Negotiations between government and union officials in an effort to avert a crippling strike by the nation's air traffic controllers passed a midnight deadline with no indication of whether a walkout at 7 a.m. today could be avoided.

The two sides, headed by Transportation Secretary Drew Lewis and Robert E. Poli, president of the 14,800-member Professional Air Traffic Controllers Organization, talked for more than four hours yesterday before breaking for dinner, then resumed their discussions at the Federal Mediation and Concilation Service at 9 a.m.

The talks were described late last night by D.J. Yount, an FMCS spokesperson, as "very active."

Poli had said his membership would be polled on a strike if there was no contract settlement by 11:59 last night. Pressed by members of Congress on Friday, however, he said a walkout could be delayed if "meaningful" talks were continuing and an agreement seemed possible. Shortly after midnight, a PATCO spokesperson said the strike vote was preceeding.

At the dinner break, neither side would predict the outcome of the contract talks but both emphasized that the discussions were continuing. "Me being here means that there's progress," Poli said.

"A number of things are on the table," Lewis said. "But it all depends on the total packasge. . . Each piece is dependent on the other."

A spokesman for the administration said that President Reagan was monitoring the talks closely through White House chief of staff James A. Baker III, who was in regular contact with Lewis. "The fact that they are continuing to talk is a sign of encouragement; we have the strongest hope that we'll be able to avert a strike," the spokesman said.

As the talks continued, the government, the airlines and the public nevertheless were bracing for a possible walkout by the controllers at 7 a.m. today. The Federal Aviation Administration was preparing to put into effect a contingency plan that could reduce the airlines' 14,000 daily flights by 75 percent if all of APTCO's members honored a strike call.

On Saturday, the FAA beefed up security at its 23 air route traffic control centers and 500 airport towers as a precautionary measure to protect the machinery equipment the controllers use.

There have been some allegations of sabotage during the last few weeks at FAA facilities -- charges of false flight plans entered into computers and deliberate power outages. However, Poli has denied the charges and pledged his cooperation in any investingation of the allegations.

The FAA's contingency plan depends both on the number of controllers who join a walkout and the number of people who walk out of a specific center. Flights would be handled by nonstriking PATCO controllers, 2,200 non-union controllers and about 3,000 supervisors the FAA says are qualified controllers. An additional 400 military controllers could be made available if the FAA requests them.

In general, international and transcontinental flights would be relatively unaffected under the FAA's complex plan. Most airline flights under 500 miles would be grounded under the FAA's worst-case scenario because travelers going a short distance may have options such as trains or buses that may be unavailable to long-distance travelers. The plan gives priority to military, medical emergency and presidential flights.

The government also was prepared to take action against controllers who participate in a strike, which would violate federal law and a permanent injunction. Backed by the White House, Lewis has warned that the Justice Department will move quickly to seek penalties against striking controllers, including fines and imprisonment.

The airlines, whose losses could exceed $80 million a day if a strike occurs, were warning travelers who hold reservations on the normal schedules to seek new bookings on the contingency flight schedules, should a walk-out occur.

On the surface, the government and the union appear to be far apart. The union wants an additional $770 million a year in wages and benefits, and the government insists it cannot go higher that a $40 million annual package. PATCE was seeking an across-the-board increase of $10,000 in the annual pay of controllers, whose salaries now range from $20,000 to $50,000 with an average of $34,000; a reduction of their five-day, 40-hour work week to four days totaling 32 hours, and improved pension benefits and working conditions, including a voice in FAA police decisions affecting controllers.

The government's $40 million package included money the controllers would get under a scheduled pay raise for federal employes, higher pay for controllers who work as on-the-job instructors and for those who work at night; a 6 percent reduction in the workweek through a guaranteed half-hour lunch period or the equivalent in overtime pay; a change in severance pay for controllers forced to retire for medical reasons, and a provision allowing PATCO members to sit on advisory committees that make recommendations on changes in FAA controller procedures.

There appeared to be no change in the government position that it wouldn't offer any money beyong the $40 million package but wasn't opposed to restructuring it to meet the economic demands of the union. The controllers' last contract expired March 15; the government is seeking a new contract that would expire March 15, 1985.

The controllers have been under pressure to avoid a strike, particularly from members of Congress who must approve the controllers' wage and benefits package.

"These are good people," Sen. Robert J. Dole (R-Kan.) said yesterday on "Issues and Answers" (ABC, WJLA). "But, on the other hand, if they go on strike, they're in violation of the law. I would just say, as a member of Congress, that they'd lose a lot of friends in a very short time."