His father was a locomotive engineer and senior member of La Fraternidad, the main railway workers' trade union at the time.

This opportunity led labor leaders Ángel Borlenghi and Juan Atilio Bramuglia to seek allies in the new regime, which they first found in Mercante - familiar to the rail workers' unions through his father.

[1] This role led to his appointment as comptroller of La Fraternidad and of its rival, the Unión Ferroviaria; as such, he proved central to Perón's achieving political dominance of the regime after October 17, 1945.

[4] Governor Mercante pursued a vigorous social agenda, becoming a key contributor to the Peronist program of enhanced labor rights and public works investments.

Reelected governor in 1950 by over 25% over UCR nominee Ricardo Balbín, Mercante became increasingly thought of as a potential successor to the president, whose second term would, in theory, end in 1958.