Here is video of Bob Barr's announcement:

Barr was also interviewed by the National Review:

NRO: But you don’t believe that Americans would experience significantly less liberty if a President Obama was to, say, raise taxes, spend more, and enact a series of regulations that curtail Americans’ liberties?

BARR: I think anybody that does that would be by definition curtailing Americans’ liberties, but why wouldn’t the American people legitimately blame Sen. McCain for that? Because Sen. McCain, for example, if he loses to Sen. Obama, did not put forward an agenda that appealed to a sufficient number of Americans, did not convince them of his bona fides in terms of being a so-called conservative candidate. Why blame Bob Barr? Why not blame Sen. McCain for not putting together and moving forward with an adequate platform?

NRO: What convinced you that it would be impossible to reform the Republican party from within?[

BARR: The fact that the Republican party has evidenced absolutely no interest whatsoever in moving back toward concepts such as the rule of law and away from concepts such as the so-called unitary executive, which flies in the face of both common sense and constitutional history in terms of separation of powers — a Republican party that for several years while the party controlled both houses of Congress and the White House did not move in the direction of shrinking the size, the scope, or the cost of government and has moved significantly and consistently to increase federal spending.