Syriza led the opposition conservative New Democracy party by as much as 15.2 percentage points in May. But the gap has been gradually whittled down since and it dropped to 1.8 percentage points in an MRB poll for weekly newspaper Agora published on Saturday, but with more than 20% of the respondants undecided.
Other polls also showed the lead narrowing, suggesting momentum may be shifting towards New Democracy.
Tsipras said he wanted to complete what he started when Syriza won national elections in January.
“Against us is the old political system that pushed the country into a tragedy, which built the regime that led to the bailouts,” he told a gathering of the party’s central committee in Athens. “We want to demolish this regime.”
He urged supporters to fight back against the old and “hated” political system he held responsible for Greece having needed bailouts, and justified his decision to agree to a third rescue.
“We do not regret having fought nor having chosen at the end to avoid catastrophe,” he said.
“Whoever wants to escape has the right to do it but we are moving forward, we have not seen our best battles yet,” he said in a reference to a breakaway Syriza faction that has formed the anti-bailout Popular Unity party.