House Democratic leaders yesterday abruptly postponed today's scheduled floor debate on the Democratic-sponsored bill to overhaul the welfare system.

The decision raised speculation the bill is in serious and perhaps fatal trouble because of Republican and southern Democratic opposition.

But Rep. Thomas J. Downey (D-N.Y.), who is expected to handle most of the floor-managing chores, said the decision to put off debate until the week of Dec. 7 does not indicate it lacks the votes to pass.

"We know we have the votes to pass it. We have the votes for the rule, too. We just decided we wanted to do it after the budget conference, since the leadership is preoccupied with Central America and the budget negotiations," Downey said.

According to several sources, some Democrats who might be willing to vote for the bill at a later time feel reluctant to add spending now when the outcome of the budget-cut conference is uncertain.

The bill, which would cost $5.5 billion over five years, would require welfare parents with small children to participate in job training, education and job search programs with a view toward getting off the welfare rolls.

Republicans have charged that the bill is loaded with benefit increases. At a House Rules Committee meeting Tuesday where the bill was cleared for floor debate, Rep. Hank Brown (R-Colo.) said the House should not consider such a costly bill at a time when White House and congressional negotiators are straining to cut a few billion dollars from the budget to reduce the deficit and reassure financial markets.

"It's a multibillion-dollar piece of legislation," said an aide to GOP Leader Robert H. Michel (R-Ill.) yesterday. Michel, Brown and others are sponsoring a work and training bill that costs $1.1 billion over five years, without any benefit improvements.

The Democratic welfare bill was included in the omnibus budget reconciliation bill but was removed on the floor after opponents insisted it be considered separately.