John McCain suggested that Barack Obama is a lightweight celebrity who isn't ready to lead the country by releasing a commercial comparing the Democrat to Britney Spears and Paris Hilton. Obama counter-punched with his own ad showing images of the Republican contender with US President George W. Bush and accusing his 71-year-old rival of practicing "the politics of the past." While Obama tries to depict McCain as offering a mere continuation of Bush's unpopular policies, McCain wants the presidential campaign to be about Obama. To that end, the Republican is helping frame a not-so-flattering portrait of Obama for voters. His ads have become increasingly tough; a third of his commercials portray Obama negatively, a new study concluded. A new ad launched Wednesday by the McCain campaign suggests Obama is nothing more than a lightweight celebrity. Images of him speaking to a 200,000-strong crowd in Berlin last week are interspersed with shots of Spears and Hilton.