Joe McKnight and Stafon Johnson rushed for scores for the Trojans (3-1, 1-1 Pac-10), whose national title hopes were seriously dampened by last week's 16-13 loss at Washington. USC's offense still showed many of the problems exposed last week in Seattle, even with Barkley's return from injury.
The Trojans' formidable defense forced three turnovers in the second half and held the rebuilding Cougars (1-3, 0-2) scoreless until Dwight Tardy's TD run with 22 seconds left.
Barkley, the freshman who led USC's game-winning drive against Ohio State two weeks ago, looked healthy in his third career start after missing last week's loss. He went 13 for 22 with a handful of sharp downfield throws to Damian Williams, who had five catches for 100 yards.
USC scored three touchdowns in the first 12 minutes, with Barkley throwing a 29-yard TD pass to Brice Butler and a 57-yarder to Williams on consecutive plays. Kicker Jordan Congdon adroitly recovered his own onside kick in between.
But Washington State suffered nothing close to the embarrassment of last season's 69-0 loss to the Trojans. The Cougars defense even twice stopped the USC offense on fourth down near the goal line in the second half, preventing a blowout.
Freshman quarterback Jeff Tuel took over for Marshall Lobbestael in the second quarter and immediately led a 19-play drive for the Cougars, who dropped to 3-14 in coach Paul Wulff's second season. Tuel got all but 5 of his 130 yards passing in the fourth quarter, when he led a 13-play scoring drive.
USC's trip to Berkeley next week lost much of its luster with No. 6 California's loss at Oregon, but now the Trojans and Golden Bears will be playing to avoid near-elimination from the conference title race.
McKnight finished a three-play drive by scoring just 2:36 into the first quarter, in which USC outgained Washington State 195-13.
Early in USC's third drive, Barkley threw a beautiful fade to Butler, his talented fellow freshman. Congdon then pooched his kick and fell on it before the Cougars knew what happened, and Barkley promptly hooked up with Williams over the middle for a rambling score.
Barkley even took a mean hit while releasing the ball, but pointed his hands skyward in excitement while flat on his back.
Tuel, a Fresno high school product, nearly redshirted this season before the coaching staff became impressed with how quickly he picked up their offense. The mobile freshman replaced the 2-for-9 Lobbestael, who himself took over for Kevin Lopina last week, and immediately led the Cougars on a clock-consuming 58-yard drive over the final 8:35 of the first half.
But Nico Grasu missed a 34-yard field goal at the halftime whistle, visibly deflating the Cougars.
USC gambled on a backbreaking score on fourth down from the Cougars 2 on its opening drive of the second half, but tight end Rhett Ellison dropped a pass that hit him between the numbers.
Trojans defensive end Devon Kennard forced and recovered a fumble deep in Cougars territory on the last play of the third quarter to set up Johnson's 6-yard TD run.