The top issue on the Senate agenda to kick off the new congressional session in 2015 will be confirmation hearings for Ash Carter, President Obama's nominee to replace Chuck Hagel as his defense secretary, a process that is expected to move quickly — but not necessarily calmly.

Carter, Hagel's former deputy, is well known on Capitol Hill for his previous work at the Pentagon and has easily sailed through previous confirmation processes.

But his nomination could be the first voted on by the new Republican-controlled Senate, whose leaders have voiced serious concerns not about his record but about Obama's overall national defense strategy.

One of Obama's harshest critics — his 2008 campaign opponent, Sen. John McCain, R-Ariz. — is taking over as chairman of the Senate Armed Services Committee and will lead the questioning of Carter. McCain already has praised Carter but promised to use the opportunity to ask pointed questions about the White House's response in Iraq and Syria, and its long-term Afghanistan plans.

The hearings also will echo fights to come on the fiscal 2016 defense budget, which Carter likely will have to present and defend to Congress in early February. Pentagon planners have pleaded with lawmakers for years to undo automatic sequestration funding cuts mandated under the Budget Control Act of 2011, but stalemated political disagreements have left the policy in place.

McCain said in early December that he thinks the entire nomination could be wrapped up in January, despite the full agenda already planned by incoming leadership.

Senate Democrats have offered strong support for Carter's nomination, and publicly stated their desire for a fair and quick confirmation process.