As Congress struggles to finish work on the federal budget for the fiscal year that began Oct. 1, a fierce political battle is raging once again over Social Security. Eager to maintain control of the House, congressional Republicans are running television ads accusing Democrats of "raiding" the Social Security trust fund to finance extra spending. Democrats say the charge is a lie and contend GOP leaders are doing exactly what they accuse the Democrats of doing.

What follows is a primer on the Social Security debate on Capitol Hill:

Q. What is the Social Security trust fund and how is it used?

A. The 60-year-old Social Security program, the bedrock of retirement security for Americans, is a pay-as-you-go operation that uses payroll tax revenues to provide retirement and disability benefits. Like a bank that books deposits and lends out the cash, Social Security for years has been lending its huge surplus cash reserves to the Treasury to help cover day-to-day government costs.

Q. Is there a problem with Social Security?

A. Once the baby boomers begin to retire, people live longer and the work force that supports the system shrinks, the long-term future of Social Security will be imperiled. The program's trustees forecast that without fundamental changes in the system, the program will be unable to cover all of its expenses starting in 2034. When Social Security needs the money, the government will have to make good--by drawing on non-Social Security surpluses, cutting other programs, raising taxes or running deficits again--or cut down on benefits.

Q. Is that the problem being dealt with by President Clinton or the GOP-controlled Congress?

A. No. Both the White House and various congressional Republicans have put forth proposals for how to shore up the system, but none of them have gone anywhere this year.

Q. So what's the fuss over the budget?

A. To blunt Clinton's use of Social Security as a campaign issue, Republicans have promised to finance the 13 annual spending bills without dipping into surplus revenues generated by Social Security. By so doing, they say, they will be "protecting" Social Security from the Democrats. Clinton has also promised to keep his hands off Social Security.

Q. Is it risky for the government to be using Social Security revenues for other purposes?

A. Most experts say no. The lending practice has no effect on the benefits retirees receive. In effect, interest-bearing IOUs are building up in the trust fund that are backed by the full faith and credit of the government. Since 1983, in fact, both parties have spent more than $600 billion in Social Security surpluses on other government programs.

Q. Why have politicians waited until now to volunteer to keep out of Social Security?

A. It simply wasn't politically feasible until now. For decades, Congress and the White House used the huge surpluses in Social Security to mask the true extent of the budget deficit. As the economy improved and the Social Security trust fund ballooned, the size of the overall deficit began to shrink until 1998, when it evaporated.

Q. What do the Republicans need to do to meet their pledge?

A. Of the overall $161 billion surplus forecast for the current fiscal year, $147 billion comes from Social Security. So to do what they promise, Republicans must use only $14 billion of the surplus to finance new programs. That means that all 13 spending bills, covering everything but the giant entitlement programs like Medicare and Medicaid, must not exceed $594 billion.

Q. So are they going to meet their pledge?

A. It depends on whom you ask. Both Clinton and the Republicans want to spend more than $594 billion this fiscal year; in fact, the 12 spending bills already approved--plus the one remaining measure for labor and health programs--would use roughly $614 billion, according to the Congressional Budget Office, the nonpartisan budget analysis arm of Congress.

Q. So aren't the Republicans violating their pledge?

A. GOP leaders say no. The way they add up the numbers understates the amount of actual spending through the use of several creative accounting tactics. For instance, Republicans achieved one of their biggest savings--$19.3 billion--by selectively choosing between CBO and administration assumptions to come up with the lowest possible estimate of how much money would be spent in fiscal 2000. In budget parlance, this is known as "directed scoring."

Even with such scoring, the Republicans still were over their limits by $5.6 billion. Unable to agree among themselves on additional specific spending cuts, the Republicans settled on a 1 percent cut in virtually every government agency and program.

Q. House Republicans declare they achieved their goal, but the Democrats dispute this. So who is right?

A. According to the CBO, the Republican spending bills are $17.1 billion into Social Security money. That's principally because the CBO disagrees with the Republicans' use of directed scoring.

Traditionally, Republican leaders have insisted on using CBO forecasts in writing their budgets and literally vowed to shut down the government in 1995 unless President Clinton accepted the CBO estimates. But trapped in a budget box of their own making this year, Republicans concluded they had no alternative but to pick and choose among CBO and administration spending forecasts to produce the most advantageous mix.

House Speaker J. Dennis Hastert (R-Ill.) has circulated a letter from CBO Director Dan L. Crippen acknowledging that, if the directed scoring tactic were allowed, the Republican plan indeed would not dip into Social Security. But Crippen and other CBO officials dispute the use of that tactic.

Q. How does Clinton propose fulfilling his pledge to keep out of Social Security?

A. He has proposed some of the same accounting tactics as the GOP, though not to the same extent. He has also proposed raising tobacco taxes and user fees to make up the difference--an approach rejected by Congress.