In the early 1970s, Paul H. DeMange was president of Local 204 of the Professional Air Traffic Controllers Organization when the union was struggling for government recognition.

Yesterday, he crossed the picket lines to work the afternoon shift -- defying the strike by his fellow controllers -- at the Washington Air Route Traffic Control Center in Leesburg.

In job action parlance, Paul DeMange has become a scab. What happened?

The government happened.

After not reporting for work in a 1970 sickout, DeMange was suspended without pay by the government. He brought suit in federal district court against the Federal Aviation Administration to win money he thought the government owed him. The suit went on for a decade, and, in February, DeMange finally won $1,954 when a federal judge ruled that he really was sick during the job action, and that the FAA suspension was "arbitrary and capricious."

But now DeMange, 43, has just a year and a half left to go before he will have worked 20 years as a controller -- and thus become eligible in 1988 for retirement benefits. In his heart, in his gut, DeMange is a union man. But he says he cannot afford to take the chance of being singled out again by the government and possibly fired, as President Reagan has threatened.

He says that like most of the controllers who have chosen to work at the Leesburg Center during the PATCO strike, he is sympathetic with the strikers.

"I want to be there on the picket line myself," he said quietly. "And that's the case with more than half of the controllers who are working now at Leesburg -- they're in sympathy with the strikers if for no other reason than they would benefit from the risk [the strikers] are taking."

DeMange said that few of the working controllers have any philosophical objection to the strike, or oppose it because they believe no government workers should strike.

"They're scared," he said.

Over coffee in the living room of his Fairfax house yesterday, DeMange talked about the strike quietly, speaking deliberately of his personal anguish in working at cross-purposes with the union he once led.

"I'm hurting the union -- I know that -- by doing my job as a controller. And I'm still a member of the union. But the possibility of more legal action, that's more anxiety than I wish to deal with. I just don't want to face that. The government can just wear you down."

DeMange's voice dropped.

"By working, I feel inadequate," he said. "Because in the '70s, my wife was out there on the picket line [during the sickout], and I was angry at the scabs then because they didn't have the courage to stand up and be counted.

"But I just can't get on that picket line now. I have -- and many of the controllers working now have -- used my mind and my body for 18 1/2 years expediting air traffic in my sector, through sometimes horrendous conditions. And to risk all that and to risk my pension as a payoff is more than I'm willing to jeopardize. I'm just not willing to make that effort [and go out on strike] for the possible loss of my job."

DeMange said he will collect between $1,200 and $1,400 a month in pension starting in 1988 -- money he will need to make house payments and meet other expenses.

"It is selfish decision," he acknowledged. "But I didn't think I had any real choice. If I was a striker I'd be very vulnerable."