Unicameral legislatures exist when there is no widely perceived need for multicameralism (two or more chambers). Many multicameral legislatures were created to give separate voices to different sectors of society. Multiple houses allowed, for example, for a guaranteed representation of different social classes (as in the Parliament of the United Kingdom or the French States-General). Sometimes, as in New Zealand and Denmark, unicameralism comes about through the abolition of one of two bicameral chambers, or, as in Sweden, through the merger of the two chambers into a single one, while in others a second chamber has never existed from the beginning.
The principal advantage of a unicameral system is more efficient lawmaking, as the legislative process is simpler and there is no possibility of deadlock between two chambers. Proponents of unicameralism have also argued that it reduces costs, even if the number of legislators stays the same, since there are fewer institutions to maintain and support financially and that it is more democratic. Proponents of bicameral legislatures say that having two legislative chambers offers the opportunity to re-debate and correct errors in either chamber in parallel, and in some cases to introduce legislation in either chamber.
The main weakness of a unicameral system can be seen as potential lack of restraint on the majority, particularly noticeable in parliamentary systems where the leaders of the parliamentary majority also dominate the executive. There is also the risk that important sectors of society[specify] may not be adequately represented by the elected singular body.
Approximately half of the world's sovereign states are currently unicameral. The People's Republic of China is somewhat in between, with a legislature and a formal advisory body. China has a Chinese People's Political Consultative Conference which meets alongside the National People's Congress, in many respects an advisory "upper house".
Many subnational entities have unicameral legislatures. These include the state of Nebraska and territories of Guam and the Virgin Islands in the United States, the Chinese special administrative regions of Hong Kong and Macao, the Australian state of Queensland as well as the Northern Territory and the Australian Capital Territory, a majority of the provinces of Argentina, all of the provinces and territories in Canada, all of the German states, all of the regions of Italy, all of the Spanish autonomous communities, both the autonomous regions of Portugal, most of the states and union territories of India and all of the states of Brazil. In the United Kingdom, the devolved Scottish Parliament, the Senedd Cymru, the Northern Ireland Assembly and the London Assembly are also unicameral.
Though the current Congress of the Philippines is bicameral, the country experienced unicameralism in 1898 and 1899 (during the First Philippine Republic), from 1935 to 1941 (the Commonwealth era) and from 1943 to 1944 (during the Japanese occupation). Under the 1973 Constitution, the legislative body was called Batasang Pambansa, which functioned also a unicameral legislature within a parliamentary system (1973-1981) and a semi-presidential system (1981-1986) form of government.
The ongoing process of amending or revising the current Constitution and form of government is popularly known as Charter Change. A shift to a unicameral parliament was included in the proposals of the constitutional commission created by President Gloria Macapagal Arroyo. Unlike in the United States, senators in the Senate of the Philippines are elected not per district and state but nationally; the Philippines is a unitary state. The Philippine government's decision-making process, relative to the United States, is more rigid, highly centralised, much slower and susceptible to political gridlock. As a result, the trend for unicameralism as well as other political system reforms are more contentious in the Philippines.
While Congress is bicameral, all local legislatures are unicameral: the Bangsamoro Parliament, the Sangguniang Panlalawigan (Provincial Boards), Sangguniang Panlungsod (City Councils), Sangguniang Bayan (Municipal Councils), Sangguniang Barangay (Barangay Councils) and the Sangguniang Kabataan (Youth Councils).
The Nebraska Legislature (also called the Unicameral) is the supreme legislative body of the state of Nebraska and the only unicameral state legislature in the United States. Its members are called "senators", as it was originally the upper house of a bicameral legislature before the Nebraska House of Representatives dissolved in 1937. The legislature is also notable for being nonpartisan and officially recognizes no party affiliation, making Nebraska unique among U.S. states. With 49 members, it is also the smallest legislature of any U.S. state.
A 2018 study found that efforts to adopt unicameralism in Ohio and Missouri failed due to rural opposition. There was a fear in rural communities that unicameralism would diminish their influence in state government.
Local government legislatures of counties, cities, or other political subdivisions within states are usually unicameral and have limited lawmaking powers compared to their state and federal counterparts.
Some of the 13 colonies which became independent, such as Pennsylvania, New Jersey and New Hampshire had initially introduced strong unicameral legislature and (relatively) less powerful governors with no veto power. Pennsylvania's constitution lasted only 14 years. In 1790, conservatives gained power in the state legislature, called a new constitutional convention, and rewrote the constitution. The new constitution substantially reduced universal male suffrage, gave the governor veto power and patronage appointment authority, and added an upper house with substantial wealth qualifications to the unicameral legislature. Thomas Paine called it a constitution unworthy of America.
Seven U.S. states, Arizona, Idaho, Maryland, New Jersey, North Dakota, South Dakota, and Washington, effectively have two-house unicamerals. In these states, districts in the upper house and the lower house are combined into a single constituency, a practice known as nesting.
The U.S. territory of Puerto Rico held a non-binding referendum in 2005. Voters approved changing its Legislative Assembly to a unicameral body by 456,267 votes in favor (83.7%) versus 88,720 against (16.3%). If both the territory's House of Representatives and Senate had approved by a 2⁄3 vote the specific amendments to the Puerto Rico Constitution that are required for the change to a unicameral legislature, another referendum would have been held in the territory to approve such amendments. If those constitutional changes had been approved, Puerto Rico could have switched to a unicameral legislature as early as 2015.
The United States as a whole was subject to a unicameral Congress during the years 1781–1788, when the Articles of Confederation were in effect. The Confederate States of America, pursuant to its Provisional Constitution, in effect from February 8, 1861, to February 22, 1862, was governed by a unicameral Congress.
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