As the head of Generation Ecologie, a party founded only a year ago, he has become a pivotal figure in French politics, where the fault-lines between left and right have become blurred after 12 almost continuous years of Socialist government.
Passionately pro-European and pragmatic when it comes to pushing the green agenda, Generation Ecologie is tapping a wellspring of support.
Under France's first past the post electoral system, even though the ecologists are rivalling the Socialists for second place in the polls, with 15-19 per cent, they can count on only a handful of seats in the National Assembly. Their supporters are thus being vigorously wooed by Michel Rocard, the former Socialist prime minister, who has urged the ecologists to join a centre-left coalition. Mr Lalonde is not keen.
He was co-opted by the Socialists as environment minister until he resigned last year, and has since forged a strategic alliance between Generation Ecologie and the more fundamentalist green party, Les Verts, sometimes known as the Khmer Verts because of their uncompromising stance and preference for protest over politics. Generation Ecologie has mitigated their worst excesses to offer a united ecological front.
Mr Lalonde's vision is one of sustainable development through the market economy. As he told a meeting of candidates and supporters in the Parisian suburb of Massy on Friday night: 'Without a coherent ecological approach to problems, we will all suffocate.'
He called for a war on unemployment, but believes that conventional solutions of straight- forward economic growth, in which 'capital is invested in machines that put people out of work, so there are fewer and fewer consumers to buy goods', cannot be sustained.
He sees reduced working hours and job-sharing as one solution, and calls for a more sensitive approach to industrial and urban development that recognises quality of life as a key ingredient in the country's well-being.