The Dallas Cowboys' principal worry in the Eastern Division of the National Football Conference is themselves. Despite their usual success, they occasionally slump in midseason.

Coach Tom Landry, in his 19th year, wants the Cowboys to become the first team to win three Super Bowls. He says it is "a No. 1 challenge," following last year's 27-10 demolition of the Den ver Broncos in Super Bowl XII.

A lineup change was necessitated when placekicker Efren Herrera held out and was traded to Seattle for a draft choice. Jay Sherrill, free agent from North Carolina State, has taken over.

The St. Louis Cardinals will be handicapped in their run at the Cowboys. They have a new coach, Bud Wilkinson, trying to bring order to a badly disorganized squad, and they have lost quality personnel.

Much has been made of Wilkinson's being a rookie head coach in pro ball at age 62, after 15 years away from the coaching sidelines at the University of Oklahoma.

But before he took over, Terry Metcalf, the all-purpose back, jumped to Toronto of the Canadian Football League.Wide receiver Ike Harris and guard Conrad Dobler were traded to New Orleans. Safety Ken Reaves played out his option and was let go. Then tight end J. V. Cain was lost in training camp with a ruptured Achilles tendon.

The Philadelphia Eagles were 4-10 in 1976 and 5-8 in 1977 under Dick Vermeil. If they don't improve sharply in his third year as coach, the former assistant to George Allen at Los Angeles may be looking over his shoulder at his old boss.

The Eagles have a tough schedule, facing 11 winning teams.

The New York Giants, 5-9 last season, don't figure to improve much. They have three second-year men at quarterback -- Joe Pisarick, Jerry Golsteyn and Randy Dean. The Giants had the third fewest yards passing in the NFL. The offense produced only 181 points.

Larry Csonka came back from knee surgery and had a 100-yard rushing performance on the ice against the Chicago Bears. Central Division

Surprisingly, the Minnesota Vikings last year permitted almost as many points, 227, as they scored, 231. Yet, defense-oriented Coach Bud Grant says, "I think the basic point is that we don't have a deficient position."

The Vikings won their fifth straight NFC Central title with a 9-5 record, despite losing 17-season veteran quarter-back Fran Tarkenton with an ankle fracture in the ninth game.

Observers wondered last year about the Vikings' wisdom in using a No.1 pick to draft quarterback Tommy Kramer of Rice. But he showed winning poise after Tarkenton went down in 1977 and has been impressive in this year's exhibitions.

The Chicago Bears will beat the Vikings, some oddsmakers say. The Bears had a similar record, 9-5, in landing a wild-card spot, but they are being reorganized by a new coach, Neill Armstrong, former defensive coordinator for the Vikings.

Running back Walter Payton is concerned about two issues. First he will play out his option unless he receives more than the $375,000 reportedly offered by the Bears.

And Armstrong has said he envisions passing to running backs. Payton prefers to run the ball. He led the NFL in rushing with 1,852 yards and wants to top O. J. Simpson's record 2,001.

The Detroit Lions hired an offensive line specialist as head coach and Monte Clark will have an immediate challenge: Lions' passers were sacked 54 times, a league high.

He will have back from the injured list Lawrence Gaines and Dexter Bussey as running backs, David Hill as tight end, and J. D. Hill, Ray Jarvis and ex-Colt Freddie Scott as wide receivers. The Lions' kicking game figures to improve through the signing of Ohio State product Tom Skladany, who punts and placekicks.

The Green Bay Packers probably will make up Bart Starr's mind whether to continue as head coach. Their performance in his fourth season will decide it. The former Packers' quarterback could coax only 134 points out of the Packers' offense. The defense yielded 219.

Lynn Dickey apparently is sidelined again with injuries. Second-year quarterback David Whitehurst has the assighment to better a 4-10 record that resulted in a fourth-place finish for the third consecutive year.

The Tampa Bay Buccaneers finally won two games in 1977 and the coaches of the losing teams, Hank Stram of New Orleans and don Coryell of St. Louis, were fired. The Bucs scored only 103 points and were last in the league in passing, with 30 throws intercepted.

The Buccaneers drafted All-America quarterback Doug Williams of Grambling as an eventual solution. They also drafted fullback Johnny Davis of Alabama and traded for veteran defensive linemen Wally Chambers and Ernie Holmes. Western Division

Will the Los Angeles Rams find happiness under Coach Ray Malavasi, former defensive coordinator, with the abrupt dismissal of George Allen?

Owner Carroll Rosenbloom will be on the spot for letting 1977 Coach Chuck Knox go if the Rams do not surpass their 10-4 record of last season and/or make it to the playoffs.

Pat Haden survived the challenge of Joe Namath and finished second to Roger Staubach in NFL passing ratings. The Rams drafted running back Elvis Peacock of Oklahoma, only to lose him for the season with a knee injury. They acquired punt returner-defensive back Eddie Brown from the Redskins and wide receiver Harold Jackson was traded to the New England Patriots.

The Atlanta Falcons had their best record since 1973. Under new Coach Leeman Bennett they finished 7-7 and set a modern NFL record by allowing only 129 points. The defensive unit set a club record with 42 sacks. Falcon passers were sacked 40 times.

Atlanta still hopes to improve its attack by getting a full season out of quarterback Steve Bartkowski. In 1975 an injured left elbow sidelined him; in 1976 and 1977 it was knee injuries.

The San Francisco 49ers have undergone major changes under General Manager Joe Thomas and new Coach Pete McCulley.

O. J. Simpson was acquired from Buffalo, but has to demonstrate he has recovered from the knee surgery he underwent at midseason in 1977. Running back Delvin Williams was dealt to Miami.

The 49ers acquired speed at wide receiver and for their special teams by landing Freddie Solomon in the Williams deal and Larry Jones from the Redskins. Tight end Ken MacAfee was drafted from Notre Dame to give quarterback Jim Plunkett more options. Plunkett threw only nine touchdown passes in 1977 and in nine games threw no scoring passes.

The New Orleans Saints have a new head coach, Dick Nolan, who won three NFC West titles at San Francisco. Before that he handled Tom Landry's defense at Dallas, and defense is what the Saints need.

A trade for wide receiver Ike Harris and guard Conrad Dobler of St. Louis should help quarterback Archie Manning and running backs Chuck Muncie and Tony Galbreath.

The No. 1 draft choice was wide receiver-kick returner Wes Chandler of Florida and already he has shown he can start effectively.