TAREK AMARA and ANGUS MCDOWALL TUNISIA'S president is using a referendum on a new constitution to move back towards dictatorship, the head of the biggest Tunisian opposition party, the Islamist Ennahda, warned. Rached Ghannouchi, who was parliament speaker until President Kais Saied shut the chamber and seized most powers last summer, said the atmosphere in the build-up to the July 25 vote was "undemocratic and funereal". Saied's focus on political changes while the economy slid further towards crisis also meant Tunisia faced a "Sri Lanka situation" with a looming social explosion, Ghannouchi told Reuters in an interview. "Saied's speeches can…