In the 5th Congress of Synaspismos (February 2008), he was elected President of the Party.

In the national elections of 2009 he was elected Member of the Greek Parliament in the first district of Athens and became chairman of the parliamentary group of SYRIZA.

In the 3rd Congress of the European Left Party (December 2010) in Paris, he was elected Vice President.

In the 4th Congress of the European Left Party (December 2013) in Madrid, he was nominated as candidate for the Presidency of the European Commission and he was re-elected Vice President of the European Left Party.

Since the general election of 2012, when he was re-elected Member of the Parliament in the first district of Athens, he had been the Leader of the Main Opposition until the parliament was dissolved on Dec. 31, 2014.