During the postwar era, party systems in established democracies enjoyed considerable stability. The standard explanation for this phenomenon was provided by Lipset and Rokkan's seminal work, which suggested that patterns of party competition in Western Europe were ‘frozen’ from the 1920s until the mid-1960s between the block of Social Democratic, Labour, and Communist parties on the left and the block of Christian Democratic, Liberal, and Conservative parties on the right, in a mold established with the expansion of the working-class franchise decades earlier. Yet from the late 1960s or early 1970s onwards, challenges to established party systems emerged sporadically at intervals in some Western democracies, with occasional electoral successes catalyzed by diverse issues, movements, and parties. One of the first indicators occurred with the sudden success of the radical right Fremskridtspartiet (FP, or Danish Progress Party), which became Denmark's second largest party in 1973. This led observers such as Mogens Pedersen to detect evidence of greater electoral volatility in some European democracies, notably in France, Germany, Denmark, and Norway. Yet stability continued elsewhere, such as in Austria, Switzerland, and Sweden, where the relative strength of parties hardly changed from one election to the next. Since the mid-1980s, many indicators have confirmed observations of growing party fragmentation and electoral volatility following the rise of new challenger parties, including those on the radical right.