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The two faced off in Wisconsin over barely distinguishable health care plans and nearly identical economic plans. They also traded jabs in TV ads that experts said hurt Mrs. Clinton more.

Sarah A. Fulton, political science professor at Texas A&M; University, said the Clinton campaign appeared “a little desperate” in its TV spot that criticized Mr. Obama for ducking debates, despite 18 previous debates and two more this month in Texas and Ohio.

“This doesn’t seem to me to be a very compelling or credible issue that is gaining much traction,” said Mrs. Fulton, who specializes in political campaigns and the role of women in American politics.

Another scholar said the ad played into the hands of Mr. Obama, who presents himself as the anti-establishment candidate.

Stephen Dinan and Christina Bellantoni contributed to this report.