The Reagan administration plans to ask Congress for a $100 million increase in civil defense spending for 1983, to reinvigorate a program that has languished since the decline of Cold War tensions and the advent of detente.

Sources familiar with the plan say most of the increase would be spent initially on accelerated planning for the mass evacuation of major American cities and areas near likely military targets.

This strategy of "crisis relocation" dates back in some form to the 1950s and was endorsed by the Carter administration in 1978, but until now the Federal Emergency Management Agency (FEMA) received only small budget increases for the evacuation program, which is expected to cost at least $2.5 billion over several years.

Another likely element, sources say, is money to study how to identify and ensure protection of "key" workers in "essential" industries, people who refine petroleum and fix military aircraft, people bringing food to evacuation sites and people guarding evacuated cities. Protection might mean fallout shelters or more expensive "blast" shelters designed to withstand the shock wave and heat generated by a nuclear explosion.

"It's got to be something more than a do-it-yourself, head-for-the-hills program," one Defense Department official said.

The aim of the program is to protect 80 percent of the population--180 million people, most in designated "high-risk" areas--major cities or military installations. One key assumption underlies it: that the United States would have two or three days to evacuate its high-risk areas during a period of escalating international tensions, or after an evacuation of Soviet cities began.

Since 1973 FEMA has worked with state and local officials, some of whose salaries are paid in part by FEMA, on plans for relocating 145 million people in high-risk areas to "host areas" 60 to 100 miles away.

FEMA, which inherited civil defense responsibilities from the Defense Civil Preparedness Agency in 1979, would have a 1983 budget of between $230 million and $240 million if Congress approves President Reagan's request, according to those familiar with the proposal.

Building shelters for essential workers could push costs up in later years, but one Office of Management and Budget official said there are too many variables to permit reliable long-term estimates.

With the new commitment, FEMA is likely to be caught in the middle of a renewed debate over civil defense's effectiveness. Already, both sides of the debate are offering variations on the themes they have sounded for 25 years while civil defense spending soared and dipped, according to the mood of the time.

Those favoring more civil defense point to the Soviet Union, saying that intelligence estimates indicate that the Soviets spend $2 billion annually on civil defense and have developed a sophisticated program to protect 110,000 key government officials in hardened blast shelters.

Another 12 to 24 percent of the work force could be protected in shelters at "key economic installations," and evacuation plans could lead to the survival of tens of millions more Soviet citizens, according to a 1978 CIA report.

"If our people are sitting here as a tempting target while their people are protected, it might make a nuclear war more likely," said Russell Clenahan, a civil defense expert and FEMA spokesman. "It creates a different sort of 'window of vulnerability.' "

Leon Goure, a long-time observer of the Soviet program, said that as recently as Jan. 19 the Soviets ran a front-page editorial in the Soviet Army newspaper "Red Star" urging a greater civil defense effort.

"The Soviets regard civil defense as an essential element of their national defense posture . . . . The United States' lack of it undermines the credibility of deterrence," Goure said.

But for every study cited by civil defense advocates its critics have one of their own: studies by the Arms Control and Disarmament Agency, the congressional Office of Technology Assessment and private experts.

"These are third-rate people reinventing wheels that were well considered and discarded 20 years ago," said Jeremy Stone of the Federation of American Scientists. He drew up a mass evacuation plan for the Defense Department in 1962, then recommended that it not be followed.

"Even if you get people out and they survive, if the cities and the economy are gone there will be mass starvation and epidemics . . . . What this signals is a greater American interest in limited nuclear war," Stone said.

Stone and others at organizations such as the Center for Defense Information point to the same 1978 CIA study of Soviet civil defense. That study, they say, shows the limits of Soviet capabilities.

"The Soviets almost certainly believe their present civil defenses . . . would enhance the U.S.S.R.'s chances for survival following a nuclear exchange," the CIA study said. "They cannot have confidence, however, in the degree of protection their civil defenses would afford them, given the many uncertainties attendant to a nuclear exchange. We do not believe that the Soviets' present civil defenses would embolden them deliberately to expose the U.S.S.R. to a higher risk of nuclear attack."

Reagan has long been a civil defense advocate. In his Oct. 2 news conference he listed it as an essential element in the U.S. strategic posture. In July, a member of the White House congressional liaison staff asked Rep. Donald J. Mitchell (R-N.Y.) to withdraw legislation to increase civil defense spending, since Mithcell sought less than the White House was likely to.

"There are a lot of unknowns associated with this," one Defense Department official said. Among them, he said, is the degree of cooperation that would come from states and 3,300 local civil defense directors who would bear the brunt of getting people out and preparing rural host areas to receive them.

"There's a lot of variety of opinion on this on a local level," he added. Last March, the Cambridge, Mass., City Council told its civil defense director not to participate in such planning.

"This whole thing can seem bizarre," he said. "All of the solutions in the situation of nuclear war are unsatisfactory. This seems to be the least unsatisfactory."