Antti Rinne made the announcement that he is in the running for the SDP chairmanship on an Yle TV1 discussion programme on Saturday morning.
A recent Yle poll of SDP congress delegates found Rinne, who heads the private sector white-collar Trade Union Pro, to be the second most popular possible contender for the post following Urpilainen.
The Social Democratic Party is to hold leadership elections in May. Urpilainen, now finance minister and deputy prime minister, has led the party for nearly six years, during which time it has slipped into fourth place in the polls.