President Carter has embarked on the highest-risk venture of his presidency by tightening the screws on Iran as hard as he can and announcing that the probable next step is United States military action.

The developments and rhetoric of the last two weeks and especially of the last several days, if carried to their logical and seemingly inevitable conclusion, would bring American warships and planes into action to shut off Iranian commerce by mines or blockade in mid-summer, if not before.

It would be the first such use of American combat military power in the Carter presidency. If undertaken in the circumstances that are now foreseeable, it is likely to be the most hazardous use of American power in many years, the first steps on a road whose end can not be foreseen.

The president's associates acknowledged in recent days that the risks involved in taking military action are very high. They said, however, that the costs of failing to act, while the holding of the hostages continues, are also high in international as well as domestic terms. "Doing something may not get the hostages out, but doing nothing will not do so," a White House official said.

The main line of argument in favor of military steps, as put forward to the press last Friday, is that Carter believes "no other course" is open following the failure of major efforts at negotiated settlement. At the same time, senior officials acknowledged that if and when the safe release of the hostages is obtained, the mechanism will be a diplomatic one similar to that which was tried unsuccessfully before.

The question still under debate within the administration -- though Carter's public statements and White House intimations suggest the issue is all but closed -- is whether moving to military action will advance the chances for release of the hostages, or will retard if not destroy those chances.

There are cautionary notes from severl sectors of the administration that the military decision has not been taken despite Carter's repeated references to military action and his press conference statement that, if present U.S. and expected allied sanctions fail, "the only next step available" would seem to be military action.

There is little optimism, however, that the existing U.S. economic and political sanctions, augmented by expected support from Europe and Japan in coming days, will end Iran's political paralysis on the hostage question. Some high officials have spoken of a "shock effect" that might break the Iranian stalemate, but there is no sign of such a shock effect to date, and several signs of continued intransigence combined with growing domestic disintegration.

Given all of this, the chances of a change of heart or a decisive shift in political power relationships in Tehran are not considered high in the short run. All four power centers on the hostage issue -- Ayatollah Ruhollah Khomeini, President Abol Hassan Bani-Sadr, the Islamic clerical party and the militant student captors at the U.S. Embassy -- have agreed that the hostage decisions will be up to the Iranian parliament, which is unlikely to get down to business before June or July.

In this situation, Carter's decision to schedule a U.S. "reassessment" in mid-May is likely to bring matters to a head before the legislature meets. Thus if Carter's recent words about military action are not simply a public bluff -- and high officials suggest that this is not the case -- a month or so from now Carter may be faced with the choice of ordering strong action, probably of a military nature, or eating the words he has uttered recently. And he will be under pressure to step up to his big decision as Democrats and Republicans are approaching the nominating conventions.

In explaining what has brought the crisis to this pass, senior officials last week spoke of three stages of U.S. policy since the seizure of American diplomatic personnel and the U.S. Embassy in Tehran last Nov. 4. a

Stage one began with the seizure and ran through the turn of the year. It featured slowly increasing U.S. economic and political pressures along with unavailing bids for negotiation. U.S. military action was vaguely threatened -- but the threats were directed to the possibility that the hostages might be harmed or put on public trial.

Stage two began with the Soviet invasion of Afghanistan, which intensified the importance of the strategic factor in calculations on all sides, especially with the election in late January of Bani-sadr, who openly sought to resolve the hostage crisis. With the beginniing of serious negotiations, U.S. pressures and threats against Iran were deemphasized.

Stage three, the present stage of a return to pressures, originated in the collapse of the negotiations around the United States commission in mid-March, and was confirmed by the failure of a last-ditch U.S. attempt in late March to insist on transfer of the hostages to the control of the Iranian government. When this was thwarted in early April, Carter had little choice but to return to a harder line. But he faced crucial choices about the duration and nature of stage three.

One strategy might have been to space out the U.S. and allied sanctions in order to give the appearance of continuing motion over several months, bringing pressures to a head in mid-summer when the Iranian political system reaches its point of likeliest decision. This timing might also have been a way to deal with the growing impatience in the United States.

In fact, Carter chose to play most of his remaining economic and political cards at once, on April 7. A White House meeting April 11 scraped up a few remaining nonmilitary gestures and sanctions, which were announced as a package last Thursday.

The reasoning, according to White House officials, was that it was better to get all the sanctions on the table quickly for maximum impact on the Iranians and Europeans. A more dramatic strategy was also more helpful at home to a politically embattled president.

By quickly "using up" his nonmilitary actions, Carter faced the obvious question of the next step. Starting last weekend, he chose to speak with growing explicitness and clarity about military action if the present sanctions do not succeed. The clearly spoken threat of force as a near-term "last resort" is a new element of fundamental importance.

At this point, the greatest and perhaps gravest questions are about the theoretical "stage four" -- the military stage of the hostage crisis after the mining of Iranian ports and a naval blockade. Officials report that while contingency papers addressed such matters several months ago, there are few clearcut or satisfactory answers. Among the questions are:

The Soviet reaction. Would the Russians seek to run a U.S. blockade or create a path through a minefield? Would they seek to thwart U.S. action by providing Iran with its needs across the Soviet-Iranian border or by airlift? And would U.S. military action push Iran into Soviet arms?

The reaction of the Persian Gulf region and Islamic world. The United States has declared the region vital, due to its oil supplies. The reaction to the United States as the outside power in battle with Islamic revolutionaries could be explosive.

The impact on Europe, Japan and world oil markets. Cutoff of Iranian oil would be painful, but the spread of disorder to other oil-producing areas of the Persian Gulf could be catastrophic. A brief decline in Iranian production early last year triggered worldwide jitters and a doubling of world oil prices to levels which many nations -- and citizens -- can hardly afford to pay.

Finally, the impact on the hostages and the crisis. Some experts believe the use of force will make it almost impossible for revolutionary Iran to back down. And toughest of all the questions -- what is the next step up the ladder, if mining or a blockade is insufficient, and the next steps after that? l