Alexis Tsipras, whose party came second in the inconclusive May 6 polls, said there was no way to partially implement the terms of the bailout.
Presenting his Syriza party's economic program, he said he would seek to repeal the bailout terms, which have included deep spending cuts and stiff tax hikes and have been blamed for a prolonged recession.
The debt-strapped country holds its second election in six weeks on June 17. It was called after the centre-right New Democracy party won the May 6 poll but without enough votes to form a government, and coalition talks collapsed.
"The first act of a government of the left, as soon as the new Parliament is sworn in, will be a cancellation of the bailout and its implementation laws," Tsipras said.
"Let people know that in Greece there is still a democracy."
Tsipras' Syriza party has been neck-and-neck with New Democracy in most recent opinion polls. The socialist PASOK party, which came to power in a landslide in 2009 elections, saw its support crumble on May 6, coming in third place with a meagre 13 per cent compared to ND's 18.9 and Syriza's 16.8 per cent.
Tsipras said the bailout agreement "is a mechanism of definitive bankruptcy and pushing the country to a voluntary withdrawal from the eurozone."
He predicted that under the bailout terms, Greece would be unable to return to the markets, from which it has been blocked out since mid-2010 by sky-high borrowing rates, for a decade.
"There is no more or less bad memorandum," he said, referring to the bailout agreement. "You either implement the memorandum, or you cancel it. ... We will cancel it."
Tsipras said that if he wins enough votes to form a leftwing government, he will "seek a Europe-wide deal to drastically reduce the debt payments, or seek a debt moratorium. We will cancel the law ... to cut the minimum wage and benefits. Debt payment must be tied to growth levels."