The delay of the Lebanese government's long-debated plan to extend its authority from Beirut to one-third of the country further endangers prospects for a political settlement that would allow the US Marines to withdraw. The already chronic security problem, symbolized by the rising death tolls, is also in danger of getting worse.
Informed United States officials said Thursday they were still optimistic that the differences could be worked out. But pessimism mounts in the Lebanese public. As the English-language Daily Star editorialized Thursday: ''Those leaders on whom the peace of Lebanon depends . . . appear bent on continuing the quarrels that are bleeding Lebanon to death.''
Lebanese security forces were poised to take their new positions Tuesday, but the deployment had to be aborted at the last minute. Druze leader Walid Jumblatt added new demands, and the Syrian government balked over Lebanon's refusal to abrogate its May 17 agreement with Israel on withdrawal of foreign forces.
US special envoy Donald Rumsfeld was scheduled to fly to Damascus Thursday for talks with Syrian officials, possibly including President Hafez Assad. Meanwhile, Lebanese authorities scurried back and forth among various parties at home. Syria remains the key, according to Western diplomats. As the principal backer of Lebanon's opposition forces, the Assad regime can almost dictate acceptance or rejection of any mediation proposal.
In effect, Syria has linked the first step of implementing the plan with the resumption of the Geneva reconciliation conference on the formation of a national unity government of all Lebanese factions.
Damascus has told Beirut that a Lebanese offer to formally ''freeze'' the Israeli-Lebanese accord was insufficient. Syria wants it scrapped and hints that reconciliation could also be ''frozen'' if the government of President Amin Gemayel does not go further.
With each day, new demands are added, further complicating the situation. Syria has put pressure on Lebanon to have the multinational force (MNF) withdrawn. Mr. Jumblatt is calling on the Lebanese government to end press censorship and to reinstate Druze soldiers who left the Army rather than fight their own sect during last September's mountain war.
Yet the feeling is growing among diplomats from MNF countries that President Assad may be holding out for other and bigger stakes unrelated to either the security plan or the May 17 accord.
For the past year, the Syrian government has been trying to maneuver into the position of kingpin of all Middle East peace efforts. A well-informed source suggested the Syrians are using the current disputes as a vehicle to gain further recognition of their role as pivotal.
The source predicted that Assad's letter to President Reagan this week, welcoming a direct dialogue, was related to the Lebanese impasse: The Syrians want to talk face to face with the only power capable of dealing long term with both sides of the Arab-Israeli dispute, thus winning acknowledgement that Damascus is an important player, if not the leading one.
Syrian officials have expressed open concern this week with developments in Jordan, where it appears King Hussein may be trying to mobilize Palestinian moderates to join him in a peace bid. Assad often displays jealousy at the traditional US focus on Jordan in discussions of various peace plans.
Some MNF officials feel a little US ''stroking'' of the Syrian leadership now may do more good in breaking the Lebanese deadlock than appeasement of their demands.
The as-yet unpublished security plan calls for:
* Disengagement of rival Christian and Muslim militiamen in Beirut's suburbs and in the surrounding mountains, where sporadic battles have killed more than 1 ,500 since September, according to security sources.
* Deployment of an estimated 2,500 police and Army troops to patrol the regions and stabilize the repeatedly broken cease-fires.
* Normalizing disrupted services. This would include reopening schools, repairing damaged power lines, and lifting the curfew.
* Fostering an atmosphere of goodwill that will allow resumption of the Geneva reconciliation talks.