As a partner at the law firm "Irureta, Zaldivar, Briones y Hernández", he worked in his profession until he finished his studies.

In early 2006, after he lost the presidency of the PDC to Soledad Alvear, Zaldívar became a vocal critic of the Chilean economic system and voiced his disappointment with the government's "unwillingness" to amend it.

He frequently came into conflict with Alvear and in August 2007 the Christian Democrat Party's Supreme Tribunal chastised him for his criticisms.

Additionally, he was one of the harshest critics of the Transantiago public transport system, created under the leadership of the Concertación coalition to which the Christian Democrats belonged.

In November 2007, Zaldívar, together with senators from the Alliance for Chile and independents Fernando Flores and Carlos Bianchi, voted against continued funding for the Transantiago, against the wishes of his party's leadership.