President Carter threatened bluntly yesterday to take "drastic action" such as seeking a back-to-work order under the Taft-Hartley Act if necessary to end the nation's intractable 75-day coal strike.

Face with yet another apparent stalemate in negotiations between the striking United Mine Workers Union and the coal industry, Carter also said he may ask Secretary of Labor Ray Marshall to prepare "a proposal of his own" to submit to the two sides.

Carter's comments to a group of high school students during a New England campaign swing came shortly after the UMW's bargaining council unanimously rejected a revised contract offer from the Bituminous Coal Operators Association, the industry's bargaining group.

After two days of virtual round the clock bargaining under White House pressure, the BCOA offered Friday to modify proposed penalties fro wildcat strikes and to partially restore cost-of-living increases. But it dropped its earlier offe to guarantee health and pension benefits and refused other concessions demanded by the union.

When the 39-member council of the union's top officers rejected the offer and refused even to pass it along without recommendation to the 160,000 strikers. Marshall wearily told reporters at about 4 a.m. yesterday that chances of reaching a negotiated settlement appeared dim.

But about four hours later Carter, briefed by Marshall by telephone on the latest negotiating failure, said talks would continue and indicated he would not immediately ask Congress to authorize the government to seize and operate the mines - an option that White House aides were floating Friday night in an apparent attempt to intensify pressure for a settlement.

"We haven't given up yet," Carter told reporters in Nashua, N.H.

After returning to Washington, Carter met late yesterday with Marshall and other top officials on what the government - increasingly ensnarled in what many officials view as a no-win situation - will do now to prevent the strike from forcing more electric power cutbacks and job layoffs in coal-dependent midwestern and mid-Atlantic states.

The administration has been reluctant to invoke Taft-Hartley, under which a president can ask the courts for an injunction forcing strikers back to work for an 80-day cooling-off period. Miners have ignored such injunctions in the past and, in their currently rebellious mood, are considered likely to do so again. Other options mentioned by the administration - seizing the mines or submitting the contract dispute to binding third-party arbitration - would require congressional action and thus invite delay and possible defeat.

Carter has said he would consider Taft-Hartley as an option but his remarks today went further.

Asked by a student at Nashua Senior High School if and when he would seek a back to-work injunction. Carter said it is "very doubtful" tht the federal courts could force miners to go back to work. he noted the possibility that such an order would trigger violence. But he said, "If I have to I will take such drastic action."

He doesn't "want to face that prospect," Carter said, because any such drastic action would be "a major loss . . . for our country" with srious repercussions for the UMW, which, he said is "already on shaky grounds" because of internal divisions.

However, he said, "I cannot as president permit the country to suffer from a delay in the negotiations."

While pressing for negotiations to continue he said he could not predict success, and he described prospects of a negotiated settlement as "not particularly encouraging."

During the day Marshall was in contact with both sides but met only with union negotiators at the Labor Department. It appeared to be a holding action until the government could determine its next step.

Starting with all-day talks Thursday and a secret pre-dawn White House meeting Friday between Marshall and two top industry officials, the government made its big push for a settlement - a "make-or-break" effort, one aide called it.

Industry was encouraging speculation that a settlement might be imminent, and at one point union sources indicated there might be a close affirmative vote from the bargaining council despite its 30-to-6 rejection of a tentative settlement last Sunday.

As Marshall shuttled between the union's bargainers at the Labor Department and the industry command post in the Roosevelt Room of the White House, a package of industry-proposed trade-offs began to emerge, piece by piece. But by the time it took final form, the union's bargaining council gagged again.

The industry let it be known early that it was willing to abandon its controversial demand that miners reimburse benefit funds for losses during wildcat strikes, while continuing to insist on other disciplinary measures such as dismissal of strike instigators.

But later another message came through: if reimbursement was dropped, industry would also withdraw its previous offer to guarantee health and pension benefit payments, which are not now guaranteed and have been suspended since the strike began.

"At that point we just stopped talking . . . that was the crowning blow," said Joseph Phipps, a Tennessee-Kentucky representative on the bargaining council who, along with others, charged industy representatives with fudging on their earlier concessions.

"I don't think they've improved anything from the word 'go'," Phipps told reporters after the council vote.

He said the union was holding out for considerably more, including full equalization of benefits (which are now considerably higher for future retirees than for the union's existing 80,000 pensioners), full as opposed to partial restoration of cost-of-living increases, and a return to totallyfree medical care. The rejected settlement called for medical deductibles, and industry has refused to reconsider that.

After the UMN bargaining council rajected the industry's co-called "final offer, 37 to 0, and voted 26 to 11 against submitting it to the union's members without recommendation, Marshall held a 4:10 a.m. press conference in which he held out little hope for a settlement through negotiations.

"As of where I am right now," said Marshall, "it tooks like it's not going to be possible for us to have a negotiated settlement." But, he added, "There are other ways of settling short of seizure and short of Taft-Hartley."

Marshall said the industry's shifting position regarding contract trade-offs had created a "very different situation." Said Marshall: "We'd get what I consider to be a geneal understanding and then later, when they BCOA negotiators discussed it with all of their principels, they were not always able to deliver everything they had implied in our earlier conversation."

Industry, in turn, was none too happy with Marshall's suggestion that chances for a negotiated settlement were fading. "That was unfortunate," said an indsutry course.

The BCOA called rejection of the contract "incredible," accusing the union of inteocuing a "shopping list of items that were settled weeks ago."

Marshall said th shuttle negotiations were able to "improved considerably on the contract that had been rejected earlier," including reducing from 26 to 10 the number of contract items that the union was protesting. But he said the last two days of negotiations were able to deal with only four of the items.

"I think it is simply safe to say that that was not enough," said Marshall. He described the process as "trying to bridge the gap of faith" between union and management and trying to accommodate stubbornly conflicting interests.