Texas Solidly Republican John Cornyn, the Senate’s No. 2 Republican, easily beat his main primary challenger, Representative Steve Stockman, and avoided a runoff. Solidly Republican Republican John Cornyn

has held the seat

since 2002. Doug Mills for The New York Times

Illinois Solidly Democratic Richard J. Durbin is the second-most powerful Democrat in the Senate. This seat is likely to remain blue for at least six more years. Solidly Democratic Democratic Richard Durbin

has held the seat

since 1997. Stephan Savoia/Associated Press

North Carolina Could flip to Republican Senator Kay Hagan is the No. 1 target of Americans for Prosperity, the powerful conservative group backed by the Koch brothers. In November, she will square off against one of three contenders for the G.O.P. nomination: a powerful state legislator, a Tea Party candidate or a social conservative. Could flip to Republican Democratic Kay Hagan

has held the seat

since 2009. Philip Scott Andrews for The New York Times

Nebraska Solidly Republican Conservative groups are split between the two Republican front-runners to replace retiring Senator Mike Johanns: former State Treasurer Shane Osborn and Ben Sasse, the president of Midland University, who served as an assistant secretary of health and human services under President George W. Bush. Solidly Republican Republican Retiring Senator Mike Johanns has held the seat since 2009. No Incumbent

West Virginia Could flip to Republican Republicans are optimistic that their likely nominee, Representative Shelley Moore Capito, can break the Democrats' longtime lock on the state's Senate seats. Natalie Tennant, West Virginia's secretary of state and the likely Democratic nominee, will distance herself from President Obama. Could flip to Republican Democratic Retiring Senator John D. Rockefeller IV has held the seat since 1985. No Incumbent

Arkansas Could flip to Republican Senator Mark Pryor is trying to hold on to his seat in what will be a difficult race against Tom Cotton, a freshman representative who is supported by both Tea Party and establishment factions. Could flip to Republican Democratic Mark Pryor

has held the seat

since 2003. Danny Johnston/Associated Press

Georgia Could flip to Democratic Michelle Nunn, the daughter of former Senator Sam Nunn, could be the Democrats' best chance to pick up a seat. Ms. Nunn, who led the Points of Light Foundation, the volunteer group inspired by the elder George Bush, is positioning herself as a centrist. Some Republicans fear that a crowded G.O.P. contest will produce a nominee too far to the right to win in November. Could flip to Democratic Republican Retiring Senator Saxby Chambliss has held the seat since 2003. No Incumbent

Idaho Solidly Republican Senator Jim Risch should easily win re-election against his Democratic opponent, a lawyer who spent most of his career outside Idaho. Solidly Republican Republican Jim Risch

has held the seat

since 2009. J. Scott Applewhite/Associated Press

Kentucky Could flip to Democratic The Kentucky G.O.P. primary has gotten a lot of national attention, but Senator Mitch McConnell is expected to survive a challenge from Matt Bevin, a Louisville businessman with endorsements from several conservative groups. At the same time, Bluegrass Democrats are united around a candidate for the first time in decades. Polls indicate that Alison Lundergan Grimes, Kentucky's secretary of state, could give the most powerful Republican in the Senate a strong challenge in November. Could flip to Democratic Republican Mitch McConnell

has held the seat

since 1985. Win McNamee/Getty Images

Oregon Likely Democratic Senator Jeff Merkley could be in for a tougher-than-expected race in deep-blue Oregon if a Portland pediatric neurosurgeon, Monica Wehby, wins the Republican nomination. She is a moderate who could resonate with Oregonians suffering from Obama fatigue. Likely Democratic Democratic Jeff Merkley

has held the seat

since 2009. Mark Wilson/Getty Images

Alabama Solidly Republican Jeff Sessions is likely to be re-elected. Solidly Republican Republican Jeff Sessions

has held the seat

since 1997. Chris Kleponis/Getty Images

Iowa Could flip to Republican The race for the G.O.P. nomination to replace retiring Senator Tom Harkin is a test case for the battle between Tea Party and establishment Republicans. Representative Bruce Braley, running unopposed for the Democratic nomination, is hoping that a bruising Republican primary will help keep the seat on the Democratic side of the aisle. Could flip to Republican Democratic Retiring Senator Tom Harkin has held the seat since 1985. No Incumbent

