First term, 1995–2001; EDSA II and III Defensor Santiago was first elected senator in 1995. She was among the yearly topnotchers by the number of bills filed. In 2000, she filed in the Senate the first version of the anti-discrimination bill which would later become the
SOGIE Equality Bill. She was one of the few senators who were against the opening of the second envelope—an envelope which allegedly contained evidence against then-President
Joseph Estrada—during Estrada's impeachment trial; he was her foe in the 1998 presidential elections. She said afterwards: Estrada was ousted in the
Second EDSA Revolution in January 2001 and succeeded by Vice President
Gloria Macapagal Arroyo, which Defensor Santiago considered illegal under the Philippine Constitution due to the lack of a formal resignation from Estrada. In her support for Estrada, she once mentioned that she would jump off an airplane without a parachute if Estrada is arrested. After Estrada's arrest on April 25, 2001, Defensor Santiago joined other politicians in attending and speaking at pro-Estrada rallies held in the succeeding days at
EDSA Shrine, where she demanded the resignation of Arroyo and her entire cabinet and the reinstatement of Estrada as president lest the protesters storm
Malacañang Palace. The rallies she spoke in were soon followed by the
May 1 riots initiated by the rallyists going to Malacañang, where reporters from the
Philippine Daily Inquirer noted their attempts to storm the institution's premises being preceded by statements from Santiago the previous night that went: "I am bored. Let's not wait for the military and police. Let's do it, just us. Now!" After the May 1 incident, Santiago remained defiant in her stance against the Arroyo presidency, and was denounced by her cousin, Governor
Arthur Defensor Sr. of
Iloilo, for her rhetoric. Defensor Santiago lost re-election to the Senate in the 2001 elections, gaining 15th place with 9,622,742 votes.
Second term, 2004–2010 Defensor Santiago ran for senator in 2004 and was elected as a part of the
People's Reform Party. The Renewable Energy Act of 2008 mandated the government to shift the energy source of the country from coal and oil into solar, wind, and other renewable sources; Defensor Santiago included a proposal in the bill. Senate Bill No. 2583, known as the Climate Change Act of 2009, was passed by the Senate on June 3, 2009, incorporating amendments from Defensor Santiago. The bill highlights the action needed in solving
climate change.
Third term, 2010–2016 She ran again for senator in 2010 and won, gaining third place with a vote count of 15,231,194 votes. During this term, she served as the chair of the Foreign Affairs Committee of the Commission on Appointments. She was the first Asian from a third-world country to be elected to such a post. She submitted her resignation on June 4, 2014, due to "personal reasons" according to the ICC; according to them, she did not assume her functions in the court. She was one of the three senators who voted against the conviction of Supreme Court Chief Justice
Renato Corona during
his impeachment trial on May 29, 2012; Corona was ultimately found guilty of his failure to disclose to the public his statement of assets, liabilities and net worth. On December 4, she exposed that the Senate president,
Juan Ponce Enrile, used Senate funds to give away as cash gifts. Every senator, except Defensor Santiago and two others, received ₱2 million. This led to the
Priority Development Assistance Fund scandal, which charged the Senate president with charges of plunder from Defensor Santiago's live Senate hearings. On December 17, the Reproductive Health Act of 2012, which instilled reproductive health education throughout the nation, was passed. The law was created by
Pia Cayetano along with Defensor Santiago. The Philippine Act on Crimes Against International Humanitarian Law proposed to safeguard human rights in the nation. Defensor Santiago was the senator who filed the act. She exposed and named numerous
jueteng (illegal gambling) lords and illegal-logging lords throughout this term. She was the first Filipino elected as a commissioner for the
International Development Law Organization (IDLO) in 2016. Her role in the organization was advisory to the international law community. == 2016 presidential campaign ==