A professional football bettor builds a bankroll the same way an NFL coach constructs a winning team: by stressing defense.

Offense is wonderful to watch. And, yes, the Cardinals' front five does afford impeccable pass protection. But St. Louis, at its best, is a riskier proposition against the point spread then several of the league's duller outfits because its defense is so unreliable.

The good defenses are dependable. They show up almost every week of the season. Offense as offered in pro ball today, tends to be a sometime thing. Defense dominates.

Leading defenders of the betters' faith currently are Dallas, Denver, Atlanta, Houston, Pittsburgh and Oakland, with Los Angeles, Minnesota, San Diego and Washington one out below.

"Playing Football" is 19 for 22 this year. In our 19 winning selections, the opposition has averaged exactly nine points a game.

Defense, the home field and emotion are the three most important considerations in forming a judgment. If a certain team is faced with an important game at home and has a good defense, it is a team worth serios study. Often, however, the Head Linesman in las Vegas takes these factors into consideration and moves the line in or out, accordingly.

He has for example, reduced the home-field "advantage" this season to 47-49-2, against the spread. Favorites, with the points, are 44-50-4. Only one team is 6-1 in the 11-for-10 League in 1977. The Jets. Complete standings, halfway through the 14-game schedule, show:

Jets 6-1, Cleveland 5-2, Dallas 5-2, Chicago 5-2, Atlanta 5-2, Denver 5-2, Tampa Bay 5-2, Miami 4-3, Kansas City 4-3, Houston 4-3, San Diego 4-3, Oakland 4-3, Pittsburgh 4-3, New Orleans 4-3, Philadelphia 3-3-1, Green Bay 3-3-1, Baltimore 3-4, Seattle 3-4, New England 3-4, Los Angeles 3-4, Giants 3-4, St. Louis 2-4-1, Cincinnati 2-5, Buffalo 2-5, Washington 2-5, Minnesota 1-5-1, Detroit 1-0, San Francisco 1-6.

Cleveland is a prime pick this weekend. The Browns' defense is getting better and Greg Pruitt makes the running game go. Don't aske me where Cincinnati's passing attack has been, I don't know. Seven weeks of wondering is a long time. Give the a points and take the Browns at home for a mythical $500. They are in excellent position to take the AFC Central title from Pittsburgh.

I'll also take a mythical $250 on Houston getting 2 points at home against Chicago and $250 on Denver getting 2 points at home against Pittsburgh. The reasoning: the three criteria mentioned earlier. Also, the Steelers' Franco Harris appears to be hurting. Without Harris, Terry Bradshaw is a seriously wounded warrior.

Other games of more than passing interest concern Minnesota giving 4 points at home to St. Louis: Baltimore, giving 10 1/2 at home. Monday night to Washington; Dallas, giving 12 (up from 11) to the New York Giants, and the New York Jets, getting 3 1/2 at home against Miami.

Completing the card are New England giving 17 to Buffalo, kansas City giving 5 (up from 4) to Green Bay, Philadelphia giving 3 1/2 to New Orleans. Detroit getting 4 against San Diego, Atlanta giving 4 to San Francisco and Oakland and Los Angeles giving 21 to Seattle and Tampa Bay respectively.