In the United States, divided government describes a situation in which one party controls the White House and another party controls one or both houses of the United States Congress. Divided government is suggested by some to be an undesirable product of the separation of powers in the United States' political system. Earlier in the 20th century, divided government was rare, but since the 1970s, it has become increasingly common, mainly in part because of the Watergate scandal, which popularized the idea that a divided government is good for the country.
Some conservative and libertarian groups see divided government as beneficial, since it may encourage more policing of those in power by the opposition, as well as limiting spending and the expansion of undesirable laws.
In parliamentary systems such as the United Kingdom, the executive relies on Parliamentary support for its existence. In the United States, however, the constitution is designed to create conflict between the executive and legislative branches of government.
Despite the perceived problems of divided government, the President and Congress are often able, out of necessity, to establish a effective working relationship.
Unified and Divided Party Control of the US Government since 1901 
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*The 2000 election resulted in a 50-50 tie in the Senate, and the Constitution gives tie-breaking power to the Vice President. The Vice President was Democrat Al Gore from January 3, 2001 until the inauguration of Republican Richard Cheney on January 20. Then on May 24, Republican Senator Jim Jeffords of Vermont left the Republican Party to caucus with the Democrats as an independent, resulting in another shift of control.