Madrid — "Why did Adolfo Suarez resign?" This is the question many Spaniards are asking now that Spain's longest-serving prime minister this century -- the man who presided over the transition from the Franco dictatorship to democracy -- has presented his "irrevocable" resignation.
Mr. Suarez's decision to quit apparently was prompted by at least five considerations:
* The right wing of his own party, the Democratic Center Union or UCD, has been openly seeking to undermine his leadership. A showdown had been expected at the second UCD congress in Mallorca, schedules for Jan. 29 to 31 but canceled at the last minute.
* Mr. Suarez was reported to be incensed at criticism that he canceled the congress because of an unexpected air traffic controllers' strike, when in fact if was not his decision.
* Mr. Suarez's chief opponent in the UCD, who include liberals, Christian Democrats, and members of the ultraconservative lay Roman Catholic organization, Opus Dei, had been meeting in the last five months with officers in the Army and officials of the Catholic Church -- two institutions deeply uphappy with the Suarez government.
Spain's bishops criticized Suarez strongly during a Jan. 24 meeting of a conference called to discuss the government's proposed divorce bill, which would permit divorce by mutual consent.
In these circumstances the bishops criticized the government in the strongest terms ever since the return of democracy in 1977, while the recently appointed papal nuncio to Spain, Monsignor Innocenti, warned the Spanish government that he was reconsidering state-Vatican accords drawn up in 1979. Suarez realized that if he went ahead this month with debates on the divorce bill, church-state tensions might be revived in a way that has not been seen in this country since before the Civil War.
* Mr. Suarez's resignation must be seen in the context of a planned visit by King Juan Carlos to the northern Basque country. This visit, set for Feb. 3, is the first official visit by the Spanish monarch to the region and it is seen as a major act of reconciliation.It has provoked deep discontent in Spain's 260,000 -strong Army, however, especially as Suarez proposed that the King offer an amnesty to some 120 members of the Basque separatist organization ETA.
* Finally, a decisive element in the Suarez resignation was his gradual loss of favor with the King. The first public sign of this was a veiled criticism of Suarez in the royal Christmas message last Dec. 24. The showdown came Jan. 27 during a three-hour meeting at the royal palace. Mr. Suarez talked about resigning, as he had done before, but this time the King did not refuse his offer.
Politicians in spain's parties -- the UCD, the Socialists, and the Communists -- still believe Mr. Suarez's resignation may be a tactical one in which he is playing the injured and unwanted leader, going into the wilderness and waiting to be recalled.
Underlying this speculation is the fact that although Mr. Suarez has resigned as prime minister he remains the UCD's party leader. But Suarez himself has given strong hints that he will resign this post, too, at the beginning of the UCD congress now scheduled for Feb. 6. The UCD executive committee has nominated Leopoldo Calvo Sotelo, Mr. Suarez's deputy prime minister, to succeed him as prime minister.
Democratic Spain faces its biggest political challenge yet in finding a new leader at a time of deepening economic recession and serious internal strife in the UCD.
Either the UCD will continue as a minority government with another party's outside support, or a coalition government will be formed between the UCD and the Socialists. King Juan Carlos began a series of consultations with party leaders Jan. 30 that continued throughout the weekend.
The UCD's second congress, now scheduled for Feb. 6 through 8, should resolve three main points:
1. Who will be the new party leader.
2. How well the party can hold up, divided as it is into three factions.
3. How well the UCD, and the political center general ly, can hang together without Suarez at the helm.