PRESIDENT Bush's public approval rating has deflated by more than 30 percentage points since it peaked in the high 80 percent-range last spring.As a trend, that figure is an ominous sign for his reelection prospects. But how much trouble is the just-forming Bush campaign really in? With the election 11 months away, two unknowns will significantly shape the outcome. One is the direction of the economy next summer. The other - less important at this point than the economy - is who the Democrats nominate. But right now, Mr. Bush retains a strong field position and the race remains his to lose. The measure that shows Bush at his weakest is national polling that pits Bush for reelection against an unnamed Democrat. In recent weeks, Bush has roughly tied his generic opponent in such matchups. A Times-Mirror poll released last week showed him losing by 2 percentage points. Yet a generic Democrat may look better than the real thing. Against what the poll shows to be the strongest Democrat now considering a candidacy, New York Gov. Mario Cuomo, Bush wins 58 to 37. Bush can also draw some comfort from historical patterns. His 55 percent approval rating (in the Times Mirror poll), though a comedown, is still relatively high as presidents run in the polls. At similar points in their reelection efforts, Richard Nixon stood at 49 percent and Ronald Reagan at 53 percent. Both won by landslides. The historical patterns run even stronger in Bush's favor in the electoral college, where some analysts believe the GOP has developed a "lock" on the presidency. In the last six elections, points out Republican pollster and strategist Bill McInturff, the Republican candidate has won states worth 190 electoral votes, the Democrats 32. No one, however, is laughing in the face of a stagnant economy that some officials now describe as entering the second dip in a double-dip recession. For example, the Border States voted about 48 percent for Republican congressional candidates in 1980. In 1982, with the economy in a deep recession and a Republican in the White House, the same states voted only 33 percent Republican, warns Mr. McInturff. THE state of the economy will affect the votes of different voters at different points on the calendar, points out Kevin Phillips, the analyst and author who developed the winning Southern strategy for Mr. Nixon in 1968.The most educated voters, possibly 30 percent of the electorate, are influenced by leading economic indicators that may forecast a recovery from its early signs. Such voters can be influenced by good economic news very close to an election. Most of the others are more responsive to the unemployment figures, which tend to lag behind other signs of recovery. An upturn would have to surface by the end of the summer to show progress in creating jobs by election day. If the economy is still sluggish or worse next summer, the election becomes dicey, because the strength of the Republican coalition is the perception that the GOP is the party of strength and prosperity. Bush's campaign could lose an important edge, adds Mr. Phillips, if David Duke launches a third-party candidacy that could plausibly draw 3 percent to 5 per-cent of the vote from Bush in Deep South states. Eddie Mahe, a national Republican strategist, insists that Bush is in no serious reelection trouble. "Three months ago, it was very difficult, if not impossible, to imagine Bush losing," he says. "Now, it's easier to imagine." But not very easy.