The 2008 United States Senate elections were held on November 4, 2008, with 35 of the 100 seats in the Senate being contested. 33 seats were up for regular elections; the winners were eligible to serve 6-year terms from January 3, 2009, to January 3, 2015, as members of Class 2. There were also 2 special elections, the winners of those seats would finish the terms that ended on January 3, 2013. The presidential election, which was won by Democrat Barack Obama, elections for all House of Representatives seats; elections for several gubernatorial elections; and many state and local elections occurred on the same date.
Going into these elections, the Senate consisted of 49 Democrats, 49 Republicans, and 2 Independents who caucused with the Democrats, giving the Democratic caucus the slightest 51–49 majority. Of the seats up for election in 2008, 23 were held by Republicans and 12 by Democrats. The Republicans, who openly conceded early on that they would not be able to regain the majority in this election, lost 8 seats. Democratic candidates defeated Republican incumbents in Alaska, New Hampshire, North Carolina, Oregon, and, after a lengthy recount process, Minnesota. Additionally, they won Republican-held open seats in Colorado, New Mexico, and Virginia. 
Each major party has Hill committees that work to support its candidates for the House and Senate, chiefly by providing funds. On the Senate side, the committees are the Democratic Senatorial Campaign Committee (DSCC) and the National Republican Senatorial Committee (NRSC). In this cycle, the DSCC was more successful at fundraising. As of June 30, 2008, data from the Federal Election Commission showed the NRSC with $24.6 million on hand, as compared with the DSCC's $43 million. The NRSC chair, senator John Ensign, took the unusual step of chastising the Republican Senators who, like him, were not facing re-election, and who he thought should have done more to help raise money for their colleagues.
This election cycle was the second cycle in a row in which no seats switched from Democratic to Republican, and the first since 1990 in which the Democrats retained all their seats. In addition, this was the largest Democratic Senate gain since 1986, where they also won 8 seats. These elections marked the first time since 1964 in which a Democratic presidential candidate who won the White House had a Senate coattail effect. As of 2024, this is the last time Democrats won U.S. Senate seats in Alaska, Arkansas, Iowa, Louisiana, North Carolina, and South Dakota. 
Along with 2 independents who caucus with them, Democrats held at least 58 seats as a result of the elections. The April 2009 party switch of Pennsylvania senator Arlen Specter from Republican to Democrat and the July 2009 resolution of the Minnesota election in favor of Democrat Al Franken increased the Democratic majority to 60–40 (briefly providing the Democrats a filibuster-proof majority, and thus they were able to hypothetically override any filibusters). Republicans gained a seat in a January 2010 special election in Massachusetts, thereby making the balance 59–41 before the start of the next election cycle.
Results summary.
 ;onlyinclude ;"Summary of the 2008 United States Senate elections results"
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"Sources:"
Race summary.
Special elections during the 110th Congress.
In these special elections, the winner was seated during 2008 or before January 3, 2009, sorted by election date, then state, then class.
Elections leading to the next Congress.
In these general elections, the winners were elected for the term beginning January 3, 2009; ordered by state.
All of the elections involved the Class 2 seats.
Special elections during the next Congress.
There were no special elections in 2009.
Closest races.
In nine races the margin of victory was under 10%. However, Georgia proceeded to a runoff election where the Republican candidate won by over 10%.
South Dakota was the tipping point state, decided by a margin of 25.0%.
Final pre-election predictions.
Several sites and individuals published predictions of competitive seats. These predictions looked at factors such as the strength of the incumbent (if the incumbent was running for re-election) and the other candidates, and the state's partisan lean (reflected in part by the state's Cook Partisan Voting Index rating). The predictions assigned ratings to each seat, indicating the predicted advantage that a party had in winning that seat. Most election predictors used:
Gains and losses.
Retirements.
Five Republicans retired rather than seek re-election. 
Defeats.
Five Republicans sought re-election but lost in the general election.
Post-election changes.
Four Democrats resigned and were replaced by Democrats. One Republican was switched to Democrat on April 28, 2009. Two other Democrats died on August 25, 2009, and June 28, 2010, respectively, while another Republican resigned on September 9, 2009.
Alabama.
Alabama senator Jeff Sessions sought re-election to a third term. Johnny Swanson announced his candidacy in March 2006 for the Democratic nomination.
Despite voting heavily for Bush in 2004, Alabama still had a strong Democratic presence; Democrats controlled majorities of both chambers in the state legislature. Commissioner of Agriculture and Industries Ron Sparks appeared to be preparing for a run, but on June 12, 2007, Sparks announced that he would not seek the Senate seat, in order to avoid a primary battle with state senator Vivian Davis Figures. Figures has won elections in the Republican-leaning Mobile area. In the Democratic primary, Figures won the nomination and face Sessions in November.
Not on the ballot, but running a write-in campaign, was Darryl W. Perry, the 2004 Libertarian Party nominee for Pennsylvania State Treasurer and 2007 candidate for Mayor of Birmingham, Alabama. Perry was endorsed by Alabama Statesmen, Boston Tea Party, and Christians for Life and Liberty.
Sessions defeated Figures, taking 63% of the vote to Figures's 37%
! scope="col" colspan="2" style="width: 15em" |Party
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! colspan="3" style="text-align:right;" | Invalid or blank votes
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! colspan="3" style="text-align: right; margin-right: 0.5em" | Turnout
Alaska.
