A landmark Senate bill aimed at turning the nation's Depression-era welfare system into a jobs program heads for negotiations with the House with a threatened veto still hovering over it.

The Senate voted 93-3 Thursday to pass the $2.8 billion Family Security Act after bowing to nearly all of President Reagan's requests, including adoption of a mandatory workfare provision for a small percentage of welfare recipients.Sen. Daniel Patrick Moynihan, D-N.Y., author of the bill, called it "the first legislation of its kind since the welfare system was established in the 1930s."

He described the current welfare system as "an income maintenance program with a mild jobs component" and said, "We are reversing that completely."

The House last December passed a $7 billion welfare overhaul bill. Moynihan and Rep. Tom Downey, D-N.Y., sponsor of the House bill, refused to speculate on whether negotiations to reconcile the two bills would produce a plan acceptable to the White House.

The centerpiece of both bills is a massive new jobs, education and training program aimed particularly at people who are long-term - or likely to become long-term - welfare recipients. The legislation also contains stringent new child-support collection requirements meant to assure that non-custodial parents pay their fair share.

The House would also offer financial incentives to states to raise their cash benefits to poor families, which in many states are less than half the federal poverty line.

Moynihan, throughout an 18-month campaign for his bill, decried the generally low level of welfare benefits but did not include a provision to raise them.

"We can never address the issue of benefits at the national level until we've persuaded the American public that this is a work program, a child-support program," he said after the vote. "We have started in that direction today."

Aid to Families with Dependent Children was established in 1935 as part of the Social Security system and now supports 3.7 million families. Some 7 million children are on AFDC, which would be renamed the Child Support Supplement System under Moynihan's bill.

Originally conceived as a widow's benefit when women did not work outside the home, AFDC now is dominated by divorced and unwed mothers at a time when the majority of new mothers are at work within a year of having a child.