The machinists union went on strike against United Airlines at midnight last nigh after its members voted overwhelmingly to reject a proposed contract with the company.

The airline shut down at 12.01 a.m. today, canceling its 1,600 daily flights through April 9. Passengers en route prior to the strike deadline could proceed to their scheduled destinations, however.

The tentative settlement, rejected in voting during the last two days, was the second one turned down by the 18.611 United ground employes this month.

The rejected agreement included wage increases averaging about 31.9 percent over three years plus improved benefits. It would have boosted a top-rated mechanics's hourly wage from $10 under the old contrat to $13.14 in 1981, according to a spokesman for the International Association of Machinists and Aerospace Workers.

In a statement, the union said the rank-and-file rejeccted the proposed contract because United had not offered "an economic package that would help them keep pace with soaring prices."

The union took a swipe at Carter administration programs that it said push the workers "further into economic quicksand" by allowing prices to rise at the same time corporations are reaping "unheard of" profits.

Union leaders had recommended ratification of the contract, which was hammered out in three days of round-the-clock bargaining under the auspices of the National Mediation Board after the first tentative agreement Robert O. Harris, a board member who acted as chief mediator during the last go-round, said yesterday the mediators were ready to meet again with both sides to help end the strike but noted that neither had indicated a desire to resume immediate negotia.

As of early this morning, no talks had been scheduled.

"Shutting down the largest U.S. air carrier creates great hardship on the traveling public and we are willin to work around-the-clock in an effort to mediate a peaceful settlement of this disput," Harris said.

Although the Civil Aeronautics Board has the power to authorize other airlines to serve United's routesd to fill some of the gaps, Michael E. Levine, director of the CAB's Bureau of Pricing and Domestic Aviation, said he couldn't be very optimistic.

"With all the airlines already scrambling for airplanes to fly new routes and worried about fuel supply, I don't think it's realistic to think that any significant portion of United's route system will be replaced with other carriers," he said. "The fleets are being used now and planes are pretty full."

United operates 1,600 flights a day to 109 cities, carrying 111,000 passengers on an average day. Its operations represent between 15 and 20 percent of the domestic airline system. If the strike lasts, it could severely disrupt Easter holiday travel.

The machinists struck United for 16 days in December 1975 when their contract expired. The current contract expired last Nov. 1.

In the statement yesterday, the machinists didn't go into specifics about the rejected contract but complained generally about the economy. "For three years, since their last contract went into effect, United employes have seen their real wages go down," the union said.

"United employes, like all American workers, feel they are standing alone under the staggering burden of inflation," the union said.

The machinists union represents 50,000 employes in the airlines industry.