The political turmoil within Portugal’s coalition government underscored the increasing tensions in much of Europe over whether spending cuts and other austerity measures are to blame for serial recessions and record unemployment in Greece, Spain, Portugal and other countries on the front line of the euro zone’s debt crisis. International lenders and bureaucrats once regarded Portugal as an example of the approach’s success.

“The back-to-back resignations throw the political opposition to reform in Portugal into sharp relief and pose serious questions about the country’s ability to push ahead, let alone exit, its troubled bailout program,” wrote Nicholas Spiro, the founder of Spiro Sovereign Strategy, a London consultancy that specializes in sovereign debt risk, in a note to investors. “The rug is being pulled out from under the Passos Coelho government, and Portugal is now staring at the prospect of early elections.”

Mr. Portas announced his decision just before the current secretary of state for the treasury, Maria Luís Albuquerque, was to become finance minister, replacing Vítor Gaspar, who resigned Monday. Ms. Albuquerque was expected to support the austerity program advocated by Portugal’s creditors and put in place by Mr. Gaspar, but Mr. Portas said Tuesday that he disagreed with her appointment.

Mr. Portas’s surprise resignation “opens a period of considerable uncertainty, as parties are going to try tagging each other with the blame for the political crisis,” said Antonio Barroso, a political analyst who has been covering the euro debt crisis for Teneo Intelligence, a consulting company in New York. Like other analysts, he suggested that Portugal was likely to end up holding a general election in late September, when local elections are scheduled.

Mr. Passos Coelho came to power in the spring of 2011 after the governing Socialists were forced to call early elections and negotiate an international bailout worth 78 billion euros, or $114 billion at the time, with the International Monetary Fund, the European Commission and the European Central Bank.