NOW for the hard part.It took United States Secretary of State James Baker III eight months and as many journeys to the Middle East to set the stage for tomorrow's historic peace conference in Madrid. But that's just the beginning. To keep the peace process from unraveling, Middle East experts agree, the Bush administration will have to stay engaged at the highest level for months, if not years, to come. "They'll all go home six hours after the speeches unless the US keeps the pressure on," says Michael Sterner, a former deputy assistant secretary of state who played a major role in the Camp David negotiations. "Israel, especially, but also the Arab states will continually be testing to see how committed the US is," Ambassador Sterner continues. "The president will have to be prepared to put his full prestige on the line to support whatever Baker and his envoys are doing." In letters of assurance provided to the six governments meeting in Madrid, Mr. Baker pledged that the conference co-sponsors, the US and the Soviet Union, would do everything possible to keep the peace process on track. But given the sheer complexity of the proceedings, playing that role will be difficult. With four separate negotiations in several different venues extending into the indefinite future, the US will not be operating in the kind of controlled diplomatic environment that existed, for example, at Camp David. State Department officials define the US role in the peace process as one of honest broker, helping to reconcile differences and find areas of compromise. "It's a question of keeping the Israelis and Arabs at the table and building some momentum," says an informed department source. "The longer you can keep the sides together, the more chance you have of building something." US officials are confident that the longer the talks continue the more likely they are to acquire a life of their own, breaking down the fears and suspicions that have fed the Arab-Israeli conflict for the past four decades. Longevity could also strengthen Israel's peace camp and help it pressure a reluctant Likud government to stay engaged in the peace process. For now, US intervention will be limited to pressing for confidence-building measures and to reminding the parties - as Mr. Baker has done skillfully in the run-up to the Madrid conference - that quitting the talks would be politically costly. The US is not likely to go further by setting forth its own blueprint for peace as when, in 1988, Secretary of State George Shultz recommended a specific timetable for negotiations to prod a diplomatic settlement. Convinced that the Bush administration is capable of playing a more evenhanded role in Arab-Israeli negotiations, Arab states have welcomed a background US presence at the talks. Jittery over strained relations with the Bush administration and its commitment to a "land-for-peace" deal between Arab states and Israel, Israelis would like it to be minimal. "In principle, we don't think the US should be initiating proposals," comments an Israeli official in Washington. "Anything the US says on the substance could prejudice the outcome" of the peace talks. Pressed to devote more time to other issues, Baker is expected to name a special envoy to tend to the day-to-day task of monitoring the peace talks. But no envoy will be able to substitute for the administration's big guns, Baker and President Bush himself, when deadlock threatens. The model for direct presidential intervention in peace talks is President Carter, who personally brokered the 1978 Camp David accords that ended 30 years of intermittent warfare between Egypt and Israel. Though style and circumstance suggest that Bush will play a more indirect role, Middle East analysts agree that his popularity at home and the prestige earned abroad during the Gulf war will make his selective intervention crucial to a successful outcome. "The pattern with Bush is going to be different," Sterner says. "He'll play a more indirect role but his commitment to the peace process will still be the essential ingredient." "The reality is that he will be drawn into it," says Peter Rodman, a fellow at the Johns Hopkins Foreign Policy Institute. "But he will have to be saved for the crucial moments when a breakthrough is needed." One entree into the process could come early next year when Bush will have to decide whether to try to force a halt to the expansion of Jewish settlements in the West Bank by pressing for a further delay in $10 billion in housing loan guarantees requested by Israel. In the end, there are limits to US influence, diplomatic observers caution, since no amount of persuasion will produce a diplomatic breakthrough unless the parties themselves are finally convinced of the logic of compromise. But no such breakthrough is likely without sustained US pressure. "We will be the oil that will lubricate the engine," says the State Department official. "Without us it will seize up and stop." "Baker's political future is tied to reaching some successful conclusion," adds a former US ambassador in the Middle East. "He's not in a position to walk away from it and say, 'We tried. You guys let it bog down. Call me when you're ready to get serious.