In 1973, with the encouragement of politicians Enrique Múgica and G. Peces-Barba, he took part in the formation of the Spanish Socialist Workers’ Party (PSOE).

In 1975 he was elected Secretary of Political Relations of the Socialist Association of the Province of Navarre and a member of the Federal Committee of the PSOE.

In the same year he participated in the writing of the manifesto of the Democratic Convergence (Coordinacion Democratica, or Platajunta), which coordinated the opposition to the dictatorial regime after the death of Francisco Franco.

In 1984, he was named President of Navarre because he was the head of the party with the largest number of seats in the regional parliament, and took office on May 5 of that year.

In 1994, he left the position of Secretary General of the PSN when he was accused of accepting bribes in exchange for awarding public works contracts.