Democrats hold the majority in the Senate with 53 seats, but more than 30 of those seats are up for grabs, and many of the races are competitive.
Check out our list of top contests to watch.
ARIZONA (OPEN SEAT)
After more than two decades of serving Arizona in the House and Senate, Arizona Sen. and Republican Whip Jon Kyl (pictured) is retiring. Arizona will elect a replacement from a field of candidates that include former U.S. Surgeon General under George W. Bush Richard Carmona, who, in the Democratic primary, faces former chairman of the Arizona Democratic Party Don Bivens. The winner would face Republican Congressman Jeff Flake or businessman Wil Cardon.
HAWAII (OPEN SEAT)
After serving 13 years in the House and four terms in the Senate, Democratic Hawaii Sen. Daniel Akaka (pictured) will retire this year, creating Hawaii’s first open Senate seat since 1976. Former Gov. Linda Lingle, Hawaii’s first woman governor and the first Republican governor in 50 years, is making a bid for Akaka’s seat, facing Democratic Rep. Mazie Hirono or former Congressman Ed Case. A Democrat is the favorite to win on President Obama’s home turf.
In the 2010 special election to succeed Democratic Sen. Edward Kennedy after his death in 2009, Sen. Scott Brown (pictured) managed an upset victory, becoming the first Republican to win a Senate seat in Massachusetts in nearly four decades. But the fact remains that Brown is a Republican in a Democratic state. The incumbent will face one of the following Democratic contenders: Harvard professor and consumer advocate Elizabeth Warren, who helped create the Consumer Financial Protection Bureau; attorney Marisa DeFranco; or attorney Jim King.
Democratic Missouri Sen. Claire McCaskill (pictured), who eked out a victory against her Republican rival for the office, is recovering from missteps when she failed to pay local taxes and billed taxpayers for personal travel on her private plane. In a Red-leaning state that President Obama lost in 2008, the freshman Democratic senator faces a tough Republican field that includes former State Treasurer Sarah Steelman, businessman John Brunner and Congressman Todd Akin.
Democratic Montana Sen. Jon Tester (pictured) won his seat by a thin margin in 2006 and faces another tough election this year. His opponent, Rep. Denny Rehberg—a business owner, cattle rancher and the top Republican in the state—is a member of the Tea Party, who has proven himself able to raise funds to remain competitive. Although the incumbent has, so far, outpaced his opponent in raising funds, this contest is expected to be expensive and close.
NEBRASKA (OPEN SEAT)
After much deliberation, two-term Democratic Nebraska Sen. Ben Nelson (pictured) has decided to retire. There are three Republican candidates going after the seat in this Red-leaning state—GOP front-runner Attorney General Jon Bruning, State Treasurer Don Stenberg and State Sen. Deb Fischer. No major candidate from the Democratic Party has stepped forward, although Former Nebraska governor and U.S. Sen. Bob Kerrey might enter the contest.
When Republican Nevada Sen. John Ensign resigned in 2011 amid a sex scandal, fellow Republican and former U.S. Rep. Dean Heller (pictured) was appointed to complete Sen. Ensign’s term. Now the incumbent faces stiff competition from his opponent, Democratic Rep. Shelley Berkley, who enjoys support from her base, including densely populated Las Vegas.
NEW MEXICO (OPEN SEAT)
After nearly 30 years in the Senate, Democratic New Mexico Sen. Jeff Bingaman (pictured) is retiring. Although former Rep. Heather Wilson is the Republican front-runner in fund-raising, support and popularity—followed by Lt. Gov. John Sanchez and businessman Greg Sowards – her fund-raising trails the Democratic front-runner—Rep. Martin Heinrich. Rep. Heinrich faces State Auditor Hector Balderas in the primary.
NORTH DAKOTA (OPEN SEAT)
Four-term Democratic North Dakota Sen. Kent Conrad (pictured) is retiring. Former Attorney General Heidi Heitkamp has taken up the Democratic mantle and faces the GOP primary winner—either front-runner and Tea Party favorite Rep. Rick Berg, or Berg’s fellow Tea Party Republican, retired Navy commander Duane Sand.
VIRGINIA (OPEN SEAT)
Two former Virginia governors are going after the Senate seat left open by retiring one-term Democratic Sen. Jim Webb (pictured). Republican front-runner George Allen, who lost his U.S. Senate seat in a 2006 upset victory by Sen. Webb, faces Democrat Tim Kaine, who resigned as DNC chair to enter this race. This close contest, in a swing state with two brand-named candidates, epitomizes the electoral “toss-up.”
WISCONSIN (OPEN SEAT)
Four-term Democratic Wisconsin Sen. Herb Kohl (pictured) is retiring. If elected, the Democratic front-runner to replace him—Rep. Tammy Baldwin—would be the first openly gay senator ever elected. Rep. Baldwin, who enjoys the distinction of being the first Wisconsin woman elected to Congress, will likely face Republican front-runner and former Wisconsin governor Tommy Thompson. In the GOP primary, Thompson faces former Rep. Mark Neumann and state Assembly Speaker Jeff Fitzgerald.