It was a ‘coup’ of sorts when Raj Kumar Singh joined the Bharatiya Janata Party last week. While another retired IAS officer, former petroleum secretary R.S. Pande, also joined the BJP the same day, it was Singh who hogged the limelight. The 1975 batch IAS officer of the Bihar cadre is possibly the first Union home secretary to take a plunge into politics. And it left several red faces around as the outspoken Singh declared that the BJP, to his mind, was the only political party to care about national interest. Uttam Sengupta interviews the man. Excerpts: