For most of the twentieth century, residents of Nassau County and neighboring Suffolk County primarily supported the Republican Party in national elections. However, in the 1990s, the tide of voter support began to shift toward the Democratic Party, and the county has voted for a Democrat in every presidential election since 1992. Bill Clinton carried the county in 1992 and 1996, as did Al Gore in 2000, the latter two times by margins of nearly 20 points. John Kerry's margin in Nassau County was considerably slimmer (5.6%) in 2004, where he won the towns of Hempstead and North Hempstead, but lost the Town of Oyster Bay. The county went solidly for Barack Obama in 2008 and 2012, both times by around 8%. Hillary Clinton did marginally worse in 2016, winning by 6.2%. Joe Biden in 2020 fared better than Obama at 9.5%, but still not as well as Bill Clinton and Gore.

Democratic strength is chiefly concentrated in both the wealthier and lower income sections of the county. Liberal voters dominate many of the wealthy communities of the North Shore, particularly in the Town of North Hempstead where affluent villages such as Sands Point, Old Westbury, Roslyn, East Hills, Kensington, Thomaston, Great Neck Plaza, and Great Neck Estates as well as the neighboring City of Glen Cove vote consistently Democratic. Democratic strongholds also include several low income municipalities in the central portion of the county, such as the Village of Hempstead, Roosevelt, Uniondale and New Cassel, as well as in a few waterfront communities on the South Shore, such as the City of Long Beach and the Village of Freeport.

Republican voters are primarily concentrated in the middle to upper middle class southeastern portion of the county, which developed during the "post-war boom era." Heavily Republican communities such as Massapequa, Massapequa Park, Seaford, Wantagh, Levittown, Bethpage, and Farmingdale are the political base of many county GOP officials such as Congressman Peter T. King and former County Executive Edward P. Mangano. In the western portion of the county, wealthy Garden City is solidly Republican, as is the more middle-class community of Floral Park. Additionally, some of the more rustic areas of the North Shore, particularly in the Town of Oyster Bay usually vote for the GOP.

Areas of the county containing large numbers of swing voters include East Meadow, Oceanside, and Rockville Centre on the South Shore and Mineola on the North Shore. Several areas have changed in partisan affiliation. Formerly Democratic strongholds such as the Five Towns and parts of Great Neck have trended to the GOP while previously Republican areas such as Elmont, Valley Stream and Baldwin have become Democratic bastions.

The 2nd District, represented by Republican Andrew Garbarino, includes heavily populated suburban neighborhoods like Massapequa, Levittown, Seaford, Wantagh, and Farmingdale. Nassau County is home to the former district attorney, Democrat Kathleen M. Rice, whose 4th District includes Garden City, Carle Place, Hempstead, Uniondale, East Meadow, Valley Stream, Franklin Square, West Hempstead and portions of the Village of Freeport and Rockville Centre.

Nassau County's other two congressmen are both Democrats. Representative Gregory Meeks represents the 5th District, which includes the southwestern part of the county, including Valley Stream. Thomas Suozzi's 3rd District includes Great Neck, Flower Hill, Port Washington, Jericho, Syosset, Hicksville, Bethpage, and Glen Cove in Nassau County.

Seven out of Long Island's nine state senators are Republican at the start of the 2017–2019 legislative term in January 2017, with the exceptions being State Senator John Brooks and Senator Todd Kaminsky.