BUCHAREST, Romania (AP) _ A millionaire businessman who spent 50 years in exile was picked as an opposition party's presidential candidate Friday and is expected to present a tough challenge to the ruling National Salvation Front.

Ion Ratiu, 72, was nominated as the candidate of the National Peasants' Party. It is one of two leading pre-World War II parties that have re-emerged since the December revolt toppled Nicolae Ceaucescu and ended 45 years of Communist rule.

Interim President Ion Iliescu, a former Communist and member of the National Salvation Front, is expected to declare his candidacy this weekend.

Ratiu was one of four candidates for the nomination. He won on the first round of voting during a meeting of party's 64-member ruling body, said party official Victor Isac.

Isac said he believed Ratiu was helped to victory by party elders' belief that his international connections will serve the Peasants' Party well in the May 20 elections.

For years, Ratiu campaigned against the Communists from exile. He returned to Romania soon after the revolution.

Ratiu said he would import a printing press next week to publish an opposition newspaper open to all political parties.

The opposition parties have frequently charged the National Salvation Front unfairly dominates radio, television and the press.

Isac said Ratiu also was helped by being the nephew of his namesake Ion Ratiu, who led the influential Romanian National Party in Transylvania at the turn of the century.

No accurate opinion polls have been published in Romania since the revolution, making it hard to gauge the relative strength of the parties contesting the elections.

Opinion samples and conversations with Romanians suggest, however, that the National Salvation Front, which took over after Ceausescu was ousted, will face its strongest challenge from the Peasants' Party and the National Liberal Party.

The National Liberals, also a strong pre-World War II party, are led by Radu Campeanu, who last Saturday declared he would run for the presidency.

Campeanu also said the National Liberals, the Peasants and the Social Democrats, another group powerful before World War II, were forming an alliance for the May elections.

He said the three traditional parties would cooperate in order to win a majority in the parliamentary elections, with a possible view to creating a coalition government.

However, Amalia Radut, a National Liberal Party official, said in an interview Friday that the agreement holds only for the parliamentary, not the presidential race.

Ratiu became a millionaire in shipbuilding and other enterprises in Britain. He described the three-party cooperation as a kind of ''non-aggressi on pact'' but stressed, ''We are not a coalition.''

Ratiu's wife, Elisabeth, from Liverpool, said she would have ''to collect my breath'' to get used to the idea of campaigning as the potential first lady of Romania.

Married to Ratiu for 45 years, she said she had visited Romania frequently in the late 1960s and 1970s to see her husband's relatives.

''He's wanted to come back to Romania all his life,'' she said in an interview.