CRANSTON, R.I. — Gov. Lincoln Chafee isn’t running for a second term, he said Wednesday in an announcement that surprised his political opponents and closest advisers alike and takes him out of what was expected to be a fierce primary in his new Democratic Party.

The governor, who became a Democrat in May, has struggled with poor approval ratings and is a reluctant fundraiser, who faced a strong field of challengers in next year’s race.

He said on Wednesday he liked being governor and thinks he would have won re-election. But he described campaigning as time-consuming and said the state faces so many serious challenges that he wouldn’t be able to effectively govern and run for re-election at the same time.

“I enjoy the challenge of combat. But I also enjoy the challenge of running a government, and in this moment of Rhode Island’s history, I want to put all my time, all my energy, to the task at hand. And that is running our government,” the 60-year-old Chafee said.

He left open the possibility of running for office at a later date or accepting a political appointment but said he was focused on completing the remaining 16 months of his term. He said he discussed his decision over several months with his family. He said a vacation last month in Maine and conversations he had during 6 1/2-hour drives there and back with his wife, Stephanie Chafee, helped him decide.

The governor said he told his staff Wednesday morning. But many people close to the governor who were contacted by The Associated Press before the hastily assembled afternoon news conference said they had no idea he was announcing anything.

Chafee is a former Republican and was the most liberal within the party in the U.S. Senate when he succeeded his father, the late John Chafee, in 1999. He lost the seat in 2006 to Democrat Sheldon Whitehouse, became an independent in 2007 and narrowly won a four-way race for governor in 2010.

When he switched parties, he said the change could help him raise money for the campaign, while being careful to note he switched because he shared so many views with Democrats.

The move set up what could have been a divisive primary against expected candidates Gina Raimondo, the state treasurer, and Angel Taveras, the Providence mayor, who both praised the governor on Wednesday. Taveras called him “a man of integrity.” Raimondo said Chafee “always had Rhode Island’s best interests at heart and was an unfailing optimist during very difficult times.” Chafee said he wouldn’t endorse anyone in the primary.

Cranston Mayor Allan Fung, a Republican who’s considering a run for governor, thanked Chafee for his career in public service.

Rhode Island has struggled for years with a bad economy and high unemployment, and Chafee listed among his accomplishments a decline in the unemployment rate from 11.5 percent to less than 9 percent, although he said that number was still unacceptable. He also cited reducing wait times at the Department of Motor Vehicles from hours to 20 minutes, increasing aid for education and seeking to recoup taxpayer losses from the state’s failed investment in former Boston Red Sox player Curt Schilling’s video game company, 38 Studios.

He said one of the things that surprised him about being governor was the “irrational negativity” that came along with it, citing as one example a debate about his use of the term “holiday tree” to describe the tree put up in the Statehouse rotunda during Christmas. While Chafee maintained it was the same term used by his Republican predecessor, his refusal to call it a Christmas tree because it was in a government building sparked raucous protests.

Chafee has poured millions of dollars of his own wealth into his past campaigns, and spokeswoman Christine Hunsinger said money had nothing to do with his decision. Still, he had just $377,000 in his campaign account as of June 30, according to a filing with the Rhode Island Board of Elections, compared with Raimondo, who had more than $2 million, and Taveras, who had just under $700,000.

Brown University political science professor Wendy Schiller said Chafee’s move paves the way for what she called a titanic battle between Raimondo and Taveras. She said Taveras probably gains more because he would’ve competed with the governor for the support of organized labor. But Raimondo may be able to pick up female voters who supported Chafee, Schiller said.

Chafee said his late father, who also served as governor, would have supported his decision.

“Rhode Island’s got its challenges,” Chafee said his father would say. “Put your nose to the grindstone, and take care of the issues.”

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