WASHINGTON (CNN) -- Sen. Chuck Hagel won't run for president in 2008 and will leave the Senate when his term ends in early 2009, a source close to the Nebraska Republican told CNN Saturday.

Nebraska Sen. Chuck Hagel is expected to announce his intentions for 2008 on Monday.

The source said Hagel, 60, a vocal critic of the Bush administration, will announce his intentions at a press conference in Omaha on Monday.

Hagel would be the second Republican senator in 10 days to announce a decision not to seek re-election. On August 31, Sen. John Warner of Virginia said he would not seek a sixth term.

The Republican departures could provide opportunities for the Democrats to solidify their majority in the Senate at the start of a new administration.

Warner and Hagel, a decorated Vietnam War veteran, are among several Republican senators who have parted ways with President Bush's Iraq war policy.

Hagel supported the 2002 congressional resolution that authorized the invasion of Iraq the following year, but early this year he called Bush's plan to send thousands of additional U.S. troops to Iraq "the most dangerous foreign policy blunder in this country since Vietnam."

Hagel was the only Republican on the Senate Foreign Relations Committee to back a measure condemning the decision to deploy an additional 30,000 troops in an effort to end the sectarian violence.

Don't Miss Warner to retire from Senate

In July, Hagel co-sponsored a Democratic measure for U.S. forces to pull out of Iraq by April.

"The war in Iraq has pushed the U.S. military to the breaking point," he said on the Senate floor in support of the amendment. "... Our troops are being deployed longer than they should be, more frequently than they should be, and without full training and equipment.

"We are eroding our military power at a time when our country faces an increasing arc of challenges and threats across the globe. We are abusing our all-voluntary force in a dangerous and irresponsible way."

The Nebraska maverick even raised the prospect of Bush's impeachment over the war in Iraq during an interview in April's Esquire magazine.

Despite his break with the party on Iraq, the North Platte native has an otherwise orthodox GOP voting record.

He supported the Bush tax cuts in 2001, received a perfect score on abortion issues from the National Right to Life Committee and backed efforts to open part of Alaska's Arctic National Wildlife Refuge to oil exploration.

However, Hagel also was among the first Republicans to call for Attorney General Alberto Gonzales' resignation over several issues, including an anti-terror surveillance program and the firings of federal prosecutors.

In a written statement in May, Hagel said the "honesty and capability" of the attorney general must be unquestioned, and that Gonzales "can no longer meet this standard."

"He has failed this country. He has lost the moral authority to lead," Hagel said.

That same month, Hagel told CBS News' "Face the Nation" that a third-party candidacy for president might be good for the country.

Hagel did not rule out being the one to do it, perhaps together with New York City Mayor Mike Bloomberg.

"I am not happy with the Republican Party today," Hagel said on the CBS show. "It's been hijacked by a group of single-minded, almost isolationist insulationists, power-projectors," he said.

Hagel said he would decide by late summer whether to run for president.

"I think a credible third party would be good for the system," he said. "It would force both parties that have been hijacked by the extremes of their two parties -- and I think we would want something like that.

"I would hope this country has some options like that. I think it shakes the system up. The system needs to be shaken up."

Hagel was elected to the Senate in 1996 and re-elected in 2002. He previously was an investment banker, a businessman, a deputy administrator of the Veterans Administration and a lobbyist for Firestone Tire and Rubber Co. E-mail to a friend

CNN's Dana Bash contributed to this report.

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