile and a pleasing personality,” said Denver pollster Floyd Ciruli.

Mr. Schaffer set the tone early in the campaign at a July 14 debate sponsored by KUSA-TV, the local NBC affiliate. Asked to justify U.S. involvement in Iraq, Mr. Schaffer read a list of reasons defending the war, then asked those who agreed to raise their hands.

He then noted that nobody on Mr. Udall’s side of the aisle had done so, and announced, “What I just read was Mark Udall’s resolution supporting a declaration of war in Iraq.”

Then there was the July 28 debate sponsored by KDVR-TV, the local Fox affiliate, in which Mr. Schaffer predicted House Democrats would vote to adjourn instead of taking up an energy bill. Mr. Udall insisted that he would vote to stay in session.

Three days later, however, Mr. Udall, bedeviled by a busy campaign schedule, missed the vote by minutes, and the adjournment passed 213-212, meaning that his vote would have kept the House in session.

It wasn’t long before the National Republican Senatorial Committee had launched ads accusing Mr. Udall of breaking his promise and missing “a critical vote that could have led to lower gas prices.”

Mr. Schaffer again was in full pounce mode at the Sept. 28 candidates debate on “Meet the Press.” During a discussion on the financial crisis, Mr. Udall, as expected, criticized Mr. Schaffer for favoring less government regulation.

Mr. Schaffer responded by pulling out three 2005 House bills that would have increased restrictions on federal home-loan banks Fannie Mae and Freddie Mac, then noted that Mr. Udall had opposed all three.

Mr. Udall replied that Mr. Schaffer could have taken action during his tenure in Congress when Republicans held the majority. Mr. Schaffer served three terms in the House, ending in 2002. Mr. Udall was elected in 1998.