Former Republican Sen. Chuck Hagel, President Barack Obama's nominee for defense secretary, faces tough questions as his confirmation hearing gets underway Thursday.

Republicans are concerned about some of his past comments on Israel, Iran, and nuclear weapons.

Six Republicans have announced they will oppose Hagel's nomination. Sen. Jim Inhofe of Oklahoma, the top GOP lawmaker on the committee, said they are "too philosophically opposed" on defense spending, nuclear weapons and the Middle East.

Other critics are concerned he would cut defense spending too much.

Conservatives question Hagel's support of Israel and believe he has not been tough on Iran.

In a 112-page questionnaire he addressed several of these issues.

"I understand what it is like to be a soldier in war," Hagel wrote. "I also understand what happens when there is poor morale and discipline among the troops and a lack of clear objectives, intelligence and command and control from Washington. I believe that experience will help me as secretary of defense to ensure we maintain the best fighting force in the world, protect our men and women in uniform and ensure that we are cautious and certain when contemplating the use of force."

The hearing will be Hagel's first time publicly addressing his critics on accusations concerning his lack of support for Israel and his softness towards Iran.