It is hard to believe that barely two months ago Manmohan Singh was having quintuple heart bypass surgery. The Indian prime minister appears robust, if a little tired, as he pursues a punishing schedule that yesterday involved taking time out from campaigning in hotly contested parliamentary elections to fly to London for the Group of 20 summit.

The hurly-burly of the world's largest democracy might not seem the natural stage for a cerebral 76-year-old technocrat credited with transforming India's economy when he was finance minister in the 1990s. But with the ruling Congress party facing a stiff challenge from the Hindu nationalist Bharatiya Janata party, the mild-mannered Mr Singh has been displaying a rare combative streak.

Releasing the Congress manifesto ahead of India's month-long elections starting on April 16, the prime minister made a scathing attack on his main challenger, the BJP's L.K. Advani. He also slammed the “negative mindset” of his former coalition partners, saying those leftwing parties – which opposed a new round of economic reforms and New Delhi's nuclear deal with the US – could “not take the country forward”.