Erdan was first elected to the 16th Knesset in the 2003 elections. During his first term in the Knesset, Erdan was one of the most outspoken opponents of the unilateral Disengagement Plan from the Gaza Strip lead by Prime Minister Ariel Sharon from the Likud. In November 2005, Faced with harsh opposition to his plan, Sharon left Likud with several other MKs, and created a new party, Kadima. Erdan remained in Likud and after winning fourth place in the party's primaries, retained his seat in the 17th Knesset in the 2006 elections despite Likud's collapse from 40 to 12 seats in the Knesset. Erdan was re-elected to the 18th Knesset in 2009.