2010 United States Senate elections (Blue Decade)

The 2010 United States Senate elections were held on November 2, 2010, from among the United State Senate's 100 seats. A special election was held on January 19, 2010, for a mid-term vacancy in Massachusetts. Thirty-four of the November elections were for six-year terms to the Senate's Class 3, while other three were special elections to finish incomplete terms. Those 37 November elections featured 19 incumbent Democrats (six of whom retired or were defeated in the primary) and 18 incumbent Republicans (eight of whom retired or were defeated in the primary).

After the 2008 elections, the Senate was composed of 57 Democrats, 41 Republicans, and 2 independents who caucused with the Democrats (with the Democrats increasing their majority by one during the 111th Congress when Arlen Specter switched parties in 2009). Despite Republican gains, Democrats retained control of the Senate after the election.

Republicans won two seats held by retiring Democrats and also defeated an incumbent Democrat, for a Republican net gain of three seats. This was the first time since 1994 that Republicans successfully defended all of their own seats. This was also the fourth consecutive election of Class 3 senators where Democrats failed to gain seats, and the third consecutive midterm election held in a president's first term where Republicans picked up seats. Despite the Republican gains, the Democrats retained a majority of the Senate with 55 seats plus the two independents who caucused with them, compared to the 43 Republican seats.

, this was the last time Republicans won a U.S. Senate seat in Illinois.

Results summary
Shading indicates party with largest share of that line. Does not include the January 2010 special election in Massachusetts.

Source: Clerk of the U.S. House of Representatives

Retirements
Five Democrats and five Republicans (includes two interim appointees to finish the current 6-year term) retired rather than seek re-election. Additionally, two Democrats retired rather than seek election to finish the unexpired term.

Defeats
One Democrat and one Republican sought re-election but lost in either the primary or general election.

Closest races
Seven November races, as well as the Massachusetts special election in January, had margins less than 10%: