No one konws when a pennant race begins. The Baltimore Orioles hope, almost desperately, that for them it began with today's 12th-inning victory, 5-4, over the Milwaukee Brewers.

For almost 100 games, the Birds have looked at a succession of ill omens. Whenever they have needed to win a game considered a crisis point, that is when they have played their worst and lost.

Finally, today, the O's danced off the field with an important victory that buoyed them with a feeling of skill rewarded, plus dumb luck.

With men on first and second base, one out, in the tied-up 12th, pinch hitter John Lowenstein drove a liner to center field. The crowd of 22,571 began its roar.

"I had the whole play in front of me," Lowenstein said. "It was awful looking. Everybody was running in a different direction."

Milwaukee center fielder Gorman Thomas was running in, but not as fast as he should because of the tough light of evening. Oriole runner Ken Singleton, on second after reaching base on a Robin Yount error and moving up on Eddie Muray's single, was totally confused. Convinced that Thomas would make the catch, Singleton was barreling back toward second.

"If Thomas makes the catch," Lowenstein said, "he may double Murray off first base for a double play. If he plays it on one hop, he can force Singleton at third by 70 feet. He might be able to run to third for the force.

"I thought," Lowenstein said "that I'd singled into a double play." Instead, he had hit a game-winning double.

Thomas was caught betwixt and between. Not only did he miss the catch, he missed the short hop. The balled rolled all the way to the fence.

Had right fielder Dick Davis hustled to back up the play, Singleton, pratically nailed to second, could never have scored. Instead Davis never moved.

"Sometimes an outfielder is just mesmerized, rooting for his teammate to make a great play," offered Singleton, a right fielder himself. "On the other hand, you are supposed to be hustling."

As the Brewers gave each other accusitory glares, the ball trickled to the warning track and Singleton scored, amidst cheers and laughter.

This game had many an overtone.

For once, the Orioles won a tough series -- two out of three -- from the Brewers, while the New York Yankees were losing two out of three in Kansas City. The Birds' deficit is now eight games, three games fewer than a fortnight ago.

Baltimore also won its fifth game in six against Milwaukee this month, capturing the season series, 7-6, and moving a half-game ahead of the Brewers into third place.

"If we win our season head-to-head series with both Milwaukee and New York," Manager Earl Weaver said a month ago, when the Oriole record against the Brewers and Yanks was 3-9, "then there'll be a race in September. If we don't make up those games head to head that we gave away, we won't get there."

The next crucial order of similar business is eight O's-Yankees games in the first three weeks of August.

This victory, finally moving Baltimore 10 games over .500 for the first time this season at 50-43, has several other emblematic strong points.

With Mike Flanagan trailing Billy Travers, 4-1, going to the bottom of the eighth, the Birds railed for three runs to tie, the first two coming on a dramatic two-run homer by dependable Benny Ayala.

"Benny's slugging .650," said Singleton. "Why don't we give him 600 at bats some year instead of 100 and see what happens."

Singleton was half-kidding, since such a move would mean Ayala replacing his good buddy Lee May (.238) as designated hitter.

Ayala, whose two run homer in the seventh Saturday night broke up a 1-1 tie and was the game-winner for Steve Stone was in his typical good spirits. "We can catch those Yankees," he said. "When they were 14 games behind the Red Sox (in '78), when did they start their comeback?"

On July 19, he was told.

"And how far back were they still on Sept. 1?"

Still six games behind, was the answer.

"See," Ayala said. "We got plenty of time. We're straightened out now. Just wait."

Certainly the O's looked straighter than in the recent past. On the three previous weekends in July, they lost important home series after returning from good road trips. Against Boston, Texas and Kansas City here this month, they were a miserable 3-7. Instead of making hay, the Birds were making mistakes.

From the moment Ayala's homer landed in the Oriole bullpen in the eighth, cutting the deficit to 4-3, the home team couldn't do anything wrong.

Doug DeCinces, infatuated with a new, lighter bat that brought him a homer Saturday evening, greeted reliever Bill Castro with a line single off the left field wall that was inches from being a game-tying homer. Instead of seeing this as ill fortune, the Orioles bore down. Pinch hitter Pat Kelly singled DeCinces to third and Lenn Sakata greeted reliever Bob McClure with a solid, game-tying sacrifice fly.

For the next four innings, McClure and Tim Stoddard were equally brillant in relief. The 12th-inning breakthrough gave Stoddard (1-3) his first victory in a year and brought the collective mark to the Baltimore bullpen to a still-atrocious 4-15.