THE Clinton administration is persevering with a new Balkans peace plan even as military tension worsens between the West and Bosnian Serb forces. While NATO contemplates how much to punish Serb gunners after their mortar attack on Sarajevo, talks continued in Paris to see if the US proposal might succeed. The Aug. 28 attack left at least 39 people dead and more than 80 wounded. Most of the casualties were caused by a single shell that exploded outside the city's main market, where 68 people died in a similar attack in February 1994. US officials say their new initiative stands a good chance of success because recent military and political developments may compel Bosnian Serbs to accept a modified version of the plan drafted last year by the ''contact group'' of US, Russian, British, French, and German mediators. Much of the new plan remains secret, but it is known to preserve as its cornerstone the awarding of 51 percent of Bosnia to the Muslims and Croats and 49 percent to the Serbs. Among other things, US officials say the Bosnian Serb army has been hurt by recent territorial losses. They say they believe it has also been seriously demoralized by the defeat this month of rebel Serbs in neighboring Croatia, which recaptured most of the territory it lost in a 1991 war. ''The Bosnian Serbs don't seem so invincible anymore. They are facing a potential of a much more vigorous response on the part of NATO,'' says one US official. ''So, you could say that their position has eroded considerably, and they have been put in a position to bargain.'' US officials also say that Serbian President Slobodan Milosevic can be persuaded to accept a demand that he recognize Bosnia's international borders in exchange for a lifting of UN sanctions that have stifled his economy. By cutting a peace deal, Mr. Clinton would be pulling off a stunning coup that would improve his image as a foreign policymaker and restore US credibility that has been badly tarnished by three years of broken pledges on Bosnia. Success would also avert an embarrassing repudiation of Clinton's Bosnia policy on Capitol Hill, where lawmakers are set to override his veto of a bill requiring the US to unilaterally end the UN arms embargo on the Bosnian government. But of greater concern to Clinton is the need to prevent Bosnia from interfering in his 1996 reelection campaign. Should unrelenting fighting prompt a pullout of UN peacekeepers, the US would have to provide troops to a NATO-protected extraction operation. That raises a potential for US casualties that could cost Clinton votes. Accordingly, the chief US negotiator, Assistant Secretary of State Richard Holbrooke, plunged on with the peace effort, holding talks on Aug. 28 and 29 in Paris to persuade Washington's European allies, Russia, and the Bosnian government to buy into the US initiative. Mr. Holbrooke was then to fly to Belgrade in an attempt to win the backing of President Milosevic of Serbia, the main patron of the Bosnian Serb rebels' conquest of some 70 percent of Bosnia. But critics of the US initiative are lambasting the US effort as a sellout of Bosnia's Muslims. Despite administration denials, they claim the Muslims will be asked to trade Gorazde, their only remaining enclave in eastern Bosnia, for rebel Serb-controlled areas of Sarajevo. More seriously, critics charge, the plan embodies an ethnic-based partition of Bosnia, something the US has pledged never to support And they dismiss US warnings of concerted NATO action. First, they say, the British, French, and Russians oppose such a step. They also point to administration statements that NATO will act if the plan is blocked by the Bosnian Serb rebels. But, they add, the Bosnian Army has already spurned it, providing the US with an ''out.'' ''There are so many dominoes that would have to fall before this took effect,'' says Marshall Harris of the US Committee to Save Bosnia. ''The administration will do everything it can to prevent them from falling.'' The UN appeared poised to fulfill a pledge made in June with NATO at a London conference to retaliate for rebel Serb attacks on Bosnia's remaining UN-designated ''safe areas,'' of which Sarajevo is one. Still, UN officials in the city expressed uncertainty over the consequences. At the London Conference, attention was focused on Gorazde, the last remaining safe area in eastern Bosnia, as Western leaders vowed to protect it after the fall of Srebrenica and Zepa. But just a few weeks later, the UN announced it was pulling out the remaining 200 UN peacekeepers from Gorazde in order to lessen the number of troops vulnerable to being taken hostage. On the night of Aug. 28, the UN withdrew the last remaining 87 peacekeepers from Gorazde, leaving no potential hostages for Bosnian Serbs. Two observers remain. And UN officials in Sarajevo quietly conceded Gorazde would not be protected. ''The number of troops on the ground in an area is a clear indication of the resolve to protect it,'' a UN official said. ''In Sarajevo, there are 5,000 troops. In Gorazde, there are none.'' Because NATO airstrikes could destabilize the US plan, the UN may use the British-French Rapid Reaction Force, deployed in the hills west of Sarajevo. ''The use of artillery of the Rapid Reaction Force could be the compromise we need,'' said one UN official. ''On the other hand, it could anger the Bosnian Serbs and not appease the Bosnian government and still jeopardize the peace process.'' The timing of the Aug. 28 attack convinced some analysts that it was the Bosnian Serbs' response to US warnings that they would face NATO airstrikes if they refused to participate in the talks. ''This was an in-your-face'' action, said a US official. But if the attack was aimed at scuttling the US effort, it appears to have failed, as administration officials pressed on with their struggle for a foreign policy success that has become crucial to Clinton's political future.