Gail Forsythe of Selmer, Tenn., is a single mother of two, and a teacher. She went to Capitol Hill yesterday to tell members of Congress just how tough it is for single mothers to collect child support from deadbeat fathers and just how rough it is on the children. She's seen it in her classroom and she's been through it at home.

"For years I have dealt with the children of mothers working in low-paying jobs who can't afford to leave work to care for sick children," she told the House subcommittee on public assistance and unemployment compensation. She told of single parents who couldn't afford optical and dental work for their children and of latchkey children who cared for younger siblings while their mothers worked.

She said she has seen cases in which mothers who have remarried have tolerated child abuse because they were afraid that leaving would plunge them into economic disaster.

And she testified about her own efforts to support her family and to collect child support from her former husband, "a noted professor of nuclear physics at an Ivy League university," who "swore he would never pay a dime in child support." She testified that she went to court five times between 1972 and 1979, was forced to travel out of state and pay attorneys fees in an effort to collect. One trip, to Media, Pa., cost her $1,300, and while the court ruled in her favor, including a $2,000 lump sum in arrears, her former husband did not obey the order.

Finally, with the help of a district attorney in Tennessee, she tried to obtain support through the federal Uniform Reciprocal Support Act. She testified that a court in Philadelphia, without notifying her of the hearing, reduced the support awarded months earlier by the Media, Pa., court, and allowed her former husband to pay off his arrears at $2 a week, "giving him 18 years to pay off." She testified she now receives $60 a week, "less than he was paying for psychological counseling and marriage counseling when we were living together."

Forsythe said she worked up to 85 hours a week until school was out, supplementing her full-time teaching income by teaching a night class and working nights and weekends at a department store. "I was off Christmas Day and Easter to be with my children," she said.

Forsythe is one of the 8.4 million American women raising children alone. The Census Bureau has estimated that 30 percent of those women and their children are living in poverty, and, while most should receive child support, more than half receive only partial payment or no payment at all. The Census Bureau has estimated that the children are cheated out of nearly $4 billion a year.

Margaret Heckler, secretary of Health and Human Services, called it "a national disgrace" at the hearing yesterday, and she outlined an ambitious administration proposal to use federal financial muscle to encourage states to collect child support from all delinquent parents.

Current state efforts to collect child support are erratic, she said, with only six of them doing a cost-effective job. The administration proposes to set penalties for the states that don't collect and a $200 million incentive pool to reward those that do. A significant breakthrough in the administration's proposal came during a meeting Wednesday afternoon between the Republican women in Congress, President Reagan and top administration officials. Rep. Marge Roukema (R-N.J.) argued persuasively that the incentive pool, as well as other compliance efforts such as mandatory deductions from wages, should apply equally to the state's efforts to collect support from all delinquent parents, not just those whose children end up on welfare.

Millions of mothers, like Gail Forsythe, are struggling to raise children alone. The old image of the merry divorcee who keeps the house while her husband lives in a garret no longer applies. A California study found the income level of women dropped 73 percent after divorce while the man's income rose 42 per cent. A Denver study found that two-thirds of the fathers paid less in court-ordered child support than they spent on monthly car payments.

It is clear from both the administration proposals and the tenor of the hearing that the nation, faced with alarming data about the feminization of poverty, is about to take a long, hard look at the tolerance it has shown to deadbeat parents. "We have made a sea change of difference here," said Roukema.

It's about time.