In 1997, he was once again elected a deputy. He left New Space after personal differences with Carlos Raimundi, and in 1998 joined the Popular Socialist Party, remaining as vice-president of FrePaSo until 2000.<ref>Ex-canciller Caputo joins socialism, Clarín, 1998-02-13</ref> In 1999, he sought the FrePaSo nomination for election as Mayor of Buenos Aires, but lost to Aníbal Ibarra, who went on to win in 2000.
In 1997, he was re-elected as a deputy. He left New Space due to personal conflicts with Carlos Raimundi and joined the Popular Socialist Party in 1998, remaining vice-president of FrePaSo until 2000. In 1999, he ran for mayor of Buenos Aires on the FrePaSo ticket but lost to Aníbal Ibarra who won in 2000.