WASHINGTON (AP) — Republican Chuck Hagel, President Barack Obama’s nominee for defense secretary, is facing GOP critics who have challenged his past comments on Israel, Iran and nuclear weapons.

The former two-term senator from Nebraska is the lone witness at a Senate Armed Services Committee hearing today that could be crucial in determining whether he will win Senate confirmation to succeed Defense Secretary Leon Panetta in Obama’s second-term national security team. Two former committee chairmen — Democrat Sam Nunn and Republican John Warner — will introduce the nominee.

If confirmed, Hagel, a decorated Vietnam combat veteran, would be the first enlisted man and first Vietnam veteran to serve as defense secretary.

Hagel has the announced backing of about a dozen Democrats and the tacit support of dozens more who are unlikely to embarrass the president by defeating his Cabinet pick. One Republican — Sen. Thad Cochran of Mississippi — has said he will vote for his former colleague.

Six Republicans, including four members of the Armed Services panel, have said they will oppose Hagel’s nomination. Sen. Jim Inhofe of Oklahoma, the top GOP lawmaker on the committee, has said he and Hagel are “too philosophically opposed” on issues such as defense spending, nuclear weapons and the Middle East.

Crucial for Hagel will be the questioning by Sen. John McCain, R-Ariz. Hagel and McCain are fellow Vietnam veterans who once had a close relationship during their years in the Senate, but politics and Hagel’s opposition to increased troop numbers in Iraq divided the two men.

McCain has praised Hagel’s military service but said he had serious concerns about positions the nominee has taken on various issues. He said he is reserving judgment until after the hearing. Sen. Lindsey Graham, R-S.C., earlier this month described Obama’s selection as an “in-your-face” pick but was a bit less critical this week.

“Who are we getting — the guy today or the guy who said things before?” Graham said Tuesday after a 20-minute meeting with Hagel. Graham said he doesn’t doubt Hagel’s “personal integrity, but I do have real concerns about his policy positions.”

He addressed several issues in a 112-page questionnaire to the committee in which he said his wartime experience would shape his decisions about using military force.