In his most ambitious initiative since leading Argentina's return to democracy more than two years ago, President Raul Alfonsin has proposed moving the nation's capital from Buenos Aires to the vast, sparsely settled southern region of Patagonia to serve as both symbol and impetus for the creation of "a second republic."

The Argentine leader coupled this startling proposal, delivered in a nationally televised address last night, with a call for constitutional reform that would strengthen congressional powers and establish a new balance between president and parliament patterned more after European models than the U.S. one.

He also spoke of the need to modernize public administration and perfect the judicial system to make it "more efficient, agile and accessible to all sectors of society."

The speech was praised here today as bold and imaginative. But many Argentines also said they doubted the transfer plan ever would take place.

Opposition leaders said the presidential project was meant as a political diversion and called it inopportune given Argentina's weak economic condition and other more urgent concerns.

Alfonsin offered no timetable for the move, which requires congressional approval, nor any suggestion of how Argentina, already struggling to cover a $50 billion foreign debt, would pay for it.

Past Argentine governments have studied the feasibility of relocating the capital to decentralize the nation's political structure and reduce the influence of Buenos Aires. The metropolitan area has a population of more than 10 million and today contains 35 percent of the country's population, consumes 39 percent of its energy and employs 48 percent of all industrial workers and 45 percent of all commerce and service workers.

Nothing came of the earlier studies, the last one of which was commissioned in 1972 under military rule.

Alfonsin said the growth of Buenos Aires into "an excessive megalopolis" has paralyzed and distorted the strength of the whole nation, "destroying the foundations of federalism."

He suggested that a new capital be built at the northern edge of Patagonia in an area currently encompassing two towns -- Viedma in the province of Rio Negro and Carmen de Patagones in the province of Buenos Aires.

The two towns, which lie opposite each other across the Rio Negro, have a combined population of about 50,000 and are located about 500 miles south of Buenos Aires.

As for what would become of Buenos Aires, Alfonsin pointed to the example of Rio de Janeiro, which has remained a commercial, cultural and tourist hub of Brazil despite the construction of Brasilia, which became the capital in 1960.