Hagel, when he first ran for the Senate in 1996, promised he would serve no more than two terms. Aides said he is scheduled to formally announce his retirement Monday in Omaha, The New York Times reported.
A conservative in most matters, Hagel, a Vietnam veteran, broke with President George W. Bush on the Iraq War.
Hagel’s retirement, like that of John Warner in Virginia, makes the Republican effort to regain control of the Senate more difficult. While the Democrats have only a one-vote majority, 22 Republican seats are at stake in 2008.
The retirements, and the recent announcement by Sen. Larry Craig of Idaho that he intends to resign this month, mean the party must spend effort and money defending what would likely have been safe seats.
Former Nebraska Sen. Bob Kerrey, also a Vietnam veteran, has said he might seek the Democratic nomination to succeed Hagel. Nebraska Attorney General Jon Bruning had already announced a primary challenge to Hagel, while U.S. Secretary of Agriculture Mike Johanns might also be considering a run.