The recently appointed leader of the Social Democratic Party, Mona Sahlin, has declared that she is negotiating with the leaders of the Left Party and the Green Party on a common platform in order to challenge the ruling liberal-conservative Alliance the 2010 general election.

In the election of 2006, the Alliance stood united and presented a common political platform. The Social Democrats, the Left Party and the Green Party, however, did not seem very united or even well-coordinated. This was a mistake they obviously don’t plan to repeat.

There are, of course, many issues that the three parties disagree on. Some can probably can be overcome by negotiations. But others could prove to be more complicated.

Ever since Sweden held a referendum whether or not to join the European Union in 1994, the ”E-word” – Europe – has been somewhat infected in Swedish politics. All the major political parties agree on that Sweden should stay a member (in accordance to the outcome of the referendum), save for the Left Party and the Green Party.

But being pro-European is no election winning position in Sweden. The EU is still a popular scape-goat for un-popular political decisions, no matter how domestic. ”The EU made me do it”, is a used as an excuse for everything from raising taxes, to allowing TV commercials aimed at children and shipping conscripts overseas.

For the Left Party and the Green Party, insisting on Sweden leaving the EU has proved to be their unique selling point in a political landscape where all the major parties are virtually identical.

And herein lies the problem for Mona Sahlin. Because if a red-green coalition wins the election in 2010, how could a Social Democratic prime minister have Left and Green cabinet ministers who openly oppose the EU and wish Sweden to leave?

One could argue that this has already been the case, since there have been several anti-EU ministers in former Social Democratic governments. But there is a difference between a government made up by one single party, and a coalition. In the latter case, tensions are un-avoidable and tend to come to public attention. In the former, ministers are expected to stay loyal.

A recent survey shows that the current Swedish support for staying a member of the EU is at a all time high; 43 percent. This should give the leaders of the Left Party and the Green Party something to think about.

Jonas Morian is chairman of Socialdemokratiska pressföreningen (the Social Democratic Press Association). He also runs his own Swedish-language blog, PromeMorian