ITHACA, N.Y.--Cornell's Dave Rupert had done everything right. He had relieved himself of all Harvard defenders, caught a perfect halfback option pass from John Riley and was now breaking free down the right sideline, headed in the direction of the Crimson endzone.

Only when he was caught, tackled one yard shy of the goal by Paul Halas and Fred Cordova, did Rupert do something wrong. In a vain attempt to sneak the nose of the football over the goal stripe, he forgot to hold onto it, and as he lay on the hard polyturf of Schoelkopf Field for that one instant in the fourth quarter here Saturday afternoon, a man without a ball, he struck an image for all Cornell. Rupert was down and out, and Bob Baggott had the football for Harvard.

The Crimson had delivered the knockout punch early, withstood numerous Big Red efforts at resuscitation and walked off the field with an important 17-7 triumph that sets up a first place battle of unbeaten Ivy leaders next Saturday at the Stadium. Dartmouth will be in town, and that means war.

The Cornell game was more of an exhibition skirmish. The Crimson defense, which can be taken for one legitimate touchdown a game but little else, proved early that it could stop the Big Red charge when it so desired. It so desired when Cornell reached the Harvard 27 on the opening drive of the game, when the Big Red advanced on the Crimson 14 right before the half, and on all but one occasion in the final quarter, when Cornell scored its lone points of a winter-like afternoon.

For the most part the Harvard defenders played the role of teaser, allowing Cornell ample yardage but never the yardage that counted. It bent, one might say, but never broke.

Five turnovers--four fumble recoveries and a Halas interception--helped in this regard. Three of the fumbles can be attributed to Baggott, who led the team in both recoveries (seven) and interceptions (four) a year ago and who must make more out of turnovers than the owner of Rowinsky's.

In the second quarter, Charlie Kaye jumped on a pigskin that Baggott had jarred free. In the fourth, on the play that killed Cornell, Baggott raced 40 yards downfield to recover the loose goods. In the third, he had some fun.

The Big Red's Craig Jaeger was running up the middle for the third successive down when the ball popped free. Baggott pounced on it. Tom McDevitt pounced on it. Dave Walton pounced on Baggott and McDevitt.

"Who's got it?" Baggott yelled, his mouth full of polyturf.

"You've already had your share," McDevitt replied. "I've got it."

"You had seven last year," Walton added. "Let somebody else have a chance."

And with that the ball switched bodies. Give the recovery to McDevitt.

Offensively, give the credit to Larry Brown, who, off Saturday's showing, killed the starting quarterback question. Brown is numero uno, no ifs, ands or Buckleys about it. He threw two scoring strikes in the first quarter Saturday, a 20-yarder to Jim Curry and a six-yard shortie to Scott Coolidge, and generally ran the show the way someone with more than two weeks of multiflex under his belt should.

The Crimson's other score came via a 27-yard Gary Bosnic field goal midway through the third quarter following a 45-yard drive, the key play of which was a 23-yard Brown to tight end Paul Sablock pass. Sablock, incidentally, was used extensively for the first time this season. It won't be the last.

With an early lead and a defense that proved from the outset it could hold off Cornell at will, Brown concentrated on establishing a running attack and securing his position behind center. The timid signalcaller of two weeks ago he was not.

Of course Cornell is not to be confused with a good football team. It hasn't learned how to win, and it still has trouble scoring. It just keeps coming up one yard short.