The Montreal Expos cast the crucial vote today that gave baseball its first split season in the sport's 112-year history.

In the sort of drama usually reserved for the playing field rather than an owners' meeting, the whole structure of the second half of the season was, by circumstance, placed in the Expos' hands this afternoon.

The American League already had voted, 12-0, with two abstentions, in favor of the one-year-only plan that will institute a first-half and second-half champion in every division, with an extra tier of playoffs between the winners.

While the AL needed only a majority vote, NL bylaws necessitated a three-quarters majority. The NL count stood at 8-3 with the Expos undecided and caught in a crisis of conscience. If the split- season proposal was not agreed to by both leagues, the only other option the owners had was to continue this strike-smashed season in the usual fashion.

"We turned out to be the pivotal vote," said the Expos' president, John McHale. "We're traditionalists and conservative . . . philosophically, we're firmly opposed to the plan. But, face it, the rest of this season is 'Operation Salvage.' We decided in a directorship-level discussion that if the clear majority wanted the split season, then, since we're all partners in this, we'd do what's best for everybody."

The Expos became convinced the breakdown would be 8-3 with only the conservatives from Cincinnati, St. Louis and (amazingly) Philadelphia staunchly opposed. Thus, the Expos voted the Phillies a championship on principles -- support for the will of the majority -- and the Phillies voted to deny themselves one on principles -- belief that the plan is bad for baseball. "We were the fourth team to vote," McHale said, "but after we did, I think it was clear the proposal would pass." The vote was, in fact, 9-3.

"That scenario sounds correct, according to my understanding of the situation," confirmed Commissioner Bowie Kuhn. "I certainly don't think Mr. McHale is exaggerating his team's role."

The owners also went through the formality of ratifying the strike settlement. Only two teams voted against the compromise; the fight-to-the-last-man franchises in St. Louis and Cincinnati. Three others clubs -- the Chicago White Sox, Minnesota and Philadelphia -- expressed reservations by abstaining.

In addition, the NL owners approved the purchase of the Chicago Cubs by the Chicago Tribune; thus, the formerly self-proclaimed "World's Best Newspaper" now owns the self-incriminated "World's Worst Baseball Team" pending a stock-holders' vote on Aug. 28.

The team of the day here was the Expos, and their smokey-room moment of truth. If the two leagues had not agreed on a split season, all blazes could have broken out.

Kuhn has the power to choose between the leagues and pick "continuation" or "split season." However, Kuhn's style always has been to seek backroom unaminity, or, if he can't achieve it, then never act unilaterally in a league-wide matter. "I'd have pushed for a split season," Kuhn said. But would he have taken the decision upon himself had four NL naysayers been adamant?

"That hasn't been my history," Kuhn said.

So, if Montreal had chosen its private philosophy over its sense of responsiblity to all owners, baseball might not have its new, and traumatic, format; one already under heavy fire.

The four retroactive winners -- the Yankees, A's, Dodgers and Phillies -- already have been described derisively by Cincinnati General Manager Dick Wagner as "our new designated champions."

It was decided that the rained-out games of the first half, which could have had a bearing on who three of the division champions would be, will be played in the second half and count in its standings.

The most controversial aspect of the new format is that if any of these four teams are second-half winners, they still will have to meet their division's team with the next-best full-season winning percentage in a best-of-five playoff.

So, what motive do these designated champs have for playing with pennant-race vigor in the remaining seven weeks?

"Frankly," McHale said, "we're relying on the players' professional pride."

Before the week is out, Kuhn said, a decision will have been made on which of several methods will be used to "weight" the new playoffs in favor of a team that wins both "seasons." A decision will be announced at the All-Star Game, he said.

"We don't care for the split season, period," Cincinnati's Wagner said before the vote. "What's wrong with it? Only about 15 to 20 things. Beyond the 'integrity factor' since there's no bye, you also have the possiblity that a first-half winner could (by losing certain games) play a big role in deciding who its playoff opponent would be . . . Certain managers think that way."

Meanwhile, every tail-end club in baseball now is rejoicing that it is, until Monday at least, tied for first place again.

"Love it, love !" said Minnesota owner Calvin Griffith. "We were 18 games out of first place. If this hadn't passed, we might as well not have opened out gates the rest of the season."

Less ebullient was Commissioner Kuhn.

It was pointed out that, under this new plan, it was conceivable that the team with the best overall winning percentagek for all of 1981 could, if it finished second in its division in both halves, actually be excluded from the expanded playoffs.

"Yes, I know," Kuhn said, pausing. "Well, it's been a hard year . . . You may say that my sense of artistry is offended, to put it mildly . . . The fans will have to judge."

And, for good or ill, they can thank the Montreal Expos.