Washington — ''At this point in a presidential race,'' says a leading pollster, ''public opinion polls should be taken with a grain of salt.'' That's how analysts look at the latest Gallup-Newsweek poll that suddenly shows the Mondale-Ferraro ticket moving into a 48 to 46 percent lead over President Reagan and Vice-President George Bush.
In contrast, Reagan campaign director Edward Rollins says that private Republican polls still indicate the President leads Mr. Mondale by at least 5 to 10 points nationwide. Those polls also indicate Mr. Reagan remains ahead in every major state, including New York - Geraldine A. Ferraro's home turf.
The Gallup-Newsweek study was made July 19 and 20, while memories of the Democratic convention in San Francisco were still fresh and exciting.
The turnaround in the race seemed stunning. Only a few days earlier, a Gallup survey of registered voters gave the Reagan-Bush team a 13-point lead. Could things change that fast?
A number of analysts, including those at Gallup, say the roller-coaster polling results aren't surprising at this stage. In fact, such results are considered normal, although the movement this time has been a little greater than pollsters expected.
Andy Kohut, president of the Gallup Organization, observes that when voters are polled, they ''respond in context. Last week was a Democratic week,'' and the public reacted to that.
Mr. Kohut concedes that ''it's surprising that the race has narrowed as much as it has.... It moved around more than we expected.''
To understand the poll results, one must look at the moment in time when each was taken. Here are the four latest Gallup results:
Late June, Reagan ahead by 19 points.
July 12-13, Reagan ahead by 6 points.
July 13-15, Reagan ahead by 13 points.
July 19-20, Mondale ahead by 2 points.
Two big swings toward Mondale took place - one around July 12, another around July 19. Each is readily explained.
The July 12 survey was taken immediately after Mondale named Ms. Ferraro as his running mate. Excitement was high. Mondale enjoyed a boomlet. Gallup, however, continued to sample voter reaction. Within 24 hours the euphoria was passing, and Reagan regained much of his original lead.
Then came the Democratic convention, which many politicians considered a great success for the party. The flag-waving finale with its unity speeches galvanized support. The result: another boost for Mondale.
Analysts say they will be surprised if the lead persists. Many expect the Reagan to retake the lead in most polls by the time the GOP convention begins. That lead should be further strengthened by a show of party unity in Dallas.
All of this action in the polls, however, says some important things about the coming race, in the view of a number of analysts. It tells us that:
* Ms. Ferraro helps. At least in the short run, adding Ms. Ferraro to the ticket with Mondale has fostered unity among Democrats. The long-term effects, of course, will depend on her performance during the fall campaign.
* Reagan and Bush are ahead. Despite the latest poll, the consensus among poll takers is that Reagan is stronger. Even in the Gallup-Newsweek poll, in which Reagan was slightly behind, voters said by a 46 to 37 percent margin that the President can do a better job keeping the country prosperous than Mondale. And the economy - except in wartime - is considered the No. 1 issue with most voters.
* Opinion is fluid. Rapid changes in the polls partly reflect the uncertainty and excitement over the first woman candidate for vice-president. Even so, pollsters say that public opinion is particularly fluid this year.
* Democrats could still win. Although the GOP is favored, the public reaction - first to the Ferraro announcement, then to the unified convention - gives the Democrats hope. If economic or foreign events go badly for the Republicans during the fall, the race could turn against Reagan, pollsters say.
Meanwhile, polls continue to tell us other things about this unusual political year. A new survey by the University of New Hampshire, in conjunction with WMUR-TV in Manchester, gives Reagan a thumping 58-25 lead (17 percent undecided) in the Granite State.
Prof. David Moore, who ran the poll, says a wide difference in male-female opinion was revealed, especially among independent voters in New Hampshire. While independent men picked Reagan by a 71 to 20 percent margin, independent women had Reagan ahead by only 40 to 34 percent - a 31-point gender gap that Professor Moore called ''incredible.''
The key to that gap, says Moore, isn't Ms. Ferraro. It is the war-peace issue. Men opt for a tougher foreign policy. Women seem ''much more peace-oriented,'' he says.