ATHENS—Factions of Greece’s ruling left-wing Syriza party were taking battle stations on Wednesday, ahead of a crucial vote on austerity measures which will test whether Prime Minister Alexis Tsipras can hold his government together as he tries to satisfy the country’s creditors’ demands on terms for a new bailout.

Just hours before the late-night vote, few voices from Syriza could be heard in the corridors of parliament in support of the austerity measures, covering €9 billion of spending cuts and tax hikes over the next three years.

Government officials have appealed for unity in support of the measures, which are a prerequisite for securing a bailout package from the eurozone and International Monetary Fund worth up to €86 billion.

Officials say they don’t know how many Syriza lawmakers will vote against the measures.

Nevertheless, the measures are expected to pass, thanks to support from opposition parties. But the scale of the rebellion among Syriza lawmakers could determine how long Mr. Tsipras can continue in office without reshaping his coalition or calling new elections.