Mikko Majander has the goods:
The US Central Intelligence Agency began to fund Finland's Social Democratic party in the latter half of the 1940s, according to a book launched Tuesday by Mikko Majander, a Finnish historian. [...]
According to Dr Majander, 1949 was a key important watershed in the official US approach to Finnish politics as it marked the time when the Americans became convinced, thanks to the suppression of a Communist-led strike movement, that Finland had not turned into a Soviet satellite. The US viewed the SDP as the only significant force capable of keeping communism in check in Finland.
That the KGB funded the Communist Party is well-known and it stands to reason that the other side didn't stand pat. Still, it's good to have proper research on the subject.
The choice of parties is interesting. I'm fairly certain that the National Coalition Party also received funding, but the Social Democrats seem to have been the CIA's favorites, perhaps due to their battle with the Communists for the control of the labor unions.
I wonder, does this mean we can credit the CIA for the welfare state?