Sen. Dan Coats’ surprise announcement Tuesday that he will not seek another term will set off a scramble among fellow Indiana Republicans looking to replace him and give Democrats another chance to eat into the Republican majority in elections next year.
Mr. Coats, 71, said he would serve out his term but would be nearly 80 at the end of another term.
“Today I am announcing that I will not seek re-election to the United States Senate,” Mr. Coats said in a statement. “This was not an easy decision. While I believe I am well-positioned to run a successful campaign for another six-year term, I have concluded that the time has come to pass this demanding job to the next generation of leaders.”
Mr. Coats served four terms in the U.S. House before he was appointed in 1989 to the seat that was left open when Dan Quayle was selected as George H.W. Bush’s running mate in 1988.
Mr. Coats won re-election in 1992 but retired at the end of that term. He went on to serve as the ambassador to Germany under President George W. Bush and later worked as a lobbyist in Washington.
Mr. Coats serves on the Senate Finance Committee and the Senate Select Committee on Intelligence, and is chairman of the Joint Economic Committee.
“Dan Coats is one of the most accomplished public servants of his generation: congressman, senator, ambassador, and then senator again,” Senate Majority Leader Mitch McConnell, Kentucky Republican, said in a statement.
Political prognosticators said Democrats now have a glimmer of hope of flipping the seat in the election next year.
“Democrats will likely need some breaks to move the race from a potential gain to a top-tier contest,” said Nathan Gonzales, of the Rothenberg and Gonzales Political Report, a nonpartisan group that analyzes elections. “Coats would have started the race as a clear favorite for re-election, but now that he is retiring, his open seat could become competitive.”
Mr. Coats is the third senator to announce he will retire at the end of next year. Democratic Sens. Barbara Boxer of California and Barbara A. Mikulski of Maryland also have said they won’t seek re-election, though both of their seats are considered fairly safe Democratic territory.
Last year, Republicans took advantage of a favorable electoral map, capturing control of the Senate by flipping nine seats and giving them a 54-46 majority.
Democrats are looking for a comeback next year, when they will be defending 10 seats compared with 24 for Republicans.
Political observers say the most vulnerable Republican incumbents include Sens. Mark Kirk of Illinois, Ron Johnson of Wisconsin, Marco Rubio of Florida, Patrick J. Toomey of Pennsylvania, Richard Burr of North Carolina, Kelly Ayotte of New Hampshire and Rob Portman of Ohio.
Sen. Michael Bennet of Colorado and Senate Minority Leader Harry Reid of Nevada are widely viewed as the Democrats most in jeopardy.
Republicans have a deep bench of potential replacements for Mr. Coats, and Reps. Susan Brooks, Todd Rokita and Marlin Stutzman are among the those likely to consider bids. Eric Holcomb, a former state Republican Party chairman who now runs Mr. Coats’ state operation, Indianapolis Mayor Greg Ballard, and state House Speaker Brian Bosma also could take a look.
Democrats, meanwhile, are hoping for a repeat of the 2012 Senate race, when Joe Donnelly, a Democrat, won by taking advantage of a bloody Republican primary and his opponent’s missteps. But the Democrats’ bench is slimmer than the Republicans’.
Some Democrats hope to persuade former Sen. Evan Bayh to make a political comeback. Former Rep. Baron Hill also has been mentioned as a possible candidate.
Barack Obama won the state in the presidential election by a slim margin in 2008, but the state shifted back to Republicans in 2012, with Mitt Romney winning easily.
Sen. Roger Wicker of Mississippi, chairman of the Senate Republican campaign arm, said the party is well-positioned to defend the seat.
“We have a strong Republican bench in Indiana, and I am confident we will have another capable Republican joining us in the Senate in 2016 to continue Dan’s great work,” Mr. Wicker said.
Sen. Jon Tester of Montana, head of the Democratic Senatorial Campaign Committee, countered that the Indiana Senate race is “now one of the most competitive Senate races in the country.”