UNLOADING John Sununu, by push or jump, gives President Bush a chance to make a fresh start on his domestic agenda. He has been perceived as weak there. Samuel K. Skinner, Mr. Sununu's anticipated replacement, has shown strong administrative ability in fashioning a Transportation Department agenda.This staff switch could be a step toward recovering some of the president's recent losses. Mr. Bush, after all, is hardly alone among presidents in experiencing political slippage heading into a reelection year. Ronald Reagan had a 43 percent approval rating in late 1983, but revived by the following November. Bush's overriding need is to articulate a sense of purpose for the country. But he has trouble in this regard. The president has occasionally aired plans - like his America 2000 scheme for education - but they haven't merged into a set of basic ideas that can be identified as the Bush program. Instead, the president is too often seen as reacting to events or to shifts in the political winds, rather than leading. That perception isn't wholly fair. The president can tick off some significant achievements, particularly in foreign policy. And a president's record in world leadership shouldn't be discounted - it will weigh heavily with voters. But diplomatic laurels needs to combine with domestic activism to help the country prosper in a world that is being economically and politically transformed. Investment, building, learning, and training - these are the key concepts. Bush has a tough job ahead. Public opinion has never been more volatile. The public's assessment of whether the country is on the right track climbed 50 percentage points between fall of '90 and early spring of '91. Now it has fallen back to where it was a year ago. Such wild oscillations of mood are unprecedented. They severely test a leader's ability to communicate and guide. The president will need to find a means of convincing Americans that his will be a steady hand on the helm. Eager Democratic candidates should be quick to fill any ideas gap he leaves. And ideas - not the 30-second potshots of 1988 - should be the grist of the '92 campaign.