Beliau berbuat begitu kerana tidak bersetuju dengan polisi serta tindakan-tindakan Pak Lah sebagai Persiden UMNO dna juga Perdana Menteri. Tun Mahathir juga mengajak ahli-ahli UMNO yang lain untuk menyertai beliau, namun beliau tidak mahu ahli-ahli yang keluar menyertai parti pembangkang, sebaliknya menyertai semula UMNO apabila Pak Lah meletak jawatan.
Laporan di dalam akhbar the Star:
Monday May 19, 2008 MYT 1:35:26 PM
Dr M quits Umno (updated)
ALOR STAR: Former Prime Minister Tun Dr Mahathir Mohamad announced Monday at 12.35pm that he is quitting Umno. He said this during a talk here on Monday. Dr Mahathir joined Umno at its inception in 1946.He became a Member of Parliament in 1964 when he won the Kota Setar Selatan seat. He lost the seat in the following general election in 1969.
Following the May 13, 1969, riots Dr Mahathir was sacked from the Umno Supreme Council on July 12 because of the widespread distribution of his letter to first Prime Minister Tunku Abdul Rahman criticising Tunku’s manner of handling the country’s administration. Dr Mahathir rejoined Umno on March 7, 1972 and was appointed Senator in 1973.
He relinquished the senatorship post a year later to contest the Kubang Pasu seat in the general elections. He was returned unopposed and was appointed Minister of Education.In 1975, Dr Mahathir became one of the three vice-presidents of Umno. Tun Hussein Onn appointed Dr Mahathir Deputy Prime Minister on Sept 15, 1978. Dr Mahathir was Prime Minister of Malaysia from July 16, 1981, until Oct 31, 2003.
Umno information chief Tan Sri Muhammad Muhammad Taib said he was surprised at the decision, but added that it was Dr Mahathir's right to do so. "Every member has the right to join or leave the party," he said. Asked if Umno would convene an emergency meeting, the former Umno vice-president and Selangor Mentri Besar said that "we will wait and see. "The next Umno Supreme Council meeting is scheduled to be held within the next two or three weeks.