SANTA CLARA, Calif. -- Sam Darnold threw for 325 yards and two touchdowns and engineered a back-breaking 99-yard drive following a goal-line stand in the fourth quarter to lead No. 10 Southern California to a 31-28 victory over No. 12 Stanford in the Pac-12 championship game on Friday night.

"It was time to dig deep and win a championship," coach Clay Helton said. "When you're trying to win one, those are the plays that are going to define you and define your football team."

Darnold threw touchdown passes to Michael Pittman Jr. and Tyler Vaughns to stake the Trojans (11-2) to the lead. He then delivered one of the biggest plays of the game when he stepped up to avoid pressure in the end zone before connecting on a 54-yard pass to Pittman to spark the key touchdown drive in the fourth.

Ronald Jones finished that drive with an 8-yard run that made it 31-21 to cap a productive night where he ran for 140 yards and two touchdowns to give USC its first Pac-12 title since 2008.

The long drive came after USC stuffed Stanford (9-4) at the goal line to preserve the lead. With Bryce Love nursing an injured ankle on the sideline, Cameron Scarlett was stopped on successive runs from inside the 2, including a fourth-down try from the 1 that was stopped by Uchenna Nwosu.

"We told our guys to hunker down and this is what Trojan football is," Nwosu said. "We were able to make a play."

Stanford coach David Shaw had no regrets about his decision even though the Cardinal were unable to get the touchdown on offense or the stop on defense to make it pay off.

"The bottom line is we had opportunities and we didn't make enough plays," Shaw said. "We made enough plays to keep it close but not enough plays to win. The sequence of fourth-and-1 to take the lead in the Pac-12 championship game, there's no hesitation at all. That's what we're going to do."

Darnold sealed the game with a 15-yard pass to Josh Falo on fourth-and-2 to let USC run out the clock.

The win gives USC a season sweep against Stanford. USC holds out hope for a spot in the four-team playoff if there are several upsets in other conference title games on Saturday but the chances appear to be remote.

THE TAKEAWAY

STANFORD: Love ran for 125 yards and had his FBS-record 12th run of at least 50 yards that set up K.J. Costello's 11-yard TD pass to Kaden Smith that cut USC's lead to 24-21 late in the third quarter. Costello added a 28-yard TD pass to Smith with 2:09 remaining but wasn't nearly consistent enough to keep pace with Darnold. Costello finished 10 for 22 for 192 yards.

USC: The Trojans had a balanced offensive performance with Pittman catching seven passes for 146 yards and Jones doing solid work on the ground. The defense came up with the big stop at the goal line. But it probably won't be enough to overcome the earlier losses at Washington State and Notre Dame to lift the Trojans into the playoff even if they get help Saturday.

UP NEXT

Both teams will get their bowl bids on Sunday with USC projected to the Fiesta Bowl and Stanford expected to go to the Holiday Bowl, according to CBS Sports' Jerry Palm.