Dave Weigel covers the announcement:
The press corps wanted to know two things: Could Barr be stumped on any policy questions? Did he explicitly want to spoil the election for McCain? The answers were no and no, although Barr's "spoiler" answer wasn't entirely credible. "The thought has never crossed my mind," he said. Well, sure it has. After the conference, Barr's political adviser Russ Verney told me that Barr had tested the waters with an exploratory committee because he didn't want to run if he couldn't win. "There are substantial risks," Verney said. "We're talking about alienating life-long friends. We're talking about putting your credibility on the line." The main thing that would cause Barr to lose friends would be, of course, if he Nadered McCain.