CHARLESTON, W.Va. (AP) - U.S. Sen. Jay Rockefeller, who came to West Virginia as a young man from one of the world's richest families to work on antipoverty programs and remained in the state to build a political legacy, announced Friday he will not seek a sixth term.



The 75-year-old Democrat's decision, coming at a time when his popularity in a conservative state had been waning for sparring with the powerful mining industry and supporting President Barack Obama, told The Associated Press ahead of his formal announcement that it was time to retire.



After about three decades in elective office, it was time to "bring more balance to my life after a career that has been so obsessively dominated by politics and public policy and campaigns," he said. "I've gotten way out of whack in terms of the time I should spend with my wife and my children and my grandchildren."



Rockefeller's decision will set off a scramble for a seat held by Democrats since 1958. Within weeks of November's elections, Republican U.S. Rep. Shelley Moore Capito vowed to run for the Senate seat in 2014, even if it meant going up against Rockefeller and his storied name. Other Republicans also have been eyeing the seat in recent weeks.



Rockefeller said Capito's announcement did not influence his decision. Willing to devote millions of his personal wealth toward his campaigns - including several against Capito's father, ex-Gov. Arch Moore - the senator said he believes he would have prevailed over the seven-term congresswoman.



In a state that is the second-leading producer of coal, Rockefeller's positions rankled some who are protective of an industry that brings more than 65,000 jobs to one of the nation's poorest states. He accuses mining supporters of a combative closed-mindedness in the face of inexpensive natural gas, concerns over climate change and calls for cleaner ways to burn coal. Mining advocates accuse Rockefeller of abandoning them as Obama has ramped up scrutiny of Appalachian mountaintop-removal mining operations.



"I know the coal companies are going after me. ... I can live with that, because I know that I am fighting every day for coal miners," Rockefeller said.



Rockefeller defended his support of Obama and the president's signature health care overhaul, and insisted that their unpopularity with West Virginians did not influence his decision to retire.



"I'm proud of that work, and if people don't like it, the more it comes into effect the more they will understand that it's good," he said of the health care reform.



Rockefeller said the Senate achievement he is most proud of is the 1992 measure aimed at preserving retirement benefits for miners, their widows and children, which he credits for averting a national coal strike. He also has championed stricter coal dust limits in response to a rise in mining-related black lung disease and proposed increased safety measures after the 2010 Upper Big Branch mine disaster killed 29 West Virginians.



Other top issues he has had a hand shaping include child welfare, cybersecurity and foreign trade. He chairs the Senate Committee on Commerce, Science, and Transportation, and previously served at the helms of Intelligence and Veterans' Affairs. He co-sponsored legislation creating the states-level Children's Health Insurance Program, and helped persuade the U.S. Veterans Affairs Department to revisit disability claims arising from what has become known as "Gulf War Illness."



The great-grandson of famed industrialist John D. Rockefeller first arrived in West Virginia as a volunteer with the VISTA national service program in 1964. Within two years, he had won election to the Legislature, and then as secretary of state in 1968. After a failed run for governor in 1972 and four years as president of West Virginia Wesleyan College, Rockefeller won his first term as governor in 1976.



Toward the end of his second term, he narrowly captured the U.S. Senate seat of a retiring Jennings Randolph in 1984. He won by comfortable margins in each of his five terms.



Rockefeller hails from a family of many achievers: In addition to the successes of his oil billionaire great-grandfather, two uncles, Nelson Rockefeller and Winthrop Rockefeller, served as governors of New York and Arkansas, respectively. Rockefeller's father, John D. Rockefeller III, was a well-known philanthropist and founded the Asia Society, while his uncle David Rockefeller ran Chase Manhattan Bank.



"West Virginia has become my life and my cause," Rockefeller said. "I never, ever doubt what it is I'm trying to do. West Virginia provides that to me in the form of fantastically hard-working, tough, warm-hearted people."



Rockefeller became the state's senior senator upon the 2010 death of Robert C. Byrd, a fellow Democrat and history's longest-serving member of Congress. In his remaining time in office, he said, he plans to focus now on the fight over federal spending, taxes and the debt limit and the future of such programs as Medicaid.



"We have a whole lot of work to do for the next two years," Rockefeller said. "I'm very glad I'm going to be a part of that."



Rockefeller was to be joined during Friday's formal announcement by his wife, Sharon Percy Rockefeller, and other family members. The couple have four adult children and six grandchildren. Sharon Rockefeller was successfully treated for colorectal cancer after a 2005 diagnosis, and the senator has more recently endured torn tendons in his left knee.



"I will spend the next couple of years thinking of what I can do to continue to fight for the causes I believe in," Rockefeller said. He added, "I will not be leaving West Virginia. West Virginia will always be my home."