Two relative unknowns of college football, Southern Methodist and Purdue, could challenge for national supremacy this season.

Both have All-America quarterbacks and high-powered offenses. What they lack in defense and muscle they offset with finesse and excitement.

Other aspirants to the title are the usual powerhouses: Southern California, Alabama, Michigan, Oklahoma, Nebraska and Texas.

Upstarts such as Texas A&M and Missouri could surprise.

The Trojans, Crimson Tide, Wolverines, Cornhuskers, Sooners and Longhorns have size, muscle and top-flight defenses and their offensive philosophies are basically conservative. They would rather win by running over you, then shutting you off, than by finesse.

SMU is different. The Mustangs could be one of the most exciting teams in years. They were 4-6-1 a year ago, but Coach Ron Meyer has the nation's total-offense leader, Mike Ford, and the nation's No. 2 pass receiver, Emanuel Tolbert, returning.

Ford, who will be a junior, has passed for 5,071 yards in his career. Tolbert caught 62 passes for 1,041 yards last season.

Last year, the Mustangs were virtually all-pass. This year, they will continue to throw, but they recruited three blue-chip running backs to move it on the ground. The best of them probably is 6-foot-3, 210-pound Eric Dickerson, a 9.3 sprinter from Sealy (Tex.) High School, who rushed more than 6,000 yards in a three-year high school career.

The Mustangs recruited the Texas schoolboy player of the year in 6-2 Craig James from Stratford High in Houston.James ran for 2,411 yards and scored 35 touchdowns last season in leading his team to the state AAAA championship.

SMU will move into plush Texas Stadium, where it should draw 60,000 spectators a game.

Purdue's success revolves around 6-5 junior quarterback Mark Herrmann. Already, he is the Big 10's No. 4 career passer. He cut his interceptions from 27 to 12 last year and he has assumed team leadership on the field.

But the Boilermakers, 9-2-1 a year ago, are not a one-man team. Fullback John Macon, who rushed for 913 yards, returns and so do the other 10 offensive starters, plus seven defensive players.

Southern California, last season's national champion in one wire-service poll, is loaded again, and is probably the team to beat. Returning is tailback Charles White, who rushed 1,859 yards last season, and steady quarterback Paul McDonald.

Where USC excels, though, is along the offensive line. The anchors are 6-7, 280-pound tackle Anthony Munoz and 6-5, 253-pound guard Brad Budde.

Alabama, national champ in the other poll last season, was supposed to be a year away, but came through early. The Tide doesn't have the big names it did last year -- Jeff Turledge, Marty Lyons, Tony Nathan and Barry Krausse. But it is deeper, stronger and tougher.

The entire offensive line, led by Buddy Aydelette, Mike Brock and Dwight Stephenson, returns, and Steadman Shealy could be another in the long list of great Alabama quarterbacks.

Oklahoma is always good, and especially so with Heisman Trophy winner Billy Sims back.

Purdue's biggest challenge in the Big 10 probably will come from Michigan.

For the first time since 1975, the Wolverines will not have Rick Leach at quarterback and that will hurt. The new man on the spot is B.J. Dickey.

Defense will make or break Michigan. Eight of 11 defensive starters, led by linebacker Ron Simpkins, return on a team that yielded only 88 points in 11 regular-season games last year.

SMU is not the lone star in Texas. Both Texas and Texas A&M will field strong teams.

The Longhorns were supposed to be rebuilding last year and they ended up 9-3. This season's Longhorn edition is loaded on defense, and 6-1, 190-pound All-America safety Johnnie Johnson leads the show. Defensive tackles Steve McMichael and Bill Acker provide muscle.

The big man on offense is flanker Johnny (Lam) Jones. Also returning are running back A.J. (Jam) Jones and the entire offensive line.

A&M bagged the wishbone halfway through last season and became explosive. The stars are quarterback Mike Moseley and tailback Curtis Dickey.

Oklahoma's challengers in the Big Eight will most likely be Missouri and Nebraska.

Missouri averaged 31 points a game last season and the man who made the Tigers go, quarterback Phil Bradley, is back. He led the conference in passing and total offense.

Nebraska is seeking a quarterback to go with its wealth of runners and receivers. Tight end Junior Miller, tailback I.M. Hipp and tailback Kenny Brown are game breakers.

Other perennial powers such as Notre Dame, Ohio State and Penn State could have a tough time this season. Notre Dame doesn't scare people anymore, Ohio State doesn't have Woody Hayes and Penn State has a great defense, but a questionable offense.