IT IS NOT surprising that members of the air traffic controllers union are voting to reject the contract negotiated with the Federal Aviation Administration last month. Their leaders had been telling them that there was plenty of pie in those control towers that the government would give them if only they would hang tough. By comparison, the pay increases contained in the contract to which the leaders agreed must seem unappetizingly small.

What happens next? The controllers, armed with what now appears to be an overwhelming vote of rejection, will go back to the bargaining table and demand more. Secretary of Transportaton Drew Lewis, if he sticks to the position he has stated many times, will say the administration is not prepared to add any more money to the pot. In that event, an impasse is likely, and the controllers will look around once again to see if they have enough support to strike successfully. That, at least, is the scenario now contemplated by some of those close to the situation.

This is high-risk labor negotiations. A strike by the controllers would be clearly illegal. It would result -- if the administration keeps its word -- in monetary fines and, eventually, the loss of jobs by the strikers. Presumably, the controllers are betting that these things won't happen and that President Reagan will interene on their behalf at some point because they supported him in last year's presidential campaign.

Our own reading of the situation is that a strike by these important government workers -- or, even, another threatened strike -- is not likely to receive much support. Congressional leaders put considerable pressure on the union's negotiators to accept that contract last month, and nothing has happened since to make them more supportive of large wage increases for one group of workers. Even if the administration agreed to a contract giving the controllers more of what they want, Congress -- which has the last word on making the pay increases effective -- might find it hard to swallow. In a year of belt-tightening throughout government, it is a little difficult to make a sound case for even the pay increases the controllers have already been offered.

There is ome room inside the dollar figure set by Secretary Lewis as the maximum for adjusting some aspects of this contract in ways that might be more appealing to the controllers. The union and its leaders would be wise to focus on those adjustments and give up their more ambitious ideas.