SEATTLE — USC quarterback Sam Darnold has proven to be instinctive and elusive, and that extends to his responses when he is compared with Washington quarterback Jake Browning.

His assessment of Browning?

“I’ve watched him a couple weeks now,” Darnold said. “Obviously they’re a great team. They’re No. 4 in the nation for a reason.”

Was he excited by the matchup of arguably the two best quarterbacks in the Pac-12 Conference?

“You look forward to challenges,” Darnold said. “And they’re a great team overall. Great defense. He’s a great player, obviously.”

Today’s clash could be the only time Darnold and Browning meet in college. USC and Washington won’t play again during the regular season again until 2019, when Browning is likely to have exhausted his eligibility — although USC hopes there will be a second meeting this season, in the Pac-12 Championship game in December.

Browning is a Heisman Trophy contender. Darnold has been the catalyst for USC’s turnaround. They are the principal figures in what may be the most significant game of the Pac-12 season.

ESPN’s “College GameDay” has validated the game’s stature with a visit to Seattle — the first time USC has been a “GameDay” participant since 2013. The No. 20 Trojans (6-3, 5-2 in Pac-12 play) constitute a significant obstacle between No. 4 Washington (9-0, 6-0) and an undefeated regular season and a spot in the College Football Playoff.

USC cannot afford a loss, either. It must win out to have a chance to win the Pac-12 South Division and earn a spot in the conference title game. It could also solidify, or alter, the upward trajectory of the program.

In a private moment during the week, USC safety Chris Hawkins and linebacker Michael Hutchings discussed the magnitude of the game. “This is the biggest game of our careers,” Hawkins said. “We can’t think that way. But deep down, we all know that.”

In a similarly consequential season opener against Alabama, USC players said, the moment swallowed the Trojans. Emotions took over.

In that regard, they are happy to have Darnold, who has been unflappable.

Before games and at halftime, the coaches retreat to an office near the locker room. Helton sometimes can’t help but laugh. Through the concrete walls, he can hear offensive tackle Zach Banner’s voice booming, berating or encouraging teammates.

One voice he almost never hears — one voice few USC players hear — is Darnold’s. Judging by his expression, it is usually difficult to tell if Darnold has just woken up from a nap or is about to mount a late drive.

“I mean, I’m never the one to get hyped up in the locker room or anything,” Darnold said. “I guess I just sit there.”

Said Helton: “It’s hard to tell whether he’s excited or nervous or anything. It’s the sign of a good quarterback. He stays the same every time.”

Despite appearances, Darnold said he actually does get nervous.

“Especially when you’re playing a huge game like this,” he said. “If you don’t feel nerves, something’s wrong with you.”

But they rarely consume him. When USC was in danger of collapsing against Colorado last month, players said his composure helped keep them together. Before the game-winning drive, he said few words but assured his teammates in the huddle that they would score.

“He’s so level-headed,” tight end Tyler Petite marveled. “He’s just able to bring everyone back.”

Although they were in the same recruiting class, Darnold and Browning did not attend any showcase camps or passing clinics together. They have never met.

Darnold grew up in San Clemente, a two-sport star and a reluctant participant in the modern recruiting machine. Browning, the son of a former Pac-12 quarterback, grew up in Folsom, Calif., near Sacramento.

Darnold redshirted last season while Browning mounted a freshman campaign that would’ve garnered more plaudits were it not overshadowed by UCLA quarterback Josh Rosen’s. His opponents nonetheless took notice.

“You were like, ‘OK, this kid’s gonna be really good when he matures,’” Helton said.

He has matured. This season, Browning has passed for 2,273 yards and a school-record 34 touchdowns. He has had just three passes intercepted. One of every six Browning passes has been for a touchdown.

“He doesn’t really make mistakes,” Hawkins said.

“Decision making, timing and accuracy,” Helton said. “He has all of those.”

Despite Browning’s proficiency, Helton called Washington “a run-first team.” It leads the conference in rushing. Myles Gaskin averages 6.3 yards per carry. The Huskies’ potency on the ground functions like a champion boxer’s jab, enough to stagger an opponent on its own. Browning and two explosive receivers, John Ross and Dante Pettis, are happy to provide the knockout.

“They don’t try to trick you,” Helton said. “They’re gonna bring five or six runs to the park. They only pass the ball 25 times (a game) so there’s not a ton of concepts that come out of it. They just line up and play good, sound football. And that’s the mark of a Chris Petersen team.”

Helton has tried to prepare USC as if this were any other week. Every game, he said, was important.

“That’s why you came to USC,” the coach added, “to play the best in November in a game that’s critical to your season.”