A popular initiative in 1918 established a system of proportional representation. As a result, the Free Democratic Party (FDP) lost nearly half its seats in the elections the following year.

The main beneficiaries were the Socialists and the new Peasants', Tradesmen's and Bourgeois Party (BGB).

Despite the strong showing of the Socialists it was soon clear that the so-called "bourgeois bloc," made up of the FDP, the Conservatives and the BGB would dominate Swiss politics for the foreseeable future by keeping the Socialists on the sidelines.

However, the Socialist Party was able to use the system of referendums and initiatives to defend the interests of the workers, and gradually evolved into a party of reform rather than confrontation.

Nevertheless, an attempt by the Socialist Party to get a seat on the Federal Council in 1929 was blocked by the Conservatives.