Backup quarterbacks live by this mantra: It’s just a play away.

Just as the vice president is a heartbeat away from the presidency, the No. 2 signal-callers are one snap away from going from relative obscurity to having all eyes looking at them.

There are those who never get the chance, those that history relegates to the sidelines indefinitely. Then there are the Tom Bradys and Matt Cassels, those who prepared every game as if they were starting, so when the day came, they would be ready. And they were.

There might not be a more important backup position in sports. The second-string quarterback is like an understudy who must be able to jump into the performance at any moment and not forget any of his lines. He is an insurance policy, a reliever and the most popular guy on the team — until he makes his first mistake.

With junior quarterback Matt Barkley firmly in control of the starting job at USC, the battle over who is going to back him up has been one of the biggest story lines of the spring. With spring practices set to finish at the end of the week, it appears redshirt freshman Jesse Scroggins will eventually hold the No. 2 spot.

USC was spoiled in recent years by often having talented quarterback waiting on the sidelines: Cassel, John David Booty, Mark Sanchez, even Mitch Mustain. Though not always a guarantee of success, having that experience coming off the bench is certainly a luxury.

Scroggins, who turned 19 last month, will not have that. He’s green. He has never taken a snap in college. The two freshmen behind him, Cody Kessler and Max Wittek, are even greener.

Scroggins has continued to impress coaches, but he still has a long way to go. He completed four of five passes in Saturday’s scrimmage but threw one interception.

Those mistakes will happen as he continues to learn the playbook and adjust to the speed of the college game. His understanding of USC’s pro-style offense continues to grow as well.

More telling, though, is that Scroggins continues to get more opportunities than Kessler or Wittek. Although USC coach Lane Kiffin probably won’t reveal the depth chart until the fall, Scroggins is the clear favorite right now.

More importantly, Scroggins’ presence in the huddle appears to have improved. Last season, when he was relegated to a scout team role, Scroggins would sometimes stutter in the huddle out of nervousness as he was calling plays.

Now, Scroggins is talking about getting over his shy side and asserting himself.

“You’ve got to be the leader,” he told the Los Angeles Times last week. “You’ve got to be the guy that everybody likes.”

He could take plenty of direction from Barkley, who made his way into the starting lineup just about as soon as the ink had dried on his letter of intent. Barkley seemed to have that presence of leadership upon arrival, whereas Scroggins is still learning.

Then again, Barkley was forced into the role as a freshman. It makes more sense to ease Scroggins along at his own pace.

But in the world of sports, nothing is ever guaranteed. Whether he gets his name called tomorrow or never, Scroggins still has to be prepared. After all, Barkley has missed at least one game each of the last two seasons.

Even if Scroggins never plays a down this season, he has an opportunity to position himself to take over in the future if he continues to progress. That could happen as early as next season, if Barkley decides to skip his senior year and enter the draft.

Like all the backups who preceded him, the key for Scroggins is to remain patient without becoming complacent. He must make the most of every rep, every film session and every walkthrough.

No one knows when his time will come. But we all know how close it could be.

“Middle Ground” runs Tuesdays. To comment on this article, visit dailytrojan.com or email Josh at jjovanel@usc.edu.