As a major region of New York State, Long Island has a significant impact on state and national politics. On the local level, the region has been long dominated by the Republican Party, although the region's alignment has shifted notably over the past decade, with many Democrats being elected to major local offices.
Nassau County and Suffolk County each have their own governments, with a County Executive leading each. Each has a county legislature as well as other countywide-elected officials, such as district attorney, county clerk and county comptroller. The towns in both counties have their own governments as well, with town supervisors and a town council.
Brooklyn and Queens, on the other hand, do not have independent county governments. As boroughs of New York City, both have Borough Presidents, largely ceremonial offices with little political power since the shutdown of the city's Board of Estimate due to a Supreme Court decision which declared it unconstitutional and led to a reorganization of the city government.
Politically, Nassau and Suffolk Counties were long controlled by the Republican Party. Republican presidential candidates won both counties from 1900 until 1988, with the exception of the 1912 victory of Woodrow Wilson and the Lyndon B. Johnson landslide of 1964. In 1972, Richard Nixon won Nassau, Suffolk and Queens and came within 14,000 votes of winning heavily Democratic Brooklyn. In 1992, the suburban counties split, with Nassau voting Democratic and Suffolk voting Republican; however, since 1996, both counties, along with Brooklyn and Queens, have been Democratic, although sometimes by fairly close margins. In 2004, John Kerry won Suffolk County by just under 14,000 votes. The close 2004 margins followed large victories for Al Gore in Nassau and Suffolk in 2000, and many observers think the 2004 results were more of a reflection from a 9/11 bump President George W. Bush received through portions of the New York City metro area (as his numbers jumped quite a bit from 2000 in Staten Island, Rockland County, and parts of New Jersey as well) rather than a reversal of the Democratic trend. This hypothesis was proven false when in 2008 Long Island still gave the Democrat Obama a narrow margin of victory in both counties. In 2016, Suffolk County voted by a wide margin for Donald Trump.
Five of New York's 29 congressional districts are located on Long Island. Of them, the Republicans only represent two; Peter T. King of Seaford represents the 2nd Congressional District, Lee Zeldin represents the 1st. The other three districts are represented by Democrats.
In 2000, Senator Hillary Clinton lost both Nassau and Suffolk to Republican Rick Lazio, who had previously served as a congressman from Suffolk County. While the 2004 results did show a much stronger showing for Bush across Nassau and Suffolk County it did not hurt Democrat Charles Schumer and his re-election bid in the area. Schumer won both Nassau and Suffolk in a landslide receiving close to 70% of the vote in both counties. Republican Governor George Pataki won both Nassau and Suffolk in all three of his victories.
In 2006, Long Island continued its Democratic trend, helped by a strong Democratic win nationwide, Democrats Eliot Spitzer and Hillary Clinton won Long Island in a landslide in the Governor and US Senate race. Democratic Comptroller Alan Hevesi, despite being scandal-ridden, won Long Island and Democrat Andrew Cuomo won all of the island's counties in the Attorney General race, with Republican Jeanine Pirro narrowly losing in Suffolk. Republican Peter T. King held on to his Congressional seat in a race against Nassau County Legis. Dave Mejias by a 56%-44% margin, even as two other New York Republican Congressmen lost their seats upstate and one open Republican seat flipped to the Democrats. His 12-point margin of victory was less than half his margins in past elections.
Much of the traditional edge Republicans have had in the New York State Senate is due to dominating elections for these offices on Long Island, but the election of a Democrat to replace Michael Balboni in 2007 for the 7th District on Nassau County's North Shore demonstrated a recent weakness at that level of government as well.
On the western side, both Brooklyn and Queens are reliably Democratic, although Queens became that way fairly recently, having still been politically volatile through the 1980s. This is mainly a consequence of the recent changes in Queens demographics, that used to be a white-middle-class suburban county, and is now one of the most diverse places in the United States.
In 2001, Nassau County elected Democrat Thomas Suozzi as county executive and Democrats took control of the county legislature, marking the first time Democrats had full control over county governments. Republicans still held on to the District Attorney's office and Hempstead town government, which has not had a Democratic majority on the town council or held the town supervisor position in close to 100 years. In 2003, Suffolk County followed suit, electing Democrat Steve Levy as county executive.
The 2005 election saw Nassau move further into Democratic hands. Denis Dillon, the Republican Party District Attorney of Nassau County for over thirty years, lost his re-election bid to the Democrat Kathleen Rice. The Republicans also lost the Town of Brookhaven, long known as a bastion of Republican corruption and patronage on the Island. As a result, many critics and residents had begun to refer to Brookhaven as "Crookhaven". Republican corruption and the ensuing scandals, indictments, resignations and convictions helped usher in Democrat Brian Foley's as Brookhaven town supervisor. The Suffolk County sheriff's race also resulted in a Democratic win. For the first time in years, Democrats once again control the Suffolk County Legislature. In 2006, for the first time ever, Democrats controlled a majority of government offices in both counties including county executives, legislatures, and district attorneys. After the 2009 elections, Democrats lost heavily in various communities including losing Nassau County executive, numerous legislatures and attorneys.
In 2009 Ed Mangano, a Republican, was elected in an upset victory over County Executive Thomas R. Suozzi. In 2011 Steve Bellone, a Democrat, was elected the 8th Suffolk County Executive.