Striking coal miners were voting by a "substantial margin" to approve a new three-year contract and their record 109-day walkout, United Mine Workers president Arnold Miller announced late yesterday.

With more than half the union's locals reporting on yesterday's coal-field balloting, Miller said it appeared that most of the 160,000 striking miners - who overwhelmingly rejected an earlier contract offer three weeks ago - would be back at work on Monday.

Ratification of the contract, which includes a labor cost increase of about 39 per cent over three years, would end the nation's longest coal strike well before it seriously affected the country's economy or overall energy supplies.

Miller - a smile breaking his usual dour countenance - declined to claim victory for the cotract in a brief appearance before reporters midway through the vote count but said:

"It looks to me like the trend is that it will be ratified by a substantial margin."

Unofficial tallies by news services indicated the contract was winning approval easily. An Associated Press survey counted 24,832 votes for the contract and 17,913 votes against it - a 58-to-42 percent majority based on returns from 287 of 776 UMW locals.

A vote of more than 2 to 1 had rejected the earlier draft, which included controversial wildcat strike penalties and health benefit retrenchments that were eliminated or modified in the latest round of negotiations between the UMW and the Bituminous Coal Operators Association.

The industry had made what it considered to be major concessions in these areas to aovid mounting losses and a threatened breakup of industry-wide negotiations.

While many miners complained bitterly that the contract offer still fell short of what they wanted, reports from the coalfields indicated that financial hardships were becoming acute for many miners' families - creating a more favorable climate for ratification.

"They're disturbed about the contract," said Val Scarton, president of UMW District 2 in Pennsylvania. "But they've been out an awful long time, and you figure you'll get another crack in three years."

The contract covers roughly half the nation's coal production, involving about 300 companies and 160,000 miners. Still to be negotiated is a contract between the UMW and the Association of Bituminous Contractors covering 14,000 construction workers. A delay on this contract could keep some mines out of production, as it did after a shorter strike three years.

Yesterday's vote followed rejection of an initial package by the union's bargaining council Feb. 12 and repudiation of a second negotiated settlement by rank-and-file miners over the March 4-5 weekend.

The White House, which leaned heavily on the coal operators to win their consent to the second pact, moved immediately after the rejection vote to invoke strike-ending machinery of the Taft-hartley Act.

Contending that a serious national emergency was threatened by continuation of the walkout, the administration succeeded in convincing U.S. District Judge Aubrey E. Robinson Jr. to issue a temporary back-to-work order on March 9.

But the order was almost universally ignored by UMW miners, although a number ofnonunion mines went back into production, and the administration held back on enforcing the order in the coalfields while negotiations resumed in Washington.

Last week, after the new proposal was hammered out by union and industry bargaining teams, Robinson refused to grant an extension of the temporary order, complaining, among other things, that the government had exaggerated the nature of the coal shortage emergency.

Government statistics appeared to bear him out. The White House reported Thursday that utilities nationally were getting 77 percent of their normal coal supplies, with the heavily coal-dependent east-central region falling only marginally behind, at 72 percent. Job layoffs in east-central states had tapered off at about 22,000, far fewer than predicted.

The latest contract package was worked out without government assistance. Actually, the threat of a total breakdown of industrywide bargaining, inspired in part by governmnet efforts to seek regional or company-by-company settlements, brought the two sides back together on March 10, the day after the temporary Taft-Hartley order was issued.

In four days, they had an agreement that included some major industry concessions, principally abandonment of all proposed controls on wildcat strikes and absenteeism and reduction of new health care deductibles from a maximum of $700 a year to $200 a year. Pensions for older retirees were also increased somewhat.

Wages would rise from an hourly average of $7.80 to $10.20 over the three-year life of the contract. But the benefit changes increased the total cost of the package from roughly 37 percent to 39 percent.

This would be the largest overall increase for any major union since the last UMW contract in 1974, which contained an increase of more than 50 percent in wages and benefits.

However, there was still opposition to the contract in the coalfields, although less vociferous than before. Criticism centered on retention of the industrys demand for company takeover of the union's trustee run health care system, on the deductibles and, on unequal pension benefits for older and younger retirees.