Bargainers for United Airlines and the machinists union will meet with federal mediators today in an attempt to settle a contract dispute and avert a strike against the nation's largest air carrier.

The machinists agreed to call off a strike threatened for midnight yesterday and meet with United at the request of Labor Secretary Ray Marshall.

The machinists had set a strike deadline after the union reported that a tentative settlement reached with United last month had been "overwhelmingly" rejected by 18,000 mechanics, ground-crew employes and other workers the union represents.

Union spokesman Larry Rubin said yesterday that union negotiators were prepared to bargain round the clock if necessary.Asked if a new strike deadline would be set, he said, "We're taking it a day at a time."

The airline has acknowledged that a machinists' strike would force systemwide shutdown.United operates 1,600 flights between 109 U.S. cities a day, carrying an average 111,000 passengers daily.

The rejected settlement included wage increases averaging about 30 percent over three years, plus cost-of-living adjustments. Patterned after a contract signed last fall between the union and Trans World Airlines, the settlement would have boosted the average mechanic's hourly wages to $12.79 in 1981.

Rubin said members apparently felt the tentative contract was inadequate because inflation threatens to exced 10 percent this year.

The two sides are meeing with assistance from the National Mediation Board, a federal agency that handles labor disputes in the airline and railroad industries.