President Clinton has been daring Republicans all year to tussle with him over Social Security. House Speaker Newt Gingrich seems ready to rumble, just in time to make it an issue for the fall congressional campaigns.

Gingrich, R-Ga., has privately told colleagues that a $70 billion or $80 billion tax cut over five years is probably the biggest one that Congress could pass and the president sign this year. But publicly, he says projected budget surpluses over the next decade should be used to cut taxes by $700 billion and to strengthen Social Security with another $700 billion.Gingrich says if Clinton and Democrats continue to fight a big tax reduction, Republicans will happily use that against them this fall.

"The president will decide how big the campaign issue by deciding how big the tax cut," Gingrich told reporters on Friday. "If he accepts the entire tax cut, it will be gone as a campaign issue. If he doesn't accept the tax cuts, it will be right there on the front burner as a clean, decisive choice this fall."

But it's not going to be easy for Gingrich to get other Republicans to go along. When vacationing lawmakers return for a five-week, pre-election finale in September, the GOP will first have to resolve internal fights that have paralyzed it all year on cutting taxes and a 1999 budget.

Many Republicans - especially in the Senate - worry that Gingrich's proposal to use the surplus for a big tax cut is the political equivalent of Pickett's Charge, with GOP candidates this fall playing the role of the hapless Confederate soldiers mowed down at Gettysburg.

The reason: Clinton's call last winter to "save Social Security first" by preserving budget surpluses until Social Security's long-range fiscal health is ensured. With that one stroke, Clinton warned that he would accuse anyone looking to use the surplus this year of looting the government's most popular program - a warning he served again last Thursday.

"I don't want us to run right out and spend it before we take care of the crisis in Social Security that is looming when the baby boomers retire," Clinton said.

Gingrich says Republicans would be safe to follow his plan because it envisions using all payroll tax revenues collected for Social Security for the massive pension system. But his critics note that he would use hundreds of billions of dollars in interest that Social Security also accrues for tax cuts, opening the door to Clinton's potentially lethal political attacks.

Fearful of attacks from Clinton, Senate Republicans have pushed all year for much smaller tax cuts, perhaps $30 billion over five years. And they have been working behind the scenes on a plan for putting aside for Social Security hundreds of billions more than Gingrich has proposed.

"We are very far apart," Senate Budget Committee Chairman Pete Domenici, R-N.M., said before the Senate began its recess on July 31. "In that context, I don't know how we're going to bridge that gap."

Because of those House-Senate divisions, Congress has not approved a final federal budget for fiscal 1999, which begins Oct. 1. The budget is non-binding, and because tax and spending bills can be approved without it, congressional leaders may choose not to devote dwindling time to completing one.