This should explain a bit about it...
Rand Paul aides say Lamar Alexander ad is not an endorsement
http://www.timesfreepress.com/news/2013/jul/15/paul-aides-alexander-ad-is-not-endorsement/
“The footage that Sen. Alexander’s campaign is using is from a public press conference in regards to a bill they both cosponsored,” spokesman Sergio Gor said. “At this time Senator Paul has not made an endorsement in this race.”
If this is true, he should tell alexander to stop using the footage as an implied endorsement. Perception is everything.