Thailand has been a constitutional monarchy since 1932. The present king, Bhumibol Adulyadej, ascended to the monarchy in 1946 and became Rama IX on 5 May 1950. Until 1958, Thailand was governed under a constitution originally promulgated in December 1932. In October 1958, however, the constitution was suspended, and three months later the king proclaimed an interim basic law providing for a constituent assembly to draft a new constitution. Nine years in the making, a new constitution was promulgated in June 1968, and the first elections under it were held in 1969. In November 1971, Marshal Thanom Kittikachorn overturned the document despite being chosen by its rules.
A period of martial law under a national executive council ensued, with the military continuing in power through an interim constitution. A new constitution, promulgated in 1974, was suspended and replaced by martial law in 1976 when civil disorder ensued. The 1976 constitution was abrogated after an October 1977 coup and under an interim constitution, the king empowered a legislative assembly to draft a new governing document. This constitution, approved by the legislature on 18 December 1978, lifted the ban on political parties and eased some of the martial law provisions imposed in 1976.
On 23 February 1991, the National Peacekeeping Council (NPKC), led by the supreme commander of the Royal Thai Armed Forces, General Sundhara Kongsompong, took over the administration of the country. On 9 December 1991, the NPKC promulgated a new constitution. It provided for a national assembly comprised of elected representatives and an appointed senate, and a cabinet headed by an appointed prime minister. This charter was sympathetic to the needs of the military and gave the junta power over the senate. Protests that resulted in the deaths of pro-democracy demonstrators between 17–20 May 1992 quickly led to a constitutional amendment to provide for an elected prime minister and to curb some of the appointed senate's power. This constitutional amendment was approved by the national assembly on 10 June 1992 and required the prime minister to be a member of the house of representatives. In addition, the revised constitution significantly cut back the powers of the senate by ruling that the speaker of the lower house will be president of the parliament (previously it was the speaker of the senate). The senate is also barred from initiating, or taking part in, "no-confidence" motions. The first elections under these reforms were held on 13 September 1992.
Efforts to amend the constitution again came before parliament in April 1994, and the seven government-sponsored amendments were defeated. These amendments sought to reform Thailand's political structure by institutionalizing political parties and increase the role of the legislature. Prolonged debate and political indecision prevented the passage of these amendments until 27 September 1997, when the new constitution passed with the king's endorsement. According to this constitution, the house of represenatives would consist of 500 members, with 400 selected by respective constituencies and 100 seats allocated by proportional representation of all parties exceeding the 5% threshold of popular votes. In an attempt to stabilize the political situation and institutionalize parties, the new constitution requires representatives to resign their seat if they renounce or switch their party membership. The senate, to consist of 200 nonpartisan members, requires all members to hold at least a baccalaureate. Members of both the house and senate serve four-year terms.