One of the main layers of the social safety net, the unemployment insurance system that is supposed to help the country through hard times, has fallen on hard times.

The federal-state system is in a seriously weakened condition at precisely the time when unemployment and the need for help are the greatest since World War II.

In part because of cuts President Reagan proposed and Congress made in the program last year, and in part because the recession has lasted so long, only 45 to 50 percent of Americans without jobs are receiving unemployment benefits today, as against 68 percent in the depths of the 1975 recession.

Unless Congress acts, this percentage may fall even lower next year as the long-term unemployed exhaust their benefits. Even if the recession is bottoming out, some economists think unemployment could continue rising above the current 10.4 percent of the work force and remain high for most of 1983 and into 1984.

Twenty states owe the Treasury $8.6 billion for loans to cover their leftover unemployment benefit costs from the 1970s and part of their outlays in this recession. Although Congress voted an increase this year in the unemployment insurance tax paid by employers, the system's income is still unlikely to match its obligations for some time in many states.

That will be particularly true if Congress, which reversed itself once this year and voted a temporary extension of benefits to offset last year's cuts, votes another extension after the election.

The 1981 cuts tightened eligibility standards for the extra 13 weeks of benefits available under current law to workers who have exhausted their initial 26 weeks during periods of high unemployment.

Last summer's reversal restores six to 10 weeks, but that extension expires March 31, when the 1981 cuts are scheduled to come into effect fully, throwing millions of workers out of the extended benefits program.

Unemployment compensation is one of the few large programs whose costs sometimes decline. It was set up in 1935 as part of the Social Security Act--literally part of the social safety net.

It has always been a federal-state program, with the states financing the basic 26 weeks and retaining essential control over eligibility standards and benefit levels, and the federal governmment setting broad guidelines, paying administrative costs and paying half the costs for the extra 13 weeks.

The idea was that money would build up in good years and be paid out in bad years. There have been too few good years lately, and too many bad.

The unemployment insurance spending curve looks like a roller coaster. The 1973 program cost was $4.6 billion. By 1975, recession and inflation had lifted outlays to $19.4 billion, then they had dropped back to $9.6 billion in 1978. Last year they soared again, to $22 billion for about 11 million workers, and in the current fiscal 1983 could reach $24 billion.

The system's financial problems stem in part from the fact that recent recessions have tended to be longer and with higher levels of unemployment than in the past. On top of that, in most recessions the same big industrial states are hardest hit. Because states have been extremely reluctant to tax business too heavily, few have maintained the desired 18-month reserve against contingencies.

Congress recently has taken some steps to make the states raise more money, such as forcing up the amount of wages on which the unemployment tax is based from $6,000 to $7,000 and requiring states to pay 10 percent interest on any funds borrowed from the Treasury after April 1 of this year. Previous loans were interest-free, giving the states little incentive to pay them off.

But the system would be put to disastrous financial pressure if the current recession were to stretch out much longer.

Last year's cuts in the program were made mainly to help reduce the federal deficit. But the cuts were also supported by spokesmen for business, who say benefits are overblown. Labor unions and congressional Democrats objected.

There were three major changes, all having to do with extended benefits.

Under previous law, the states, regardless of their unemployment rates, were eligible to offer the extra 13 weeks of benefits at any time when "insured unemployment" nationwide was at least 4.5 percent. Insured unemployment is a special computation, measuring the ratio of people receiving benefits at any time to the total number of people who are working and covered by the system.

The insured unemployment rate is usually about half the regular rate. Thus, insured unemployment for the latest week reported was 5.4 percent nationally, as against total unemployment of 10.4 percent.

Under the old law, even if the national insured rate was below 4.5 percent, a state was eligible for the extended benefit program whenever its insured unemployment rate averaged at least 4 percent for a 13-week period and was 20 percent higher than for the two preceding years, or, alternatively, averaged 5 percent for 13 weeks.

One Reagan administration change was to repeal the 4.5 percent national trigger. This had the effect of excluding any state that did not qualify on the basis of its own rate.

A second Reagan change, which went into effect Oct. 23, was to raise the 4 percent and 5 percent state trigger rates to 5 and 6 percent.

The third change was to alter the way insured unemployment was computed, counting in the calculation only those drawing the first 26 weeks of benefits, and excluding those drawing extended benefits. This reduced the insured rate.

These changes combined to reduce enormously the number of workers eligible for extended benefits.

Under the old law, the national trigger would be operative today and every state would be eligible for the extended benefits program, but under the 1981 law only 24 states were eligible for extended benefits at the start of this month.

On Oct. 23, when the change in state triggers went into effect, nine of those 24 dropped out. Others will drop out in the future.

According to the most recent Labor Department estimates, federal-state outlays for extended benefits would have been $4.9 billion in fiscal 1983 under the old law, but would be cut to $1.2 billion under the new law. In fiscal 1984 they would have been $3.3 billion but the changes cut that to $302 million, with corresponding reductions in 1985.

Put another way, 3.3 million people who would have been eligible for the 13-week extended benefits in fiscal 1983 will not be eligible. Another 2.6 million will be excluded in fiscal 1984 and about 600,000 in fiscal 1985. The Congressional Budget Office says it does not think the 1981 cuts will shut out quite that many people, but its estimates are substantial, too.

These cuts still look good to some business groups. Chamber of Commerce spokesmen call some of the eligibility reductions "important" and "long overdue reforms," helping to reduce abuse of the program by seasonally unemployed and people only marginally attached to the labor force.

They add that they support unemployment insurance as a bridge for workers between jobs but would oppose anything that tends to turn it into "a program for the long-term welfare of the hard-core unemployed."

However, the 1981 legislation began to look less good to many in Congress this year as unemployment mounted, workers began exhausting their benefits and Election Day approached. So in the tax bill passed last summer, Congress enacted with Reagan administration agreement a special program that will be totally financed from the federal Treasury, will last six months only--from September of this year to March 31--and will provide from six to 10 weeks of extra benefits in every state, depending on their unemployment rates.

None of the 1981 restrictions was reversed. Instead, Congress just added six to 10 weeks to what any individual would otherwise have been entitled after the 1981 changes went into effect.

Thus, if a state had high unemployment and remained eligible for extended benefits under the 1981 law, an individual conceivably couldget 26 weeks of initial state benefits, 13 weeks of federal-state extended benefits and another 10 weeks of "federal supplementary compensation."

But in a state with low insured unemployment, the worker would get only the 26 initial weeks plus six (or perhaps eight) weeks of federal supplementary aid, losing the 13 weeks of federal-state extended benefits.

Until it expires March 31, this program is expected to cost about $2.2 billion and help about two million workers, thereby temporarily restoring part but not all of the reductions made by the 1981 law.

The decision for Congress, as March approaches, will be whether to extend the temporary program and, beyond that, how to strengthen the general financial condition of the system without simultaneously further reducing its usefulness as part of the social safety net.