Helsinki — In the end neither the horrendous complexities of the Finnish electoral system nor the disapproval of Leonid Brezhnev could stop Mauno Koivisto, the quiet man now destined to fill the shoes of the mighty Urho Kekkonen as president of Finland.
Koivisto on Jan. 18 won a landslide 145 seats in the 301-member electoral college that chooses the nation's head of state Jan. 26. The Finnish President has one of the most powerful political posts in Scandinavia, an almost monarchial position whose main duty is to keep the Russian bear from the door.
With such a plurality, nothing can stop Koivisto, whose support draws on all sections of Finnish society.
After a quarter of a century under the magisterial Kekkonen, Finland now looks forward to a more delegated power structure at home - but still linked to Kekkonen's policy of ''thinking geographically'' in foreign affairs. (''Thinking geographically'' is a Finnish political euphemism for ''bearing in mind your next-door neighbor is the Soviet Union.'')
Koivisto, a poor dock worker from the west coast, joined the Social Democratic Party in 1947 and after completing his education. He shot like a comet through the Finnish political firmament to become minister of finance in 1966, governor of the central bank the following year, and prime minister in 1979.
He has an irreverence that appeals to the man in the street (''All I know about the economy is that it will be worse than last year,'' is one of his better-known statements) but which scares the men in the Kremlin.
It was only in the latter days of the campaign that Brezhnev acknowledged the Koivisto steamroller by sending him an encouraging telegram. Before this the Politburo had been doing all it could to ensure that Ahti Karjalainen, former foreign minister and head of the Finnish-Soviet Trade Commission, would be in a position to take over.
Even after he had been ditched by the Center Party in favor of the more flamboyant Johannes Virolainen, Moscow did not completely give up hope with articles in Pravda warning darkly that the ''Kekkonen doctrine'' was endangered.
Koivisto, like the other seven candidates, pledged adherence to the Kekkonen doctrine but is likely to prove a little too independently minded to really suit Moscow. Although as president he will be above party politics, he is a devout (if understated) anticommunist, and Soviet interference in Finnish affairs is likely to meet with opposition.
Koivisto's relative lack of political experience worries both Brezhnev and home observers of the Finnish political scene.
Jan-Magnus Jansson, editor of Hufvudstadsbladet, Finland's main Swedish-language daily paper and himself a candidate in the election, warns that power may now pass to a coterie of Social Democratic advisors likely to surround the new president.
However, since taking over from the ailing 81-year-old Kekkonen as acting president last September, Koivisto has shown his ability to avoid political infighting and remain above party politics.
Koivisto's victory may dampen domestic criticism of the Finnish system of electing a president.
In a poll immediately before the election, when there was speculation that a ''compromise candidate'' might still be chosen by the electoral college despite popular support for Koivisto, nearly 60 percent of Finns said the system should be changed.
When the election was all over and the craggy, good-looking Koivisto knew that he was destined to be his country's next president, he said that he was surprised he had won so convincingly.
And when asked for a comment on his policy as president, he chose his words with all the care he puts into placing a shot in his favorite spare-time occupation of volley ball.
''I want to stress that this election will not alter our social or economic system,'' he said. Finnish election result: No. of electors Mauno Koivisto, Social Democrat 145* Harri Holkeri, Conservative 58 Johannes Virolainen, Center 36 Kalevi Kivisto, Communist 32 Jan-Magnus Jansson, Swedish People's Party 11 Veikko Vennamo, Rural Party 1 Helvi Sipila, Liberal 1 Raino Westerholm, Christian League 0 *With over 43 percent of vote.