He wanted Greek voters “to take a position, to see the choice before us in its starkness, hoping they will back the lesser of two evils, instead of letting irate reactions in the streets dominate the debate,” said one adviser to the prime minister.

But on Tuesday, it appeared that his move may have backfired. A lawmaker in his governing coalition left the Socialist Party to become an independent, another said she would not vote confidence in the government unless the prime minister formed a coalition government, and an additional six leading Socialists wrote a letter calling on Mr. Papandreou to resign and schedule early elections for a new government with greater political legitimacy. Together, the developments made it doubtful whether his government would survive a confidence vote, because the loss of even two supporters threatened his parliamentary majority.

Meanwhile, the center-right opposition New Democracy Party on Tuesday stepped up its calls for early elections. Its leader, Antonis Samaras, has opposed most of the austerity measures the government accepted in exchange for foreign financial aid. Mr. Samaras has said that if he were in power, he would try to renegotiate the terms of Greece’s arrangement with its main foreign lenders, known as the troika: the European Union, the European Central Bank and the International Monetary Fund.

“Mr. Papandreou, in his effort to save himself, has presented a divisive and extortionate dilemma,” Mr. Samaras said on Tuesday. “New Democracy is determined to avert, at all costs, such reckless adventurism.”

Mr. Samaras declined to say whether he would ask his 85 members of Parliament to resign, a move that would lead to the dissolution of Parliament and a snap election. The next general election was not due until 2013, when the Socialists’ four-year term expires. Mr. Samaras is expected to clarify his stance at a meeting of his party’s parliamentary group on Wednesday.

While the political instability took Europe and markets by surprise, it was less of a shock in Greece. Mr. Papandreou’s political capital had run out and tension has been building for months — not only with the center-right opposition, which objected to many of the terms of the austerity plan, but also within his own party.