Bring on the Cowboys, the Redskins have been saying all week, and the game they believe will convince America they are no flim-flam fluke will begin tonight at 9 o'clock at RFK Stadium.

John Riggins, for one, can hardly wait.

"We've all been looking forward to this one," the folksy fullback said the other day. "In the past, you always said the same thing, but I'm not sure how many guys believed it. You'd always make out like you wanted to whup somebody, but deep down inside you just weren't sure.

"But this year, nobody's talking loud or saying much. There's kind of a quiet confidence around this place. That's how it should be when you believe in yourself and in your team. And that's what we all believe."

The same cannot be said for the oddsmakers, who have installed Dallas (3-1) a four-point favorite in a game that will be televised locally by WJLA-TV-7. The Redskins are underdogs even though they have won four straight in 1978, seven in a row going back to last season, and never have lost in six games at home on Monday night football.

No matter what happens, the Redskins will remain in first place in the NFC East. A victory would give them a two-game lead over the Cowboys and the Giants, who lost to Atlanta yesterday and are now 3-2.

"We're going to have to play hard and play our best game of the year," Redskin Coach Jack Pardee said yesterday. "We're playing a good team with tremendous balance and it's going to take everybody. If we play the way we know how, it should be a close game. When you play the defending Super Bowl champions, you have to earn it. They don't give you a thing."

Well, not quite. After four games the Cowboys rank dead last among NFC teams in pass defense, allowing opponents an average of 185 yards a game through the air.

Pardee insists many of those yards have come after Dallas had built up large leads against the Baltimore Colts and Giants and that "nobody has been able to control the ball on them yet consistently. That's a very misleading statistic."

But the Redskins believe they can move the football against the Cowboy flex defense. "I believe the whole key to this game will be third down conversions and the play of the two lines," said quarterback Joe Theismann.

"We've got to keep our drives going and keep their offense on the bench because they can score so many different ways. It's the same thing I've been saying every week, we're not going to try to do anything different.

"I think our line can block those guys. No, I know they can block 'em. They've been doing it every week, and this is no time for them to change. I have all the confidence in the world in them, I really do.

One of the critical catchups pits Washington lefttackle Terry Hermeling, against the NFL's defensive player of the year, Harvey Martin."Harvey's the same as he always is," Hermeling said. "Tough and a great pass rusher. It's going to be interesting."

Riggins, the NFC's second-leading rusher, says he is primed to play, though the memory of last year's season-ending knee injury in Texas Stadium will not be a motivating factor, he insists.

"I'd be lying if I said I hadn't thought about it, but I sure, haven't dwelled on the thing," Riggins said."In this day and age, it's hard for me to believe that anyone would be foolish enough to try to put somebody out of the game intentionally. That's not what happened last year. To this day I'm not exactly sure how it happened. So I don't have any ill feelings about it.

"It's just one of those games you look forward to playing because it's for first place and you're playing the best team in football. That's plenty of motivation for me."

The Redskins obviously would like to establish a running game early against a Cowboy defense ranked third against the rush in the conference. "But we've got to throw some, too," said Pardee. "You have to loosen them up and not get caught into situations where they know you have to throw."

The Redskins will shuttle three wide receivers and hope none get hurt. With Danny Buggs out with a separated shoulder, the Redskins would be forced to use tight ends Jean Fugett or Reggie Haynes at wide receiver in emergency situations.

"I hope we can throw the football against them," said wide receiver Frank Grant, off to a slow start in 1978 with only four catches all season. "I've been looking for a big day the last eight weeks. But we've winning, we're 4-0, so I'm not worried about it.

"They have had some trouble in the secondary. Maybe it's the new rule change. One bump downfield in the past really helped a lot of people. Now they've got to play more man to man, and that may be part of the problem. I do know one thing, if you catch the ball, you're gonna get popped. Those people hit hard."

The Redskin defense, meanwhile, has other problems.

The Cowboys lead the NFL in scoring, total offense and passing offense and are second in rushing offense. Quarterback Roger Staubach has completed 59 percent of his passes and thrown for nine touchdowns. He has tossed seven interceptions, compared to Theismann's two.

Tailback Tony Dorsett is playing full time now and leads the league in rushing with a 5.7 yard average. Receivers Drew Pearson and second-year man Tony Hill are always touchdown threats any time they go deep.

The Redskins seem to have an edge on special teams. Kicker Mark Moseley, who says he will make up for an 0-for-3 performance in last season's second Cowboy game, adds his leg has never felt stronger this year.

A year ago, the Cowboys harassed Moseley by sending linebacker Mike Hegman on a head-hunting mission every time he kicked off. "I hope they do it again," Moseley said. "I can take care of myself and that'll leave one less guy they'll have to stop Tony Green."

Green, the rookie return man, says he is completely recovered from that sprained left knee he suffered two weeks ago in St. Louis. Pardee will use him on both punt and kickoff returns.

Cowboy kicker Rafael Septien has been having problems, hitting only one of his four field goal attempts this season, including a miss from 18 yards.