"I think fiscal union is one key objective, one key target to be achieved in the year 2012. I am happy that the Parliament has been involved from the beginning in the preparation of the treaty text," he said.
Leinen was elected president of the European Movement International (EMI) by the organisation's Federal Assembly in November 2011.
He lamented the "one-sided" treaty proposal saying that it "lacks the other components of how we come out of an economic recession and how we can generate sustainable growth and new jobs."
Leinen noted that the proposal did not provide elements for creating "solidarity" along with "stability", namely eurobonds, a role for the European Central Bank as a lender of last resort, or a larger European budget capable of great investment.
"I would like the Commission to be much more proactive, much more dynamic in legislative proposals for economic growth," he said.
Crisis and opportunity
Leinen said current efforts at integration were largely reactive, not driven by "political will" but by the external pressure of the financial markets.
However, he also argued the crisis is an opportunity for federalists to reach out public opinion, saying "I hope that the crisis will also be a chance. There has never been so much debate about Europe than in these months."
"The European Movement and civil society have a big role to play to recreate the political will for the European idea," he said.
Leinen said that the new treaty proposal had been a unifying factor for organisations like EMI and the Union of European Federalists.
"In fact the whole process of the convention for a constitutional treaty has brought the pro-European organisations in civil society onto the same line and objective: We recognise that the community method is the method to govern, to lead and to meet Europe's challenges," he said.
The MEP welcomed the European Citizens' Initiative, operational from April 2012, that will allow ordinary Europeans to make legislative proposals if they gather at least 1 million signatures.
He added that the European Movement was currently discussing several ideas for a citizens' initiative. "We have to be careful that [the initiative] not be hijacked by eurosceptics and other anti-European elements," he said.
Cameron's 'biggest mistake'
Leinen argued that Britain's decision to veto the new fiscal treaty would prove costly, saying "Cameron made perhaps the biggest mistake of his political career because he connected a question of the common market ... with a new question of fiscal union where Britain would not even be concerned".
The MEP was torn as to whether British membership in new EU institutions could be accepted if it means giving up on a financial transactions tax - a levy that has broad support in the EU but which Britain opposes.
"If you ask me as a social democrat, I say clearly no. If you ask me as the president of the European Movement, I would like Britain come back to the table and find a solution that will not split Europe," he said.