Eastern Airlines mechanics and baggage handlers rejected the carrier's "final" contract offer yesterday, and both sides began preparing for a possible strike at midnight Wednesday.

By 72 to 28 percent, the 13,000 members of District 100 of the International Association of Machinists turned down a three-year package that would have provided a 32 percent wage increase.

IAM local president Charles Bryan, who had recommended rejection, said the wage package was "backloaded in the extreme and . . . offered no retroactivity." The union's contract expired 14 1/2 months ago, and members have not had a raise since July, 1981.

Bryan said he hopes Eastern will make a new offer in a bargaining session scheduled here Monday with federal mediators. But Eastern President Frank Borman said in an emotional appeal to union members Friday that the wage package was "final" and that rejection would mean a certain strike. The union has set a strike deadline of 12:01 a.m. Thursday.

Borman told the members that a strike would be a "catastrophe" for the financially ailing airline, which has lost slightly more than $140 million in the last two years.

An Eastern spokesman said yesterday that, by using supervisory personnel, the carrier expected to be able to operate 65 to 70 percent of its flights in a strike.

Borman has threatened to hire replacements for union members who honor picket lines. "There are whole bunches of people" ready to take their jobs, he said yesterday.

Eastern has 39,000 employes, of whom about 22,000 are unionized. The last strike against Eastern, by machinists, occurred in 1966.

Prior to the contract vote, Bryan accused Borman of overstating the company's plight. "For the last four years, they've been advertising that they carry more passengers than anyone else in the free world," he said. "Does that sound like they're ailing?"

Other union leaders noted that Eastern is saving millions of dollars for every penny-per-gallon drop in the price of fuel and that it expects to take delivery of 17 new aircraft this year.

Nearly half of the Eastern employes represented by the IAM are in the company's headquarters city of Miami, with other large locals in New York and Atlanta. The remainder are along the eastern seaboard.

The union's first strike deadline last Sunday was averted when the company, bargaining under federal mediators, offered a contract proposal 12 minutes before members were scheduled to walk out.

The wage package called for 6 percent increases April 1, Nov. 1 and April 1 next year and an 11 percent raise in November, 1984. Under the proposed contract, a top-rated mechanic, now getting $13.15 an hour, would be paid $17.40 hourly after the fourth raise.

Bryan contends that Eastern employes currently get $4 an hour less than industry standards and that the backloading--future raises and no retroactivity--of the company's wage offer will keep them behind. Eastern officials have countered that the lag in wage increases over the last two years has spared it from the mass layoffs of other large carriers.

After the vote, Bryan called on Borman to "stop stonewalling . . . . There are plenty of non-economic items in this contract favorable to them. There is still plenty of room to maneuver."

Borman, a former Apollo astronaut, said Friday that "you can't get any more blood out of the company." He denied that his plan to replace union workers was union-busting, saying those replaced would be the first offered the chance to return.