President Barack Obama will nominate former Republican Senator Chuck Hagel to be his defense secretary, and an announcement could come on Monday, a congressional Democratic aide said on Sunday.

The choice will likely set up a confirmation battle in the Senate over whether the former Nebraska senator strongly supports key U.S. ally Israel. Hagel also has been criticized for comments he made questioning the effectiveness of sanctions on Iran for its nuclear program, and for his remark that being gay was an inhibiting factor for being an ambassador.

This fight would come after the Obama administration backed down from a tough Senate confirmation battle over Susan Rice, the U.S. Ambassador to the United Nations who was Obama's first pick to replace Hillary Clinton as secretary of state. Rice withdrew her name from consideration after drawing heavy fire from Republicans for remarks she made in the aftermath of a Sept. 11 attack on the U.S. mission in Benghazi, Libya.

"The administration has a lot of work to do on Hagel. He is in a weaker position now than Rice ever was because Rice would have rallied Democrats behind her," a Senate Democratic aide said. "The administration floated Hagel's name, then neglected to defend him effectively when his critics started taking shots," the aide said.

Last month Obama offered strong support for Hagel as a possible replacement for Defense Secretary Leon Panetta, who had said he wanted to leave early in the second term.

"I've served with Chuck Hagel. I know him. He is a patriot. He is somebody who has done extraordinary work both in the United States Senate, somebody who served this country with valor in Vietnam," Obama told NBC's "Meet the Press" on Dec. 30.