As diplomatic efforts continue without obvious success, the United States inexorably, perhaps inevitably, is being drawn directly toward a more active combat role in the Middle East.
The number of US marines in and around Lebanon, whose casualties continue to mount, more than doubles this week to 3,200. Warships now number 14, and the 57, 000-ton battleship New Jersey - with its 16-inch guns - is headed across the Atlantic toward the Mediterranean.
President Reagan has also decided to expand on his promise to the Marine commander in Beirut that Washington will ''provide whatever support it takes to stop the attacks on your position.'' This may now include air strikes from the attack jets aboard aircraft carriers steaming offshore. Officials say local commanders - exercising ''aggressive self-defense'' - have been empowered to call in tactical bombers and naval gunfire to defend Marine positions from attack by Syrian-backed Druze militias in the hills above Beirut.
Officials say the marines may now call on air and sea power, not only to defend their own positions directly, but also those of other members of the multinational peacekeeping force and even the Lebanese armed forces, if attacks on them also endanger the marines.
All of this reminds some observers of Vietnam in the mid-1960s, when military advisers equipped with light arms required greater and greater firepower to defend their positions. Sen. Paul E. Tsongas (D) of Massachusetts has warned that a similar ''quagmire'' may ensue in Lebanon. So far, US firepower has escalated from small arms to artillery to helicopter gunships to naval gunfire to low-flying US Navy jets on reconnaissance.
Meanwhile, the mediating effort of US negotiator Robert McFarlane (a retired Marine colonel who urged the White House to let the marines in Lebanon exert more muscle) moves at a slower pace as the political vacuum caused by Israel's internal political struggles and withdrawal from parts of Lebanon is being filled by warring Lebanese factions. The number of Palestinian fighters is increasing, and some believe that Syrian troops are directly involved as well.
Mr. McFarlane is traveling between Saudi Arabia and Syria, seeking support for a cease-fire proposal.
Through these latest moves, the US, in essence, wants to warn those who have been shelling Marine positions that a more aggressive response may not be far away. At the same time, officials here are purposely vague about the circumstances under which, for example, air strikes could be called in.
''What our response is depends on what the challenge is,'' said Pentagon spokesman Henry Catto. ''I don't want to pin down what we're going to do. I don't want to telegraph our punches.''
At the State Department, officials this week are emphasizing the diplomatic role and aims of the US in the region: the search for a cease-fire between warring factions and, eventually, the existence of a strong national government in Lebanon.
''Obviously, we are expending significant effort to the search for a political solution,'' said State Department spokesman Alan Romberg. ''We're not looking for a military solution to that problem.''
Senior State Department officials Tuesday briefed members of Congress. Nicholas Veliotes, assistant secretary of state for Near Eastern and South Asian affairs, told the Senate Foreign Relations Committee Tuesday that the Lebanese armed forces ''are performing better than some predicted.'' He reiterated the US commitment to the withdrawal of all foreign forces from Lebanon, a strong central government there, and security for Israel's northern border.
But he also emphasized that ''the President is determined to take every possible measure to defend our contingent,'' noting that 2,000 more marines arrived off the coast of Lebanon this week, ''available to the MNF (Multi-National Force) if necessary.''
While there is growing concern about the US military role in Lebanon, most lawmakers say they do not favor withdrawing the marines at this time. Even those sponsoring enforcement of the War Powers Act, which requires congressional approval for US forces to remain in hostile areas for more than 90 days, say they would vote to keep the marines there for at least a while longer.
The White House would rather find another vehicle for involving Congress more in US military efforts in the Middle East. One congressional proposal would allow the marines to stay another six months in Lebanon without further action by Congress.