With the first of three scheduled televised presidential debates between Sens. Barack Obama and John McCain set for Friday night, both campaigns are starting the time-honored strategy of building up expectations -- for the other guy.

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"Have no doubt about the capabilities of Sen. Obama to a debate. He's very, very good," McCain told voters in Ohio. "He was able to defeat Sen. Hillary Clinton, who, as we all know, is very accomplished. He was able to, with his eloquence, inspire a great number of Americans. These will be tough debates."

Obama spokesman Bill Burton delivered similar compliments right back at McCain, even less subtly.

"Given his decades in Washington, John McCain literally has more experience debating than anyone who has ever run for president," Burton said. "If he can't show the skills he's acquired debating foreign policy, it will be a massive disappointment."

Thomas Holbrook, a University of Wisconsin political science professor who has studied the impact of presidential debates on public opinion, said that while the confrontations generate headlines, and sometimes provide "gotcha" moments that journalists talk and write about, the debates themselves generally provide little swing in one candidate's or another's direction.

Public opinion polls have their "bumps and wiggles" in all presidential races, he said. But generally, no debate has ever shown a "radical change" in polling results.

Holbrook looked at 13 past presidential debates and found that the average change in candidate support once the debate periods ended was about 1 percentage point. And he said the entire debate period of a presidential race (which this year lasts from Sept. 29 to Oct. 7) is more likely to have a lasting impact than one single performance.

He also said that the debate formats play a role. Compared to political conventions, in which each side dominates news coverage for days and promotes its agenda while tearing down its opponent, debates are balanced affairs in which each candidate has an equal opportunity to advance an agenda and line of attack.

Perhaps more important, Holbrook said, is how campaign operatives afterward spin the results in the news media.

Holbrook pointed to a general consensus in 2000 that Vice President Al Gore won his first debate with Texas Gov. George W. Bush. But Holbrook noted that the Bush-driven, post-debate spin campaign resulted in a news story arc that Gore was a "serial exaggerator," and the vice president was viewed through that prism in the following two debates -- to his detriment. He eventually lost the race to Bush.

But Holbrook also cautioned that while debates have some impact on public opinion, their impact must be weighed within the context of other dynamics at work.

He cited one question posed to Democrat Michael Dukakis in 1988 as an example. During the Oct. 13 debate that year with Republican George H.W. Bush, CNN moderator Bernard Shaw asked Dukakis hypothetically whether he would support the death penalty for someone convicted of raping his wife, Kitty. Dukakis, a staunch opponent of the death penalty, said he would not, and then offered a cool-headed reasoning for his reply. His lack of passion was exploited later as a sign of weakness, and often cited as a crucial turning point in the election.

"That election was over long before that," Holbrook said.

The theme of the first debate this year, to be held Friday at the University of Mississippi, in Oxford, Miss., is to be foreign policy and national security. The third, on Oct. 15 at Hofstra University in Hempstead, N.Y., will focus on domestic and economic policy.

The second meeting, in town-hall format on Oct. 7 at Belmont University in Nashville, Tenn., will be guided by issues raised by audience members.

The Republican vice presidential candidate, Alaska Gov. Sarah Palin, will debate her Democratic counterpart, Delaware Sen. Joe Biden, on Oct. 2 at Washington University in St. Louis, Mo.