BUENOS AIRES -- More than 19 million Argentines are expected to cast their votes Sunday in national elections in which President Raul Alfonsin's Radical Party is facing a stiff challenge from a Peronist opposition that has moved toward the center in recent years.

Polls last week showed that the largely middle-class Radicals may lose their razor-thin majority in the House of Deputies, which could jeopardize Alfonsin's legislative program.

The Radicals are also struggling to retain control of the crucial governorship of Buenos Aires, Argentina's richest and most populous province. The governorship, considered a springboard to the presidency, historically has been in the hands of the party holding the highest national office.

This year, former labor minister Juan Manuel Casella, the Radical candidate, holds a slim lead in most polls over his Peronist rival, former economy minister Antonio Cafiero. But a progovernment news weekly, Expreso, suggested last week that Cafiero is picking up support among the unusually large numbers of people who had remained undecided until late in the campaign.

Cafiero, a potential presidential candidate in elections scheduled for 1989, is a leader of Peronism's reform wing. In the past two years, the reformists have wrested control of most of the party apparatus from politicians and labor bosses with ties to the right-wing death squads that killed hundreds of opponents in the mid-1970s.

The elections are the second since Alfonsin came to power in 1983 after eight years of military rule. Twenty-one governorships, half the seats in the national legislature, 914 seats in provincial legislatures and 10,299 mayoral and city council positions are at stake.