The United States, in an attempt to force Third World nations closer to its position on mining vast mineral resources on the oceans' floor, has begun playing the diplomatic equivalent of "chicken" in the long-running United Nations Law of the Seas Conference.

The first phase of the high-stakes pressure play involves a new, hardline posture on the issue - one that not so subtly calls into question the absolute need for the kind of world treaty the conference has been working on intermittently for 10 years.

The thrust of the warning is that failure of the conference to adopt a seabed mining treaty at its March 28 meeting in Geneva might not be as calamitous as most delegates think.

And in case anyone should miss the point, Congress is moving along swiftly on a bill that would authorize and encourage U.S. mining companies to unilaterally begin to mine the trillions of dollars of cobalt, nickel, manganese and copper on the bottom of the oceans.

If the State Department had its way, the bill would hit President Carter's desk square in the middle of the six week Geneva conference and give ambassador Elliot Richardson a negotiator's dream come true.

Richardson could stand up in the middle of conference and say, in effect, put up or shut up.

On Wednesday the House International Relations Committee reported out the bill, and although a floor fight is certain over whether the mining companies' investments will be guaranteed by the government, full House approval is expected before the conference convenes.

Whether Senate approval can be timed to coincide with the Geneva meeting is less certain, but even the consideration of the measure is viewed by administration officials as valuable leverage for Richardson and the other U.S. negotiators.

One State Department official acknowledged the value of the bill's timing, but added, "I wouldn't want the administration quoted as saying we want to use it as leverage."

A congressional source involved with legislation said, "What greater leverage could there be than for Richardson to announce near the end of the session that the bill is on the President's desk and he has 10 days to sign it?"

Another source close to the Law of the Seas Conference said Richardson "is obviously playing a little hard to get. The bill would significantly strengthen his hand."

At the core of the negotiating strategy is a proposed seabed mining treaty that was secretly rewritten by a handful of Third World delegates at a session last year and is regarded as unacceptable to the United States and some other developed nations.

Among other things, the proposed pact would force U.S mining companies to transfer privately developed technology to an international seabed "authority" in return for mining rights. Moreover, the authority could control deepsea scientific research and could give the monetary benefits of ocean mining to countries that did not accept the treaty.

For a decade - largely out of view of an uninterested public - the Law of the Seas Conference has been engaged in a global tug-of-war on the issue of deep ocean mining, trying to reach agreement on a treaty whose foundation is based on the principle that all resources under the sea are the common heritage of mankind.

Developing nations, promoting a new world economic order, and industrial nations, defending entrepreneurship and protecting their technological advantage, have debated one negotiating text after another while failing to reach agreement on the key issues.

The object of the deliberations is how to govern the harvesting of potato-sized nodules of minerals lying on the seabed floor at depths of up to 15,000 feet. Scientists say ther may be 1 1/2 trillion tons of high-concentration nodules, mostly in the Pacific between Hawaii and Mexico.

A number of mining consortia have invested millions of dollars in research and have committed more toward exploration, estimating they can go into commercial by 1983-84.

One consortium has a mining ship at sea and soon will begin test operations.

The harvesting systems include a vacuum cleaner-like device that sucks that nodules up to ore carriers, or a conveyor-like chain of buckets to collect the minerals.

The negotiations so far have centered on the creation of an international authority that would control exploitation and distribution of profits among nations.

Generally, the Third World has favored the creation of an operating arm, called "The Enterprise," that would harvest the minerals for all mankind, while the United States has favoured a "parallel" system under which "The Enterprise" and private mining firms would operate simultaneously.

Squabbling has repeatly broken out over such issues as production limitations, price controls and the extent of the authority's control, and the last Law of the Seas session, in New York, recessed in disarray.

Since that meeting - and especially recently - Richardson has been articulating a tough posture in which the possibility of a conference collapse and unilateral U.S. mining operations are frequently raised.

He recently rated the conference's chances of success as 1 in 3, and has repeatedly suggested obliquely that the United States has other options besides waiting for a Law of the Seas treaty.

One of these options was presented in an article in the January edition of Foreigh Affairs. The article was written by Richard G. Darman, former vice chairman of the U.S. delegation to Law of the Seas Conference but it was widely regarded as reflecting - intentionally or not - Richardson's views.

(In a footnote, Darman disclaimed any connection between his views and those of the government.)

Darman wrote that conference advocates generally regard the absence of a seabed mining treaty as leading inevitably to "the biggest smash and grab" since the 1885 Berlin Conference, in which European powers divided Africa. This notion, he suggested is "fallacious".

The proposed international "regime," Darman complained, would "impose an arbitrary governmental system of production and price controls . . . without favorable regard to what may be the economic advantages of new systems of production or the dictates of consumer demand."

The U.S. strategy, Darman wrote, should include development of a seabed "mini-treaty" outside of the United Nations. The pact, he said, would create a "licensing regime" with limited powers but would nonetheless provide some form of revenue sharing with participating nations. "The Enterprise," however, would be eliminated.

Darman conceded that "a large number of states would not be expected to participate," but he said loan guarantees to underwrite mining operations could be offered as a lure, and that seabed miners would be taxed in order to funnel money to the poorest participating nations.

"But if agreement cannot be reached on a satisfactory seabed regime the United States should hardly despair. It should wait for a more favorable world negotiating climate while continuing to mine within the framework of a satisfactory mini-treaty," he wrote.

As the rhetoric over the Law of the Seas stalemate intensified, a congressional fight appeared to be developing over the seabed mining bill - which could upset the timing of the strategy to use the bill as negotiating leverage at Geneva.

At the urging of mining company lobbyists, several members of Congress are forcefully advocating amendments that would provide investment guarantees against losses that may stem from future international treaties.

The original seabed bill proposed guarantees of $350 million or 90 percent of a mining firm's investment, whichever was less. It was deleted by two House committees, but Rep. John M. Murphy (D-N.Y.), chairman of the Merchant Marine and Fisheries Committee, is still fighting for the guarantee.

In an unusual twist, a vice president of Inco United States Inc., a member of one U.S. mining consortium, testified before the House International Relations Committee on Jan. 24, that because commercial seabed mining is still several years away, he did not think investment guarantees were necessary now.

The statement by Alfred P. Statham came as a surprise, since it followed appeals by Kennecott Copper Corp. that investment guarantees be given. Statham stressed he was not speaking for other members of the mining consortium.

Congressional sources said Murphy has begun rallying House supporters of investment guarantees, and that the issue can be expected to be debated heatedly on the House floor.