NO United States president since Franklin Roosevelt in 1935 has had so commanding a margin over his opponents on the eve of a presidential campaign as George Bush now enjoys. He has come a long way since the winter of 1988, when most pundits expected his campaign for the GOP nomination to collapse in the face of Bob Dole's challenge.Publicly Democratic strategists must claim Bush is vulnerable - arguing he has no domestic agenda. But they know better. As John Adams remarked in his defense of the British soldiers accused in the "Boston Massacre" of 1770, "facts are stubborn things." Facts show the Democrats' hurdles in 1992 to be unusually daunting. That Democrats know the seriousness of their predicament is shown by the state of the contest for the party's presidential nomination. Democratic campaigns just haven't gotten going. Prospective presidential nominees may be thought of in terms of three classes. "Class 1" contenders have clearly established themselves in the public's eyes as "presidential possessed of attributes suited to the nation's highest office. Not one such candidate has yet come forward. A number, including Richard Gephardt and Lloyd Bentsen, say they won't run. Mario Cuomo hints the same. Of "Class 2" prospects - not yet established as true contenders but having attributes suited to the "Big Dance only West Virginia Sen. Jay Rockefeller has begun. Thus, 15 months until the election, such "Class 3" hopefuls as former Massachusetts Sen. Paul Tsongas and Iowa Sen. Tom Harkin are receiving press attention - though they have no national following. A year ago, there was some basis for thinking that the 1992 campaign might be conducted in the midst of an economic downturn - which would be harmful to the incumbent Republican. But the recession of 1990-91 has been mild. A year ago, the public might have accepted new government initiatives. However, the latest round of tax increases - the federal hike last fall, and the major new increases introduced in many states - has brought much of the country to a tax revolt. Try in 1992 to convince the public that the US needs costly new programs. Then there's Mr. Bush's personal standing. Polls show his approval, strong already, soared in the last stages of the Gulf war. Gallup put Bush's approval at 89 percent in its survey of February 28 to March 3. Such numbers were bound to fall. What's striking, though, is how high they have remained. Polls taken this spring and summer show Bush's approval rating declining from his post-war high 80s, but down only to the mid to low 70s. His support among Democratic partisans remains unprecedentedly high. Trial heats - polls in which respondents are asked how they expect to vote - are tricky things this long before the election. It's widely expected Bush will run again in 1992. And the Democrats don't have a candidate. Nonetheless, every polling effort finds the president in an enviable position. In early June, ABC News and the Washington Post asked, "If the 1992 election were being held today, would you be inclined to vote for George Bush or for the Democratic nominee for president?" Some 54 percent said Bush. Just 29 percent "the Democrat." Some Democratic strategists profess to see hope in such findings. A popular incumbent president should be able to do better than roughly 50 percent, they argue, on such "are you prepared to re-elect him?" questions. But this reasoning is baseless. No president has ever polled as large a margin over "anyone from the other party" at this stage in his office. It can't truly be said, a year before an election, that a president is "unbeatable." The public thinks and cares about the office too much. In the scrutiny of a campaign's stretch run, the field having been narrowed to just two nominees, new judgments get made. Big events, questions of health, and the chemistry of a campaign can quickly change a race. Still, no modern president has begun his re-election campaign as well placed as George Bush is today.