Washington (CNN) – President Barack Obama plans to announce former Sen. Chuck Hagel as his nominee to become the next defense secretary and Counterterrorism and Homeland Security Adviser John Brennan as the nominee for CIA director on Monday, administration officials told CNN.

Two sources close to Hagel have also been told to expect the defense secretary announcement on Monday, and additional sources - a senior administration official and a source familiar with the nomination - said Obama spoke to Hagel Sunday by telephone.

The White House was calling senators' offices Sunday to inform them Hagel's nomination is imminent and to help build support for it, a source familiar with the nomination said. CNN reported Friday that the White House had told some senior members of Congress to expect the tapping of Hagel, and another source with knowledge of the nomination called it "locked down."

At the CIA, Obama plans to nominate Brennan to replace retired Gen. David Petraeus includes and acting CIA Director Michael Morell, according to a senior administration official. The announcement is expected Monday afternoon at the same time Obama nominates Hagel, the official said.

Petraeus stepped down from his post in November amid revelations he had engaged in an extramarital affair with his biographer. Morell, a career intelligence officer who was serving as the spy agency's deputy director, stepped in as acting CIA director upon Petraeus' resignation.

For Hagel, getting to the Pentagon will mean overcoming already vocal opposition from pro-Israel groups and others who object to his stance on Iran and Hamas. He has also faced opposition from gay rights groups, who were strong supporters of Obama's election campaigns, for a comment Hagel made in 1998 in which Hagel questioned whether a nominee for ambassadorship was suitable because he was "openly, aggressively gay." He apologized for that remark in December.

On Sunday, Republican Sen. Lindsey Graham labeled Hagel's expected nomination as "in your face," expressing concerns on CNN's "State of the Union" about Hagel's past positions on Iran and Israel, which have been the subject of scrutiny since his name was first floated as a potential defense secretary five weeks ago.

Republicans have also struck upon comments in a 2007 interview that some perceive as anti-Jewish, when Hagel said the "Jewish lobby intimidated lawmakers."

They've also lambasted positions Hagel took as a GOP senator, including his opposition to unilateral sanctions against Iran, as well as votes opposing the labeling of Iran's Islamic Revolutionary Guard Corps as a terrorist organization. He also opposed the "surge" of troops in Iraq favored by then-President George W. Bush and members of his administration.

Sen. Mitch McConnell, the Republican leader in the Senate, said Sunday that Hagel would receive a "thorough vetting" just like any other presidential nominee.

"Whoever is nominated for secretary of defense is going to have to have a full understanding of our close relationship with our Israeli allies, the Iranian threat, and the importance of having a robust military," McConnell said on ABC's "This Week." "So whoever that is I think will be given a thorough vetting. And if Sen. Hagel is nominated, he'll be subjected to the same kinds of review of his credentials as anyone else."

A veteran of the Vietman War, Hagel served, by a clerical mistake, side by side with his younger brother, and earned two Purple Hearts, one of those for saving his brother's life.

His time in Vietnam would end up forging his thoughts about combat for the rest of his life, and defining his tenure on Capitol Hill as a U.S. Senator with an independent streak, often sidestepping his Republican colleagues.

"Not that I'm a pacifist, I'm a hard-edged realist, I understand the world as it is, but war is a terrible thing. There's no glory, only suffering," he is quoted in his 2006 biography.

If he is confirmed, Hagel will face the challenge of closing the final chapter on the U.S.-led war in Afghanistan and oversee the continued footprint of a smaller U.S. training force there.

Hagel has been critical of U.S. policy in Afghanistan. In 2009 he opposed Obama's move to surge 30,000 troops into Afghanistan, telling the National Journal, "I'm not sure we know what the hell we are doing in Afghanistan."

Perhaps the most immediate issue facing Hagel if confirmed as defense secretary would be the future of the Pentagon's budget. Hagel has said in the past that be believes the Pentagon's budget is overweight.

"The Defense Department, I think in many ways, has been bloated," Hagel said in a September 2011 interview with the Financial Times. "So I think the Pentagon needs to be pared down.”Hagel, Brennan Nominations Planned For Monday