NO, George Bush isn't in trouble. But he and those around him are beginning to think the unthinkable: That his reelection isn't a lead-pipe cinch.President Bush had this conclusion written all over his face in his early-morning press conference following the trouncing of his former attorney general, Dick Thornburgh, by Harris Wofford in the Pennsylvania Senate race. In the final tally, Mr. Thornburg was beaten by a hefty 10 percentage points, after having led by over 40 percentage points. The president was somber. He obviously was worried. He admitted that the outcome of the contest in Pennsylvania contained an important political message: "People are concerned about their livelihood." He said the message was for Congress as well as for himself. But the Pennsylvania contest was not a good example of this mixed interpretation. Bush's man, Thornburgh, for whom he had diligently campaigned, had just lost out to an incumbent senator. Certainly, in Pennsylvania, the public discontent was aimed mainly at the president. And Mr. Wofford's campaign took direct aim at the Bush administration's lack of a clear policy in such critical domestic areas as health care. The significant political development is that the Democrats, citing the sluggish economy and the stunning upset of Thornburgh, have come alive once again. They had pretty much conceded the president's reelection. The unwillingness of major Democratic figures to oppose Bush has been the best evidence of this concession. And now? Well, the Democrats may well mount a credible presidential campaign in coming months, with a strong candidate emerging from the primary process. Indeed, this shift in the wind may be enough to persuade well-known Democrats like Mario Cuomo or Lloyd Bentsen - or even George Mitchell or Thomas Foley - to end their reluctance and challenge the president. Could it be that Jesse Jackson might even reconsider his decision not to run? Bert Lance, close friend of Jimmy Carter and a most astute reader of the political scene, told the Monitor breakfast group the other day that he thought Mr. Jackson would become heavily involved in presidential politics next year throwing his support behind one of the candidates" and thus helping that person be nominated. Mr. Lance talks frequently to Jackson - indeed, he is one of Jackson's chief advisors. Could it be that Jackson is shaping a course in which he may be offered the vice-presidential slot by the nominee? Lance didn't go that far. But his words made it sound like that was part of the current Jackson strategy. This burst of Democratic hope of regaining the White House has been reinforced by the president's latest jittery response to the economy's continuing sluggishness. He has postponed his upcoming trip to Asia and Australia to stay in Washington and deal with legislation aimed at jump-starting the economy. Mr. Bush said he wanted to "protect taxpayers who don't need another tax increase." But the postponement obviously reflected a presidential awareness of the criticism he was receiving for allegedly traveling abroad too much and neglecting domestic social and economic problems. But back where we started: Is the president really in trouble? Is his reelection really in danger? The answer to both questions is, at this point, "no." Dark, threatening clouds hang on the president's horizon. That's as far as current developments can take the political forecaster. Some Democrats have been so carried away by a new euphoria that they are saying Bush could be another Herbert Hoover, the Republican president who was sunk by the Great Depression. I lived through the depression and vividly remember the extensive suffering of that period: little children going through garbage cans looking for food (not just the occasional homeless adult, as now); out-of-work travelers, knocking on the kitchen door in search of a meal. And all over the country, in every community, long soup lines and people starving. Those grim times aren't here, nor are they even likely to recur. Indeed, while the economists differ, it seems that the majority believe that better days are slowly - very slowly - on their way. Also, look at the president's standing. At this writing he still is in the high 60s in the approval ratings - higher than any previous president at this point in his term in office. No, this is not a moment for Bush or the Republicans to throw in the towel. Nor is it the time for Democrats to get too hopeful. But there has been a decided shift in the political wind.