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While the city of Denver, media organizations and Washington lawmakers and insiders have been busy preparing for the Democratic convention, the two presidential campaigns have been busy fighting an on-air ad battle. In the last 48 hours, the candidates have released a combined eight ads -- five from Barack Obama and three from John McCain, including a hard-hitting spot released soon after Obama's running-mate announcement of Sen. Joseph Biden, D-Del., early this morning.

The ad shows Biden expressing his support for McCain and casting doubt on Obama's ability to lead the country. The footage is from an early presidential debate where Biden said he stood by a statement he made that he didn’t think Obama is ready to lead the country. The spot closes with footage of an interview where Biden said he supports McCain: "I would be honored to run with or against John McCain, because I think the country would be better off," Biden says.

The McCain campaign also released this statement by spokesman Ben Porritt: "There has been no harsher critic of Barack Obama’s lack of experience than Joe Biden. Biden has denounced Barack Obama’s poor foreign policy judgment and has strongly argued in his own words what Americans are quickly realizing -- that Barack Obama is not ready to be President.”

Indeed, gone are the days of positive ads -- all the other spots the campaigns have released in the last two days have gone on the attack as well. McCain is continuing the "celebrity" theme, casting doubt on Obama's ability to lead the country, while the Illinois senator is hammering home a comment McCain made that he didn't know how many homes he owned.

Here's a rundown of all the ads.

Barack Obama

• "Better Off" shows Midwesterners reacting to McCain's contention that the economy is in good shape.

• In "Dangerous," Nevada residents respond to McCain's support for a nuclear waste site in Yucca Mountain.

• Obama links McCain to former Christian Coalition Executive Director Ralph Reed, who has been implicated in the Jack Abramoff scandal, in "Never."

• "Out Of Touch" claims that McCain has a distorted view of wealth and the state of the economy.

• The Obama campaign responds to McCain's statement in an interview that he did not know how many houses he owns in "Seven."

John McCain

• In "Housing Problem," the McCain camp highlights financial help that Chicago power broker and convicted felon Tony Rezko gave to Obama when he was buying a home.

• Reiterating the continuing "celebrity" theme, "Higher" says Obama is going to raise taxes at a time when families can't afford it and claims he is not ready to lead.

• "Biden" airs footage of the Delaware senator casting doubt on whether Obama is ready for the presidency and showing his support for McCain.