While it's the Democratic Convention this week in Denver, some big-name Republicans are in the Mile High City to cast doubt on Barack Obama's ability to lead the country.

Former Republican Presidential candidate Mitt Romney was at his party's "Rapid Response Headquarters" just about a mile from the arena where the Democrats are conducting their business.

Romney seemed to slip easily into the attack-dog role. "Barack Obama is a charming and fine person—a lovely family—but he's not ready to be President of the United States of America." The former governor of Massachusetts, predictably, pointed to John McCain as someone who does have that experience.

Obama typically has responded to statements like this by saying experienced political leaders got the U-S into the Iraq War and it's the quality of a leader's judgment that counts.

After Romney, former New York Mayor Rudolph Giuliani is expected in Denver Wednesday. Then it's Minnesota Governor Tim Pawlenty on Thursday. All three have been mentioned as potential vice president picks for John McCain. Could they be auditioning for the job?