OMAHA, Neb. —  Republican Sen. Chuck Hagel, who has been thorn for his own party when it comes to Iraq, announced Monday he would retire from the Senate and not seek any elected office in 2008.

"I said after I was elected in 1996 that 12 years in the Senate would probably be enough, and it is," Hagel said.

His exit means one more seat the minority Republicans will be forced to defend, and both parties are expected to bring in heavy hitters to vie for the spot. The contenders could include Democrat Bob Kerrey, a former U.S. Senator and governor, and Republican Mike Johanns, the U.S. agriculture secretary and another former governor.

Potential candidates have said they'll talk about their plans after Hagel makes his decision official.

In March, many people had expected Hagel to announce he was running for president. Rumors also flew earlier this summer that Hagel might leave the GOP to join New York Mayor Michael Bloomberg on an independent presidential ticket. Hagel shot that down in July when he said he had no plans to leave the Republicans.