Mississippi Solidly Republican The Mississippi G.O.P. establishment is rallying around Thad Cochran, the second-most senior Republican in the Senate, to fend off a challenge from the Tea Party insurgent Chris McDaniel, a state senator who is backed by several conservative advocacy groups. Solidly Republican Republican Thad Cochran

has held the seat

since 1978. Brendan Smialowski/Getty Images

Montana Could flip to Republican The state offers a pickup opportunity for Republicans, but the appointment of Lt. Gov. John Walsh to serve the remaining term of Max Baucus, who resigned to be United States ambassador to China, gives Democrats a chance against the Republican candidate, Representative Steve Daines. Could flip to Republican Democratic John Walsh

has held the seat

since 2014. Eliza Wiley/Associated Press

New Jersey Solidly Democratic Cory Booker, the former mayor of Newark who won a special election last fall to replace the late Senator Frank Lautenberg, will not have a big-name Republican opponent in November. Solidly Democratic Democratic Cory Booker

has held the seat

since 2013. William B. Plowman/NBC

New Mexico Likely Democratic Tom Udall is making his first bid for re-election after winning the seat held for more than 30 years by Pete V. Domenici, a Republican who retired in 2009. Likely Democratic Democratic Tom Udall

has held the seat

since 2009. Ron Edmonds/Associated Press

South Dakota Could flip to Republican The seat of retiring Senator Tim Johnson should be a strong pickup opportunity for Republicans, but Larry Pressler's decision to run as an independent inserts a dose of uncertainty into the race. Mr. Pressler is a moderate Republican who served three terms in the Senate before losing his seat to Mr. Johnson in 1996. Former Gov. Mike Rounds is the likely Republican nominee, and Rick Weiland, a former aide to Tom Daschle, is the lone Democrat in the race. Could flip to Republican Democratic Retiring Senator Tim Johnson has held the seat since 1997. No Incumbent

Maine Solidly Republican In November, Susan Collins, one of the last moderate Republicans in the Senate, will go up against Shenna Bellows, a Democrat who has been called "the Elizabeth Warren of civil liberties." It is unclear if Ms. Bellows poses a serious threat. Solidly Republican Republican Susan Collins

has held the seat

since 1997. Chip Somodevilla/Getty Images

South Carolina Solidly Republican Tim Scott was appointed to the Senate in 2012 to replace the Tea Party icon Jim DeMint, who resigned to head the Heritage Foundation. Mr. Scott is not facing any Republican challengers. Solidly Republican Republican Tim Scott

has held the seat

since 2012. Doug Mills for The New York Times

South Carolina Solidly Republican Lindsey Graham faces a handful of primary challengers running to his right. The two-term Republican incumbent needs more than 50 percent of the vote to avoid a runoff. Solidly Republican Republican Lindsey Graham

has held the seat

since 2003. Chip Somodevilla/Getty Images

Virginia Likely Democratic The entry of Ed Gillespie, former chairman of the Republican National Committee, into the G.O.P. race has added a level of intensity to what otherwise would have been a cakewalk for the Democratic incumbent, Mark Warner. Likely Democratic Democratic Mark Warner

has held the seat

since 2009. Mary Calvert/Associated Press

Colorado Could flip to Republican Colorado’s Senate race became more competitive at the end of February, when Representative Cory Gardner, a top-tier Republican, announced he would challenge the incumbent, Mark Udall. Until that point, Mr. Udall had attracted only opponents whose campaigns were struggling to gain traction. Could flip to Republican Democratic Mark Udall

has held the seat

since 2009. Charles Dharapak/Associated Press

Oklahoma Solidly Republican James M. Inhofe is expected to be re-elected. Solidly Republican Republican James Inhofe

has held the seat

since 1994. Alex Wong/Getty Images

Oklahoma Solidly Republican Senator Tom Coburn's decision to retire at the end of the current session opened up his seat two years earlier than expected. Representative James Lankford is the Republican front-runner. Solidly Republican Republican Retiring Senator Tom Coburn has held the seat since 2005. No Incumbent

