President Barack Obama is nominating former Sen. Chuck Hagel (R-Neb.) as the next defense secretary and White House Homeland Security and Counterterrorism Adviser John Brennan as the next CIA director.

The battle lines over Hagel’s nomination are already being drawn, as at least two no votes from senators were registered before Obama’s pick was official.

If approved, Hagel, a Vietnam War veteran, would be the first enlisted soldier to ever assume the role of the Pentagon’s top civilian. Since leaving the Senate, Hagel has served on the president’s Intelligence Advisory Board and the Defense Policy Board. He is also currently the chairman of the Atlantic Council.

In nominating Hagel, Obama highlighted that experience, along with the former senator’s ability to function outside the fray of politics. As a Republican senator, Hagel bucked his party by opposing both the war in Iraq and the surge in Afghanistan.

“My frame of reference is geared toward the guy at the bottom who’s doing the fighting and the dying,” Obama says. “Our troops will always know that Secretary Hagel will be there for you.”

Now the action will move to the Senate, where Hagel’s confirmation is not assured. But the nomination did receive a signal of confidence from a key member of the Senate, Carl Levin (D-Mich.), chairman of the Senate Armed Services Committee (SASC).

“Senator Hagel is well qualified to serve as secretary of defense with his broad experience in national security affairs,” Levin says. “The Armed Services Committee will give prompt and careful consideration to Senator Hagel’s nomination for this critical position.”

But top Republicans on the committee are preparing for an aggressive confirmation process, particularly given Hagel’s comments about a “Jewish lobby” he has described as “intimidating” lawmakers, and support for negotiating with Hamas.

House Majority Leader Eric Cantor (R-Va.) may not be able to vote against Hagel’s nomination, but he characterizes Hagel’s views on Israel as “incendiary.” Cantor says, “Sen. Chuck Hagel is the wrong man for the job at such a pivotal time.”

Sen. Roger Wicker (R-Miss.), a rank-and-file member of the SASC, is prepared to shoot down the nomination. “The president is choosing a fight rather than nominating a consensus pick. With sequestration’s defense cuts averted for only two months, this will take attention away from very pressing problems,” Wicker says. “[Current Defense Secretary Leon] Panetta was approved unanimously by the Senate. The president would be wise to nominate someone else for this critical position.”

Hagel also faces questions from the gay and lesbian community for calling then-President Bill Clinton’s choice for an ambassadorial appointment “openly, aggressively gay” in 1998.

Christopher Preble of the CATO Institute writes that Hagel ultimately is likely to be confirmed, but the process will prompt a fight among Republicans.

Neoconservatives, Preble says, will resist Hagel’s inclination to avoid putting “troops into harm’s way in pursuit of unclear objectives that do not advance U.S. interests. That is a mind-set that the neoconservatives cannot abide,” Preble writes. “Republican senators wishing to put distance between the party and the neocons should be happy to confirm a nominee who shares their views on most issues, and who is supported by people who have not been so badly wrong, so often.”

The choice of Brennan as CIA director also carries with it some controversy. Brennan is a 25-year CIA veteran, and was reported to have been Obama’s choice to lead the intelligence agency during his first term. But he was not considered confirmable, given concerns about his high-ranking role at the CIA during the George W. Bush administration.

The American Civil Liberties Union (ACLU) is continuing to make that case four years later. “The Senate should not move forward with his nomination until all senators can assess the role of the CIA — and any role by Brennan himself — in torture, abuse, secret prisons, and extraordinary rendition during his past tenure at the CIA, as well as review the legal authorities for the targeted killing program that he has overseen in his current position,” says Laura Murphy, director of the ACLU’s Washington legislative office.

Brennan would replace Gen. David Petraeus, who resigned last November after acknowledging he had an extramarital affair with his biographer.

For his part, Panetta says he is retiring after 50 years of government service to tend to his walnut farm, where he will “deal with a different set of nuts.”