The Moderate Party lost 6,400 members last year, according to a study carried out by the Dagens Samhälle magazine.
Over the last two years, the party’s membership has shrunk by 20 percent, and now stands at 54,858.
Together, five of the seven parties represented in the Riksdag lost a total of 9,000 members last year,
Calculations for the Left Party and the Centre Party have yet to be completed, but both parties also forecast a drop in membership.
The Christian Democrats have lost 1,100 members, bringing their total to 22,919 at the turn of the year, and Liberal Party (Folkpartiet) membership has shrunk by 900 members to 17,799.
While the Social Democrats also continue to lose members, the rate of attrition has level-off since from 2007 when the party shed 19,000 members. In 2008, only 519 supporters left the party.
Only the Green Party experienced an upswing in members in 2008, adding a modest 65 members, for a total of 9,110.
Sweden’s traditional political parties have been declining since the early 1990s. Since then, the number of people who belong to the parties has been cut in half.
At the same time, smaller political parties not currently represented in the Riksdag have seen their memberships swell.
The Pirate Party has added a lot of members, but all that’s required to join is the click of a mouse in the party’s homepage.
As the debate out file sharing has raged over the last four months, the party has added 4,000 members, bringing its total to more than 10,000.
The far-right Sweden Democrats, meanwhile, added 400 members last year and now have a total of 3,300.
Dagens Samhälle magazine, which carried out the study, is published by Sweden’s Association of Local Authorities and Regions (SALAR), which represents governing bodies at the county and local level.