! scope="col" colspan="2" style="width: 15em" |Party
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! colspan="3" style="text-align:right;" | Total votes
! scope="col" colspan="2" style="width: 15em" |Party
! scope="col" style="width: 17em" |Candidate
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! scope="col" colspan="2" style="width: 130px"|Party
! scope="col" style="width: 17em"|Candidate
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! colspan="3" style="text-align: right; margin-right: 0.5em" | Majority
! colspan="3" style="text-align: right; margin-right: 0.5em" scope=row | Turnout
! style="text-align:right;" | Swing
Dispelling rumors that he would retire due to advanced age (he was 84 years old on election day) and ongoing federal investigations into his conduct, senator Ted Stevens filed papers for re-election for an eighth term.
An ex-oil company executive, Bill Allen, paid for part of the renovation costs on Stevens's personal residence. The FBI investigated the remodeling of Stevens home by Veco Corp., which is part of a broader corruption investigation involving Stevens's son, former State Senate President Ben Stevens. Two former Veco executives have pleaded guilty to paying the younger Stevens $242,000 in bribes. On July 30, 2007, the IRS and FBI raided Stevens's home in Alaska. On September 14, 2007, former Veco CEO Bill Allen testified at the trial of former State House Speaker Pete Kott that Veco paid people working to double the size of Stevens's home.
On July 29, 2008, a federal grand jury indicted Stevens on seven felony counts for making false statements, and on October 26, a jury found Stevens guilty on all charges.
The Democratic candidate was Anchorage Mayor Mark Begich, the son of popular former Democratic Representative Nick Begich. Begich announced his candidacy for the Senate seat on April 22, 2008.
On October 19, 2007, the AP reported that despite the allegations and FBI probe, several veteran GOP Senators—including Orrin Hatch (R-UT), Kay Bailey Hutchison (R-TX), and Kit Bond (R-MO)—donated enough money to Stevens's re-election campaign to make it one of Stevens's most successful fund raising quarters ever.
Stevens's conviction on seven felony counts of corruption damaged his re-election bid, coming just over a week before the election, though Stevens appealed the conviction. Nevertheless, Stevens was narrowly ahead in the vote count after election day, with only about two-thirds of all votes counted. It only became clear Begich had prevailed when early votes, absentee ballots, and questioned ballots were counted.
On November 18, the race was called for Begich, who won with 47.8% to Stevens's 46.5%. Stevens was the most senior U.S. Senator to ever lose re-election, defeating Warren Magnuson's 1980 record. As of 2023, Stevens still holds this record.
On April 1, 2009, U.S. Attorney General Eric Holder, citing serious prosecutorial misconduct during the trial, decided to drop all charges against Stevens—an action that vacated his conviction.
Arkansas.
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Despite being a first-term senator in a state George W. Bush won twice, Democrat Mark Pryor faced no opposition from Republicans in his re-election bid. Although Bush carried the state twice, Arkansas Democrats swept the seven state races held in the 2006 general election. Pryor is the son of longtime U.S. senator and former Arkansas Governor David Pryor. It was rumored that Lt. Governor Bill Halter would challenge Pryor in the primary, but Halter declined to file as a candidate. Rebekah Kennedy of the Green Party was Pryor's only opposition. Pryor won on election day, with 79.53% of the vote. Kennedy took 20.47%.
Colorado.
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! colspan="3" style="text-align: right; margin-right: 0.5em" | Majority
! colspan="3" style="text-align: right; margin-right: 0.5em" scope=row | Turnout
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On January 15, 2007, incumbent senator Wayne Allard (R) announced he would not seek re-election, honoring his pledge to serve no more than two terms.
Former Representative Bob Schaffer of Fort Collins was the Republican nominee. Former Denver Broncos quarterback John Elway was rumored to be considering a run, but declined to do so. Other possible Republican candidates included former Congressman Scott McInnis and Colorado Attorney General John Suthers.
The Democratic nominee was 2nd district Congressman Mark Udall of Boulder who announced on January 15, 2007, that he would seek the seat and did not draw significant primary opposition.
Other candidates included Bob Kinsey of Denver as the Green Party nominee, Douglas "Dayhorse" Campbell as the American Constitution Party's nominee, and Independent candidate Buddy Moore, unaffiliated any party.
On Election Day, Udall defeated Schaffer 53% to 43%.
Delaware.
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On August 23, 2008, the Democratic nominee for president, Barack Obama, announced that Biden would be joining him on the ticket as the vice presidential nominee. Delaware law allowed Biden to run for vice president and senator at the same time, so he would have kept the Senate seat if the presidential ticket had lost. In 1988 and 2000, the Democratic vice presidential nominees Lloyd Bentsen and Joe Lieberman, ran similarly for their seat in Texas and Connecticut, respectively. On November 4, 2008, Barack Obama won the presidential election, making Biden the next VP. Biden vacated his senate seat shortly after the election, allowing for the Governor of Delaware to appoint a successor. There was speculation as to whether the outgoing Governor, Ruth Ann Minner, or the incoming Governor-elect Jack Markell would make the appointment, and if Biden's son, Delaware Attorney General Beau Biden would receive the appointment. On November 24, 2008, Governor Minner appointed Biden's longtime Chief of Staff Ted Kaufman to fill the seat. Kaufman subsequently announced that he would not seek election to a full term in 2010, effectively making him a caretaker. Biden's Republican opponent in the Senate race, conservative political commentator Christine O'Donnell, tried to make an issue of Biden's dual campaigns, claiming that serving his constituents is not