CHARLESTON, W.Va. — 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 today he will not seek a sixth term.

The 75-year-old Democrat’s decision comes at a time when his popularity in a conservative state had been waning for sparring with the powerful mining industry and for supporting President Barack Obama.

After about three decades in elective office, he said 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.

“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 retirement was widely expected and puts the seat held by Democrats since 1958 in jeopardy for the party.

Within weeks of November’s elections,

Republican U.S. Rep. Shelley Moore Capito in December vowed to run for the Senate seat in 2014, even if it meant going up against Rockefeller. Other Republicans also have been eyeing the seat in recent weeks.