Political career
In December 2008, he resigned from Channel 10 and became a Meretz candidate in the 2009 legislative elections. In the party primaries, he gained the third slot on the joint list of the New Movement and Meretz. He said: "My goal is to continue to do what I have been talking about over the past years, from protecting the seashore to promoting more sophisticated, non-polluting public transportation." Meretz won three seats in the elections, making Horowitz the second openly gay Knesset member in Israeli history, and the only openly gay member of the eighteenth Knesset. The first, Uzi Even, had also been a member of Meretz. On 16 February 2009, he announced a plan to bring to the Knesset a bill that would allow marriages or civil unions between two partners, regardless of their religion, ethnic background, or gender. Before being sworn into the Knesset, he was told to annul his Polish citizenship, which he had obtained due to his father's origins and had used as a journalist to enter countries that Israelis have difficulty entering.
In 2009, he announced that he would boycott all of the events related to Pope Benedict XVI's visit to Israel, saying that in his opinion, the pope bore a message of "rigidness, religious extremism, and imperviousness. Of all the Pope's injustices, the worst is his objection to disseminating contraceptives in Third World countries. It's hard to assess how many miserable men and women in Africa, Asia, and South America have contracted AIDS because of this Philistine attitude, but we are talking about many". He also published a two-part opinion piece on Ynetnews explaining his position.
Horowitz was re-elected to the Knesset in the January 2013 elections. In October 2013, he ran for mayor of Tel Aviv against long-time incumbent Ron Huldai. He lost, receiving 38% of the vote to Huldai's 53%. In 2014, he was given the Outstanding Parliamentarian Award by the Israel Democracy Institute.
He did not stand in the 2015 elections.
In June 2019, Horowitz successfully challenged incumbent Tamar Zandberg for the leadership of Meretz, which made Meretz the first Israeli party to elect an openly gay person as its leader. Horowitz led the party during the September 2019 Israeli legislative election. In 2021, he became Minister of Health in Naftali Bennett's cabinet. He resigned his Knesset seat to ultimately join the public sector he served in his ministerial position. Under Israel's Norwegian Law and was replaced by Michal Rozin.
Horowitz used a government circular to ban conversion therapy in February 2022. He supports Bernie Sanders, who calls for conditional aid to Israel, and is reluctant to display his Jewishness and is completely irreligious, as noted in his speeches to the liberal Jewish movement J Street; he has been criticized for his criticism of Israel.
He also contributed to a coalition crisis in the Bennett–Lapid government by complying with an Israeli High Court decision that determined the government cannot prohibit visitors from bringing hametz into hospitals during Passover. After this action and amid secret talks with Benjamin Netanyahu, Yamina MK Idit Silman left the coalition, leaving it shy of a majority and unable to pass legislation.
Following the Knesset's dissolution and new elections scheduled for November 2022, Horowitz announced that he would step down as leader of Meretz. Placed 7th in the Meretz list on 2022 Israeli legislative election, he lost his seat because Meretz failed to pass the electoral threshold.
