The ground of public support for United States involvement in Lebanon continues its slow but steady tilt against Mr. Reagan's initiatives. A leader of course must lead. And a large share of the US public - nearly half - feels it hurts American interests to criticize the President. We observed , when Mr. Reagan sent US troops to Lebanon this fall, that the public, which clearly didn't want them sent, would go along for the time being, giving the President the benefit of the doubt. Later, however, as the wisdom or unwisdom of his policy was proved by events, he would be held accountable. Rarely does a public, at the outset or in midstream, try to call its leader's hand.
Congress shows a similar reticence. Even now, after the battleship New Jersey has begun pounding Muslim Druze positions overlooking Beirut, after US air strikes farther inland, with Marines still vulnerably encamped on shore, Congress is not ready to call for a Marine pullout, without ensuring that there will be some replacement peacekeepers or progress in negotiations to justify the US investment to date.
Some do suggest that partition of Lebanon now be considered.
But the unease over American involvement in Lebanon continues to grow, compounded both by events and by a lack of confidence in the administration's policies. The Harris Survey, taken a week ago, shows a 67-to-24 percent majority now thinks the loss of American lives in Lebanon is not justified. The latest ABC/Washington Post poll showed the public, by a 5-to-4 margin, disapproves of Reagan's Middle East policy; a month ago, by a similar margin, the public approved it. Such measures are at best approximations of public attitudes, representing a mix of reactions to other events, such as Reagan's updraft in popularity after the Grenada landing and concern over the interruption of US-Soviet arms talks. Barring further Middle East tragedy, Mr. Reagan's foreign policy fortunes could conceivably rise again in coming weeks following his exchange of visits with Chinese leaders.
Still, it isn't events alone that trouble Americans about US policy in Lebanon. Again and again, officials and public alike find themselves misspeaking and referring to ''Vietnam'' instead of ''Lebanon.'' They know the difference. But the patterns of escalation and official US rationalization from the earlier experience continue to haunt them.
The public fails to see consistency in the administration's Middle East approach. In September of 1982 Mr. Reagan outlined a comprehensive peace plan for the Middle East which favored Palestinian self-government in conjunction with Jordan. Observes Middle East expert Robert G. Neumann in Foreign Affairs: ''Only secondarily did the President demand the evacuation of Lebanon by all foreign forces, i.e., Israelis, Syrians, and remaining Palestinians. But a series of wrong predictions and wrong moves shifted the entire emphasis of US policy to Lebanon, leaving the broader peace objectives unimplemented.''
Getting US policy back on track involves acknowledging the fundamental shifts in the Middle East over the past year, including Syria's stronger position. Goals should include gradually improved US relations with Syria, less preoccupation with Lebanon, and a return to a broader peace initiative more along the lines of the original Reagan plan, Neumann suggests. ''This might worry Israel,'' he adds, ''but might not be totally unacceptable provided American diplomacy proceeds with care, skill and balance.''
Support for the military presence in the Middle East would not be falling if confidence in the coherence of US policy were rising.
The passage of time and the pattern of escalation are not helping the President's cause.