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Just out of curiosity, I calculated the vote share won by the largest two parties in the elections for the Danish Folketing from 1918 (when proportional representation was introduced) to 2007. For most of the time, the Social Democrats and the Liberals have been the two largest parties with 1935, 1943, 1968, 1971 (Conservatives), 1973, 1977 (Progress Party) and 1981-1990 (Conservatives) as the exceptions.
From 1918 to 1960 the share varied between 60 to 65 percent of the vote, but from then on the share has declined and hovered between 50 and 55 percent. The resurgence of, first, the Social Democrats and then the Liberals put the share back to the 60 percent level but in 2005 and 2007 we have been back to the levels of the 1970s and 1980s. Recent opinion polls suggest that the combined share of the Social Democrats and the Liberals is around 50 percent.
As you have probably noted, I have a serious grudge against the indiscriminate use by journalists of the term “presidential election” with regard to Danish election campaigns and I would like (again) to point out that focusing solely on the leaders of the two main parties does lead to reporting losing some important aspects of the election campaign and the political process.
While we are at it: There are many ways of calculating the fractionalisation in a parliament. Here is a graph with the effective number of parties at the start of the parliamentary terms from 1953 to 2007.