A plebiscite slated for Sunday on a Vatican-backed accord to end a century-old border dispute with Chile has turned into a crucial test for the 11-month-old government of President Raul Alfonsin, political analysts and foreign diplomats say.

For more than a month, politicians and civic leaders in this overwhelmingly Catholic country have debated whether to accept the result of a five-year mediation effort sponsored by Pope John Paul II.

The accord, initialed last month in Rome, is designed to end conflicting claims over the Beagle Channel, at the tip of the continent, which nearly led to a full-scale war in late 1978. The draft treaty awards three small islands in the channel to Chile, but gives Argentina jurisdiction over the disputed part of the Atlantic Ocean.

[The Associated Press reported that one bomb exploded and two others were found and disarmed Friday at a soccer stadium where Alfonsin was to speak in support of the Beagle accord.]

Bitterly contested by the opposition Peronists, a handful of small nationalist parties and a number of far-right groups, the treaty is being sold by the government as the best that could be achieved given past negotiating errors that damaged Argentine claims in the region.

Officials stress that the accord offers Argentina far better terms than those set down in a 1977 arbitration agreement by the British crown, or a preliminary proposal made by the pope in 1980. Their argument was bolstered earlier this week when Chile's top Navy official criticized the treaty as overly favorable to Argentina.

"This is a solution that Chile and Argentina have had to compromise," Alfonsin said in a nationwide radio and television address last night. "If Argentina hadn't made a series of errors in the past 100 years then maybe we could have gotten better terms."

Most observers agree that Alfonsin needs voter approval of the treaty in the Nov. 25 plebiscite to press ahead with his effort to reduce the role of the military in Argentina by giving it a smaller slice of the budget and by reducing areas of potential conflict with the country's neighbors.

"Alfonsin needs to win this one badly," said one ambassador. "He needs it in his battle with the military, and he needs it to maintain his image as a winner."

Despite the government's largely successful efforts to enlist support from the country's most important smaller and regional parties, as well as from the powerful Catholic Church, until last week there were signs that popular enthusiasm for the plebiscite was flagging.

At one point a near-panic developed among government officials as private polls showed that far less than 50 percent of eligible voters, considered the minimum needed to show popular support for the treaty, planned to cast ballots.

Optimism returned only after a debate last Thursday between Foreign Minister Dante Caputo and Peronist Senate leader Vicente Saadi. Polls taken after the encounter showed that Saadi, a leftist Peronist, alienated viewers with a ranting performance during which he accused the government of "treason of the fatherland."

Although Saadi's rhetoric appeared to be couched in favor of a "no" vote on the treaty, the senator was in fact representing the position of the Peronist National Council, the most important grouping in the divided movement, which in late October voted to support a position of "active abstentionism."