With unofficial returns nearly complete, air traffic controllers apparently have rejected overwhelmingly a tentative contract with the Federal Aviation Administration, raising anew the threat of a nationwide strike that could cripple commercial air travel.

The temporary, 42-month agreement has been turned down by a vote that could be as much as 3 to 1 against or higher, said Marcia Feldman, press aide for the nearly 15,000-member Professional Air Traffic Controllers Organization.

Feldman yesterday emphasized that the returns are unofficial, but added that based on results thus far, "The positive votes on the contract are minuscule numbers. From everywhere, I'm hearing that the contract is being turned down overwhelmingly, even by 100 percent of the people voting in some places."

Feldman said her organization would not release a tally until after the votes are counted and certified Tuesday. But she said that as of Friday, when three-fourths of the vote were in, PATCO officials were "operating on the assumption that the contract has been rejected."

Rejection could mean a strike "coming about very quickly, or maybe in a few weeks." It also could mean no strike, depending on the FAA's response to the vote, Feldman said.

"Our worst-case scenario is that the government comes to the bargaining table and says: 'We gave you our best offer, and that's that.' Then, we turn around and count heads to see if we have a strike vote. If we have the vote, then, presumably we call for a strike," Feldman said.

Federal law forbids walkouts by controllers and other federal workers. But PATCO President Robert Poli said he was prepared to take his members out on strike June 22 if they had given him the 80 percent strike vote required by the union's constitution.

Poli received 75 percent bargaining unit support for a walkout. With his position weakened, he agreed to the contract that is now being called inadequate by his membership.

Poli spent the last month traveling around the country speaking to PATCO leaders about the contract and the circumstances that led him to accept it. Union sources say he has strengthened his position and, now, probably would get full constitutional support for a strike, should he decide to call one.

An administration source said last night that the controllers had been "offered a fair and equitable agreement, one that is consistent with the unique responsibilities of air traffic controllers, and is also consistent with the economic problems of the time."

The source who requested anonymith said Transportation Secretary Drew Lewis was disappointed that Poli had pushed for a rejection of the tentative agreement after indicating he would work for its approval.

However, Lewis, who led governnment negotiators in last-minute bargaining that resulted in the temporary agreement, previously said he would return to the table if the temporary pact failed ratification.

But in testimony June 25 before a House transportation subcommittee, Lewis warned that the Reagan administration would not increase the $40 million wage and benefits offer.

Under the temporary agreement, PATCO members would have received an overall 11.4 percent wage increase, 6.6 points more than the 4.8 percent pay raise due federal general service workers in their next regular pay raise. Most unionized postal workers recently settled for a contract that their leaders said would yeild a 10 percent wage increase over the next three years.

But United Press International reported yesterday that postal union leaders, who must sell the agreement to their members over the next month, have revised their calculations to show the contract would provide a 38 percent pay hike over the next three years.