The election of Sahlin, 50, will also set the scene for her to become Sweden’s first woman prime minister if she leads the party to a comeback in the next legislative elections in 2010.
Sahlin is the sole candidate for the leadership, and her election is seen as a formality during the Stockholm congress on Saturday and Sunday.
She will take over from Göran Persson, the former prime minister who has headed the party since 1996 and who was ousted by a centre-right coalition headed by Fredrik Reinfeldt in September’s election.
“The congress will be a starting signal for the work we will be focusing on in the years to come,” Social Democratic Party secretary Marita Ulvskog said.
The goal is clear: to regain control of the government for the party that has served only 10 years in the opposition in the past 75 years.
Mona Sahlin is a born-and-bred Social Democrat. She joined its youth wing in her early teens and rose through the ranks to become the party’s first woman secretary in 1992.
A former deputy prime minister, Sahlin has already singled out three key priorities for the party under her leadership: climate change, labour market issues and the fight against social inequality.
“Social democracy must tackle the issue of climate change the same way it fought against the class society 100 years ago,” Sahlin said.
“We lost an election and that requires some self-examination and self-criticism,” she added.
Sahlin has however not yet indicated whether she plans radical changes for the party that has dominated Sweden’s political scene since the 1930s and founded the “Swedish model” of a generous welfare state funded by high taxes.
She has said she will only reveal detailed plans after her election.
According to some observers, Sahlin’s biggest weakness is her lack of political vision, while others say her sensitivity and ability to listen to party members are her greatest strengths.
Swedish daily Svenska Dagbladet said those qualities will help set her apart from her predecessor Göran Persson, often criticised for his autocratic, power-hungry style.
The September election loss after 12 straight years in power was a crushing defeat for the party, especially since voters opted for change despite the country’s enviably strong economy.
But some voters now seem to regret their choice, with recent public opinion polls showing Prime Minister Fredrik Reinfeldt’s government losing ground to the left bloc.
A Synovate Temo poll published in February credited the Social Democrats with 41.4 percent of voter sympathies, their highest score in four years, compared to 35 percent in the September vote.
The left bloc, consisting of the Social Democrats, the Left and the Greens, garnered 53.2 percent of votes, compared to 46.1 in September.
After 25 years in politics, Sahlin has at times found herself at the centre of scandal – most notably in the 1995 “Toblerone affair”, when she charged personal purchases to her government credit card and a public uproar led her to take a three-year time-out from politics.
But she has always ridden out the storms to mount a comeback. The question now is whether she will succeed in doing the same for her Social Democratic Party.
By AFP’s Sophie Mongalvy