Ashton Carter, President Barack Obama's choice to head the Defense Department, in December 2014 as the president announced Carter’s nomination. (Jacquelyn Martin/AP)

Facing no significant opposition on Capitol Hill, Ashton B. Carter is expected to win easy approval from the Senate to become the next secretary of defense. But when he appears for his confirmation hearing Wednesday, he’s still likely to face a grilling from lawmakers — about President Obama’s national security policies.

Carter, 60, a physicist who previously served as the No. 2 official at the Pentagon, has earned bipartisan words of praise from senators with whom he has met privately in recent weeks. Yet Republican members of the Senate Armed Services Committee have made clear that they intend to press Carter about the wisdom of the White House’s strategy for conflict zones including Afghanistan, Syria and Ukraine.

Sen. John McCain (R-Ariz.), the committee chairman, has said he thinks Carter will “do a fine job” as defense secretary. But since taking over as head of the panel last month, McCain has scheduled several other hearings designed to air a broad critique of the Obama administration’s handling of the military and of foreign policy in general. Having Carter in the witness chair will give McCain and his allies a chance to probe more deeply about potential disagreements between the White House and the Pentagon.

In written responses to questions posed in advance by the Armed Services Committee, Carter was careful to adhere closely to Obama’s stated positions. But he also indicated a willingness to keep an open mind on some contentious issues, such as whether U.S. troop deployments to Iraq, Syria and Afghanistan might need to be revisited.

On Afghanistan, Obama has pledged to withdraw all U.S. troops from the country by the time he leaves office two years from now, save for a small number assigned to the U.S. Embassy in Kabul.

That timeline has drawn fire from some lawmakers, who criticize it as too rigid, saying that U.S. troops should stay longer to advise and train Afghan security forces and to prevent a resurgence of the Taliban.

In its formal list of questions, the Armed Services Committee asked Carter whether he would consider recommending to Obama that he change the pace of the Afghanistan withdrawal if security conditions deteriorate over the next two years.

Carter replied with a one-word answer: “Yes.”

In response to other questions about that war, Carter said that he was “encouraged by the positive strides made in Afghanistan” but added that “it is clear that much work remains to be done. We must stay engaged with our Afghan partners and support them, as they own the fight.”

Lawmakers also are expected to quiz Carter on whether he would consider deploying U.S. Special Forces closer to ground combat zones in Iraq and Syria, embedding them with Iraqi units or using them as spotters to call in airstrikes. Obama has resisted such approaches so far, but military commanders have raised the possibility that they might be necessary in the coming months.

Carter did not address his thinking on the subject in his written answers, saying only that he would consult closely with military commanders, civilian advisers and lawmakers before giving any strategic advice to the White House about fighting the Islamic State in Iraq and Syria.

Although Carter is a well-known figure inside the Pentagon and on Capitol Hill after holding several positions in the Defense Department over the past two decades, he has never been faced with sending troops into combat or having the final word on specific military operations.

He did not serve in the uniformed military but joined the Pentagon in 1981 as a civilian analyst, working on missile defense, the nuclear arsenal and programs to ensure the continuity of government in the event of nuclear war.

Among his jobs, he has previously served as the Pentagon’s chief weapons buyer and as deputy defense secretary. He was passed over for the top post in 2013 when it went to former Nebraska senator Chuck Hagel (R). But Hagel eventually fell out of favor with the White House, prompting Obama to nominate Carter after all.

Carter, a Rhodes scholar with a doctorate in theoretical physics from Oxford University, has taken some hawkish positions in the past. In 2006, he co-authored an op-ed column in The Washington Post in which he advocated a preemptive military strike against North Korea if the country moved to test a long-range ballistic missile capable of carrying nuclear weapons.