WASHINGTON – As if there wasn’t already enough to fight about in Congress, the departure of Secretary of Defense Chuck Hagel is setting up a big nomination fight in the Senate. Whoever is named to replace Hagel will have to make it through a Republican-controlled Senate, where leaders have pushed for a more aggressive fight against the Islamic State group.

The situation in Washington is becoming so toxic that virtually anything requiring congressional approval in the next two years will be a wearying process. The fight to get a new defense secretary approved could take months.

President Barack Obama is "going to have a hard time getting any nominee through," Rep. Buck McKeon, the outgoing chairman of the House Armed Services Committee, told CNN.

And while the House won’t have any say in the process -- only the Senate weighs in on nominations -- the anger from conservatives in the chamber who don’t want to cooperate with the president is sure to be felt across the capitol. McKeon cited the failing relationship between the president and the White House, saying since the midterm election when the GOP won control of Congress, Obama has made no effort to improve the relationship. Instead, he’s only made it worse -- as with the executive orders on immigration announced last week.

Sen. John McCain, who is expected to be the chairman of the Armed Services Committee when Republicans take over in January, said in a radio interview Monday morning confirming a replacement for Hagel will be the top priority for the new Congress. That is the earliest a nomination process could begin since Congress has only 10 working days left in December.

McCain, a military hawk, will be the key figure in moving the nomination. As the head of the committee that will hold nomination hearings, he will set the times for all hearings, dictate the rules of debate and control when the nominee could move to a full Senate vote. But a vote likely wouldn’t come until McCain has had a chance, along with the other members of the Armed Services Committee, to grill the nominee on what happens next with ISIS and in Syria.

There is a lot of speculation the departure of Hagel will come with a shifting strategy in Syria. Republicans have called for combat troops to be sent, instead of the current situation involving U.S. airstrikes, plus troops training Syrian and Iraqi fighters.

"It is imperative that the next secretary of defense possess a sharp grasp of strategy, a demonstrated ability to think creatively and the willingness and ability to work with Congress," Senate Majority Leader-elect Mitch McConnell said. 

Obama will need at least four Republicans to approve whomever he nominates -- maybe 14 if Republicans change the rules back. Frustrated by the number of nominees blocked in the last Congress, Sen. Harry Reid changed the needed votes from 60 to 50. Republicans, when they take control, will have to decide how many votes will be needed going forward.

Hagel is currently the only Republican in Obama's cabinet. But Senate Republicans weren't very keen on him from the beginning and enthusiasm hasn't grown. 

McCain could also work in the White House’s favor. An old-school senator when it comes to respecting the process, McCain is unlikely to want to see the nomination bogged down in fights about other topics. As he has said many times, "elections have consequences" -- including the sitting president's entitlement to pick Cabinet members.

The trick to expediting the process would be to select a nominee who is already well known among members of the Senate and who carries little or no political baggage. If Obama were to be seen as trying to nominate someone with a larger political future or whose role would be to message for Democrats, Republicans would balk.

But if he nominates someone who has already been through a nomination process with no red flags or who has a past working relationship with Congress, the process could proceed more smoothly.

The White House quickly floated three possible nominees: Sen. Jack Reed, D-R.I.; Michele Flournoy, former undersecretary of defense for policy; and Ashton Carter, former deputy secretary of defense. Reed’s office was quick to push back on the idea of him taking the role, especially since he would be forfeiting a safe Senate seat for a two-year appointment to run the Pentagon.

Carter went through the nominating process for his role with no opposition from the Senate. Flournoy is less familiar to the Senate but would be the first woman named to the job, potentially blunting some criticism.

“Three potential replacements who have been mentioned -- Sen. Jack Reed, Michele Flournoy and Ashton Carter -- are solid choices for this important position,” Sen. Lindsey Graham, R-S.C., said in a statement. Graham sits on the Senate Armed Services Committee.

But even if a less controversial nominee is selected, there is still likely to be a contentious process, since it allows Republicans to question every piece of Obama’s military policy and strategy.

If the departure of Hagel does nothing else, it could make one other nomination fight a little easier. Moving through Loretta Lynch's nomination as attorney general is starting to look more like a cakewalk.