The government combined has 176 of Parliament's 300 seats, and needs an absolute majority of those present to pass the bill. Without the Democratic Left, Samaras' conservatives and the Socialists control 160 votes â¿¿ not counting dissenters.
The main opposition Radical Left Coalition has urged demonstrators to surround Parliament during Wednesday night's vote.
"The new measures must not pass for they will turn the country into a financial and social desert," a party statement said. "They will lead us decades back, without medicine or state healthcare, without schools and universities, without a future, with endless armies of unemployed, suicides and desperate people."
The deeply unpopular measures include new deep pension cuts and tax hikes, a two-year increase in the retirement age to 67, and laws that will make it easier to fire and transfer civil servants. The country is suffering a deep recession set to enter a sixth year, and record high unemployment of 25 percent.
If Parliament rejects the package, Greece will lose access to the rescue loans from the European Union and International Monetary Fund that have kept it afloat since May 2010.
The country would then run out of money â¿¿ as soon as by Nov. 16, according to Samaras â¿¿ default on its debts and, most likely, abandon the 17-member eurozone. That would create hyperinflation as the new currency plummets in value, intensifying Greeks' misery.
A Greek exit from the euro would also have severe international repercussions, fueling fears that other troubled eurozone members could likewise leave the currency bloc.
Sky-high interest rates have kept Greece out of international bond markets since 2010. However, the country retains a market presence through regular short-term debt issues â¿¿ mostly bought up by domestic lenders that need the paper as collateral for vital European financing.
On Tuesday, Greece raised â¿¬1.3 billion ($1.6 billion) in a 26-week treasury bill auction that saw its borrowing costs ease slightly to 4.41 percent, from 4.46 percent last month. Demand was 1.7 times the amount on offer.
Costas Kantouris in Thessaloniki contributed to this report.