The coal miners' rejection of the new industry contract this weekend leaves the nation's economy on the brink of a serious setback, with the prospect of fast-mounting production cutbacks --and massive layoffs -- if the government doesn't get coal moving again.

Until now, the economic impact of the coal strike has been confined mainly to the coal industry. Most of the recent declines in economic indicators have stemmed from the winter's snow and bad weather. Labor Department estimates show only 20,000 jobs outside the coal industry have been affected by the three-month-old United Mine Workers union strike.

But now, economists say, coal supplies have dwindled to the point that any further delay in resuming coal production will have a quick and direct effect on the overall economy, with layoffs increasing dramatically with each passing week.

Otto Eckstein, head of Data Resources Inc., an economic consulting firm, says the economy has reached the point where, if the strike continues, it could result in as many as 750,000 layoffs in the next two weeks. Two to four weeks later, he says, that number could reach 2 million.

And Charles L. Schultze, chairman of President Carter's Council of Economic Advisers, told a television audience yesterday the administration's forecasts show the layoff figures could reach 3 million by "the first or second week in April."

The key question is how many of the striking miners the government can get to return to work -- either by invoking the Taft-Hartley Act or, as an alternative, by temporarily seizing the mines. If the miners continued to stay out in force, most other measures would be of little use.

Schultze said yesterday that not all the miners would have to return to work for the economy to get by. Alan Greenspan, a chief economist in the Ford administration, estimates that if between one-third and one-half of the 160,000 striking miners agreed to go back, the economy could muddle through.

James R. Schlesinger, the secretary of energy, affirmed yesterday the administration plans to continue banking heavily on production by nonunion miners, particularly in the western coalfields. But, he said, the administration did not yet plan to resort to a mandatory power allocation program among the states.

However, there also are other factors, including the still-uncertain question of how much violence there would be fit he government tried to keep coal supplies moving on its own. Threats of bodily harm could crimp efforts to enlist the help of nonunion miners and truckers.

The fact that the impact of the coal strike now is likely to be so sudden stems in part from the relative confidence that prevailed before this past weekend's voting. While industry was apprehensive, few firms made serious efforts to resure consumption. Coal stockpiles dwindled.

Now, with supplies at rock bottom in analysts figure most firms are likely to try to squirrel away what fuel and electric power is available -- intensifying the shortage that would have occurred anyway.

If the coal stoppage were to continue, the impact would be felt first in the big North Central industrial states, such as Ohio, where there have already been some cutbacks. Some estimates forecast an immediate 30 percent cutback in industrial electric power usage, resulting in a 15 percent drop in jobs.

But within a few days, the cutbacks would begin to spread to other sections of the country, where assembly plants would run out of parts and materials that are made in the North Central area. The steel industry, centered in Pittsburgh, Pa., and Gary, Ind., would be especially hard hit.

Eckstein estimates that if the coal shutdown extends through early April, it would halt the economy's growth for the first quarter; if the stoppage were allowed to go on longer, it would plunge the nation into a recession. (However, few analysts, including Eckstein, believe that will happen.)

Analysts stress that even if there is significant damage, once the stoppage has ended the economy will be able to "snap back" and make up the lost production, as it traditionally does after auto strikes and cold-weather bouts. Nevertheless, the hardship would be substantial.

A continued stoppage also could have adverse implications for the dollar. Economists say that if the coal strike continues, industry will have to make up for the fuel shortage by importing more oil -- worsening the nation's already large foreign-trade deficit.

To be sure, not everyone is quite so pessimistic. Greenspan, for example, argues that the economy is a good deal more "flexible" than the administration's estimates would imply, and that the actual impact of a continued strike may be somewhat less than is being forecast.

As evidence, he cites the economy's performance during last winter's cold spell. "When the cold weather first hit, there were widespread predictions of massive layoffs," he pointed out. "But people learned to cope with the situation."

How well the economy could adjust to a prolonged coal shortage remains to be seen. But economists seem to agree that time effectively has run out. From now on, a continued coal stoppage is likely to have a visible impact on the economy -- one that would spread quickly in coming weeks.