Kansas Solidly Republican Senator Pat Roberts faces a long-shot Tea Party primary challenger, Milton Wolf — a radiologist who is a distant cousin of President Obama's. Solidly Republican Republican Pat Roberts

has held the seat

since 1997. J. Scott Applewhite/Associated Press

Michigan Could flip to Republican The race to replace Senator Carl Levin, a Democrat who is retiring, is shaping up to be a pickup opportunity for the Republicans. Terri Lynn Land, Michigan's former secretary of state and the likely Republican nominee, has a slight lead over Representative Gary Peters, the Democrat. Could flip to Republican Democratic Retiring Senator Carl Levin has held the seat since 1979. No Incumbent

Tennessee Solidly Republican State Representative Joe Carr is challenging the two-term incumbent, Lamar Alexander, but Mr. Carr's chances to win the Republican nomination are slim. Solidly Republican Republican Lamar Alexander

has held the seat

since 2003. T.J. Kirkpatrick/Getty Images

Hawaii Solidly Democratic Shortly before he died, Senator Daniel K. Inouye, a Democrat, asked Gov. Neil Abercrombie to appoint Representative Colleen Hanabusa to his seat. Mr. Abercrombie chose his lieutenant governor, Brian Schatz, instead. Now, Mr. Schatz and Ms. Hanabusa are competing to fill the remaining two years of Mr. Inouye's term. Solidly Democratic Democratic Brian Schatz

has held the seat

since 2012. T.J. Kirkpatrick for The New York Times

Minnesota Likely Democratic Senator Al Franken may have a credible challenger with the entry of the multimillionaire businessman Mike McFadden, a Republican, into the race. Likely Democratic Democratic Al Franken

has held the seat

since 2009. Jonathan Ernst/Reuters

Alaska Could flip to Republican In 2008, Mark Begich was the first Democrat elected to the Senate from Alaska since 1974. He may not make it another term. Republicans Dan Sullivan and Mead Treadwell are competing for establishment support against Joe Miller, the Tea Party candidate who famously beat Senator Lisa Murkowski in the 2010 primary only to lose to her write-in campaign during the general election. Could flip to Republican Democratic Mark Begich

has held the seat

since 2009. Michael Reynolds/EPA

Wyoming Solidly Republican Liz Cheney's decision to drop out of the Republican primary contest means Senator Michael B. Enzi is expected to be elected to a fourth term. Solidly Republican Republican Mike Enzi

has held the seat

since 1997. Justin Edmonds for The New York Times

Delaware Solidly Democratic Senator Chris Coons, who won a special election in 2010 for the seat previously held by Vice President Joseph R. Biden Jr., is expected to win again. Solidly Democratic Democratic Chris Coons

has held the seat

since 2010. Chip Somodevilla/Getty Images

New Hampshire Likely Democratic Senator Jeanne Shaheen’s hold on New Hampshire will become uncertain if Scott Brown, a former senator in Massachusetts, enters the Republican primary. Otherwise, Ms. Shaheen is expected to keep the seat. Likely Democratic Democratic Jeanne Shaheen

has held the seat

since 2009. Chip Somodevilla/Getty Images

Rhode Island Solidly Democratic Senator Jack Reed, a Democrat, appears certain to be re-elected to a fourth term. Solidly Democratic Democratic Jack Reed

has held the seat

since 1997. J. Scott Applewhite/Associated Press

Massachusetts Solidly Democratic Edward J. Markey won a special election in June to fill the remainder of John Kerry’s term after Mr. Kerry was confirmed as secretary of state. Mr. Markey is seeking his first full six-year term. Solidly Democratic Democratic Ed Markey

has held the seat

since 2013. Josh Reynolds/Associated Press