Click here to watch "Painful."

Things turned nasty on the campaign trail this week, as Barack Obama and John McCain traded barbs and accusations, particularly on the topic of energy, and each camp released a series of negative TV ads.

Obama has been quiet about his attack spots, often not releasing details to the media, but two ads put out by the campaign popped up online this week.

"National Priorities" (subscription) slams McCain for being part of a system in Washington that has ignored the country's energy crisis for decades. McCain's "been in Washington for 26 years. And as gas prices soared and dependence on oil exploded, McCain was voting against alternative energy, against higher mileage standards," an announcer charges. Obama, on other hand, offers a "real plan" and innovative solutions to create alternative forms of energy.

The Obama camp also responded this week to a McCain ad that touted the Arizona senator's "maverick" credentials. "Original" (subscription) shows a 2003 Fox News clip of McCain claiming that he has voted with President Bush "over 90 percent of the time." An announcer asks, "The original maverick or just more of the same?" as a photo of McCain and Bush arm-in-arm flashes on screen.

The McCain camp shot back today with its third ad of the week, again casting Obama as a shallow celebrity. In "Painful" (subscription), which will be cycled into the 11-state buy of the earlier "Family" and will run during the Olympics, once again casts Obama as a shallow celebrity.

"Life in the spotlight must be grand, but for the rest of us, times are tough," an announcer says, going on to criticize Obama's tax proposals. According to MSNBC's Mark Murray, however, the ad's claims about Obama's tax plan are "dubious."

Paul Freedman, professor of American Politics at the University of Virginia, said that McCain in the last week "has redefined the negative ad" by going after his opponent on one of his greatest strengths -- his popularity. "It's akin to Coca-Cola saying: 'Don't drink Pepsi. It's too good. It's too popular.'" Given what viewers are used to seeing in political ads, Freedman points out, "It's crazy to show your opponent in a positive light -- as the recipient of the adulation of adoring throngs of the multitudes -- let alone to compare him to relatively popular celebrities.... What's interesting is that the politician of same-old same-old has given us a negative ad that's anything but."

On the other hand, Obama, nominally the candidate of change, has stuck to relatively conventional contrast ads. Many Democrats, having learned the hard way that staying above the fray doesn't always pay off in electoral politics, are worried that the Illinois senator is being too restrained in his response to McCain. "Obama hit back," said Freedman. "The question is has he hit back hard enough."

McCain does seem to be benefiting from the aggressive tone the contest has taken on this week, as polls have grown tighter over the last several days. "What John McCain has been able to do is to shake the spotlight away from Obama, to nudge him off the pedestal he was on in the aftermath of his world tour at least a little bit," Freedman said.

Some commentators have suggested that a campaign filled with negative attacks will result in an election in which many voters choose to stay at home. But Freedman contends that it is the negative attacks that make voters "tune in" and "turn on." Positive ads, he said, are "boring" and "don't get people to the ballot box."

Amy Harder contributed to this report.