He was one of the founders, alongside Dimitrije Tucović, of the Serbian Social Democratic Party (existed 1903–1918), that supported a Balkan Federation during the Kingdom of Serbia.

There he gained a reputation as a brilliant public speaker and was elected as municipal opposition president in 1893, but the Serbian government annulled it.

In polemics with the left wing of the party, headed by Dimitrije Tucović, Lapčević often adopted centrist and right-opportunist positions.

At the organization's Second Congress held in Vukovar from 20 to 24 June 1920, the less radically inclined centrist faction were led by Lapčević and Žarko Topalović; they were expelled from the party in November of the same year for issuing an anti-Bolshevik manifesto.

In the early 1920s, Lapčević left politics and withdrew from the workers' movement, not wanting to take further part in the polemics of the opposing parties.