Polls show Papademos, a former vice president of the European Central Bank, has the backing of three in four Greeks but the need to implement painful tax rises and spending cuts to secure fresh loans and stave off bankruptcy will sorely test that support.
Tension surfaced on Wednesday when the main utility union cut off power to the Health Ministry building in protest of a law imposing a tax on property owners that the government is trying to collect through electricity bills.
Papademos must secure an 8 billion euro loan tranche Greece needs to meet debt repayments next month and then must lock down a new bailout worth 130 billion euros ($175 billion). Greece needs some 80 billion euros of that second rescue package in early 2012.
His national unity government brings together bitter rivals, the conservative New Democracy of Antonis Samaras, the Socialists of fallen prime minister George Papandreou and the far-right LAOS party.
"The government is asking for a vote of confidence. This should not be taken for granted because of its big majority. It is a symbolic, political action," Finance Minister Evangelos Venizelos said in a confidence debate late on Tuesday.
"We will help ourselves and the euro zone if we do what we have to do now, quickly, responsibly, so that Greece can always be a member of the euro zone and for the euro zone to exist, to permanently overcome the risk of a default," he said.
The vote is scheduled for 1300 GMT, but may be delayed.
On the eve of the confidence motion, New Democracy lawmakers defied the European Commission's request for a written pledge from the three parties on meeting the terms of Greece's bailout, saying they would not bow to Brussels.
New Democracy MP Nikos Dendias said orders "from Brussels cannot be a legitimate policy."
Such a stance will test anew the patience and confidence of Greece's European partners, who have already begun to speculate publicly on whether the country of 11 million people has a future within the euro zone.
Further highlighting the lack of unity in a government that must prepare for an election in the first quarter of 2012, 101 Socialist deputies signed a petition on Tuesday opposing their party's cooperation with New Democracy and LAOS.