Southern California came into the season with a Heisman Trophy front-runner, some of the most dynamic offensive players anywhere and its two-year postseason ban over.

The Trojans were the preseason No. 1, their aspirations a first national title since 2004.

After a mistake-filled loss to Arizona on Saturday, those national-title hopes have quickly gone out of reach.

At this rate, if USC doesn't play well down the final stretch of the season, it might not even play for the Pac-12 title, much less the BCS championship.

"From here, we are just going to stick together," quarterback Matt Barkley said after the 39-36 loss to the Wildcats.

The Trojans went into Saturday's game in control of the Pac-12 South, their national-title chances slim, but at least still there.

USC put up some big offensive numbers against Arizona, none more eye-catching than receiver Marqise Lee's Pac-12-record 345 yards receiving on 16 catches, along with two touchdowns.

Barkley filled up the stat line, too, throwing for a school-record 493 yards and three touchdowns as the Trojans rolled up 618 yards.

USC spoiled it with common themes from this season: undisciplined penalties and Barkley's inconsistency.

The Trojans have been the most penalized team in the Football Bowl Subdivision (10.25 for 84.63 yards per game) and were at their flag-inducing worst against the Wildcats

Safety T.J. McDonald was called for a taunting foul on a third down to keep Arizona's opening drive alive, setting up Matt Scott's 8-yard touchdown pass to Johnny Jackson.

Defensive lineman Leonard Williams also got carried away on a tackle in the first half, ripping off Arizona running back Ka'Deem Carey's helmet off for another personal foul.

The Trojans had 13 penalties in all, losing 117 yards and leaving their coach unsure of what to do about their lack of discipline.

"I'm open for any suggestions," USC coach Lane Kiffin said. "I've tried it all."

Barkley had some problems of his own.

He made a surprising return for his senior season, instantly becoming a Heisman Trophy front-runner.

Like USC's national-title chances, his shot at the Heisman appears to be gone.

Barkley has had some big games this season, but also some so-so ones, held to under 200 yards passing three times.

He opened Saturday's game by airmailing a pass well over Lee's head and had two interceptions in the first half.

Barkley also missed a chance to extend USC's lead in the third quarter, overthrowing Robert Woods on what would have been an 87-yard touchdown after the defensive back trying to cover him fell.

"I'll be thinking about that play all night," Barkley after completing 31 of 49 passes. "I got a little too excited seeing how wide open he was and just didn't put enough air under the ball."

It wasn't just Barkley.

Running backs Silas Redd and D.J. Morgan both lost fumbles, and safety Jawanza Starling gave the ball right back after an interception, fumbling it away on the return.

USC's defense allowed Arizona to pile up 588 yards and chew up precious time on its final drive, giving up a pair of first downs even though they knew the Wildcats would be running.

USC got the ball a final time with 55 seconds left, but two passes over the middle kept the clock running, leaving no option but to heave the ball into the end zone from Arizona's 48-yard line. Barkley's pass sailed into a crowd of players, but fell harmlessly to the turf, sending the Wildcats jubilantly running across the field and tears streaming down Lee's face.

"We didn't win and that's why I was upset," Lee said.

The loss knocked USC (6-2, 4-2) out of the national-title picture and eight spots in the AP Top 25 to No. 18.

The Trojans still lead the Pac-12 South after Arizona State's loss UCLA at home, but lost a chance to get some separation.

The Arizona loss also knocked some of the luster off of next week's game against No. 2 Oregon at the Coliseum.

Instead of a showdown with potential national-title implications, the Trojans will be in the spoiler role for the Ducks' BCS chances while trying keep their Pac-12 title-game hopes alive.

In a season that started with much higher expectations, this isn't what they had in mind.