San Francisco — Since Franklin D. Roosevelt and the despondent GOP conventions of that period , there have been few political assemblages where delegates have been less optimistic about the nominee's prospects than they are at this year's Democratic Convention. People are happy here. Indeed, they are having a very good time. Gov. Mario Cuomo certainly gave them a big lift. So did the Rev. Jesse Jackson. But the delegates still don't see much hope for their ticket.
True, many delegates are saying that Walter Mondale just might ''pull it off'' by coming up with a campaign that sweeps the nation off its feet.
But it is clear - from polls and reporters' findings - that the predominant delegate view is that no matter what Mondale does, no matter how well he and Rep. Geraldine Ferraro comport themselves, the election is for Reagan to lose and not for the Democrats to win.
That is, the perception is that even a flawless and inspired Mondale campaign probably won't be enough to unseat the popular President Reagan. Somewhere and in some way, the Democrats are saying, the President must stumble if Mondale is to win.
The delegates are concluding what Democratic strategists are now conceding: that short of a major Reagan blunder - or an event that would react negatively on the President - the Democrats will have to win virtually all of those states east of the Mississippi and north of the Mason-Dixon line if they are to unhorse Mr. Reagan.
No doubt about it: The West and the South pretty much belong to Reagan, with the possible exception of Missouri, Arkansas, and Texas. Washington State, too, might go to Mondale.
But can Mondale win all of the Midwest (east of the Mississippi), plus all of the Northeast? For example, can he pick up states like Illinois, Ohio, and Indiana, where the Republicans are traditionally tough to beat? And can he prevail in states like Pennsylvania, New Jersey, Connecticut, and Vermont, where pro-Reagan sentiment is running quite high? To win that whole section of the United States: That's a big order.
To the realists here - and a national convention is made up of political realists - the road ahead seems one that is likely to lead to a defeat for their nominee.
That's why Sen. Edward Kennedy is content to play a somewhat secondary role here. He's waiting for 1988, when he thinks the Democrats will be better positioned to win the White House.
And that's why Governor Cuomo of New York declined Mondale's tentative offer of the No. 2 spot on the ticket. He knows that if he were part of a loss in this presidential election, it would probably erase his chances of ever heading the ticket.
Incidentally, after Cuomo's electrifying speech, many delegates were saying the New York governor had suddenly become the favorite for the presidential nomination four years hence. One delegate put it this way: ''It's hello Cuomo and goodbye Kennedy.''
What sort of an event would have to take place to turn the election around this fall? The Democrats are a little vague about this. Perhaps some sudden, major US military involvement abroad - like Central America. That's what one hears. It sounds like grasping at straws.
The Mondale strategists themselves are resting their hopes on a debate with Reagan.
But will the Democrats get even one debate with Reagan? It's a good question. Reagan has said he is willing to debate. But that's not agreeing to the specific ground rules for such an encounter.
If Reagan should stay far out in front of Mondale, as he is today, would he risk a debate? He has to remember how Richard Nixon, by agreeing to debate John F. Kennedy in 1960, gave his opponent the chance he needed before a vast national audience. Kennedy sparkled. And Nixon, as a result, lost the election.
Vice-President George Bush has said he is willing to debate Ms. Ferraro. But again, would the Republicans be willing to give her this opportunity for national visibility? And would they risk having Ms. Ferraro make Bush look bad? Not too likely, unless the race tightens up a lot.