San Francisco — It has been the best of times, and it has been the worst of times, for Walter Mondale. The Democrats' presumptive leader, now only 24 hours away from the presidential nomination, can expect a tumultuous welcome from the thousands of Democrats crowded into the Moscone convention center. The reason is clear: His choice of Geraldine A. Ferraro as his running mate has dazzled the delegates and put new zing into the party.
In the words of Democratic veteran Robert Strauss, Ms. Ferraro has given the ticket tremendous ''lift'' as the campaign takes off. All over this city, Democrats wear stickers reading: ''A woman is the ticket.''
No sooner was Mr. Mondale airborne, however, than he flew into a severe downdraft that his political radar should have warned him about.
His effort to fire national party chairman Charles Manatt (some analysts described the move as an attempt to look tough) came to a whimpering end when Mr. Manatt was promised that he could stay.
It was an erratic start that drew attention away from Ms. Ferraro. It also reminded delegates that there are still potential pitfalls for Mondale this week. The next one - the platform - will be debated tonight.
Mondale should have the votes to hold the line against unwanted platform amendments by Gary Hart and Jesse Jackson. But with Western tempers still glowing over the Manatt affair (he is from Los Angeles), an element of uncertainty has been injected into events here.
Democratic delegates, sifting through newspapers for clues just like voters all over the country, ask three things:
* How did Mondale miscalculate in his effort to replace Manatt with Bert Lance of Georgia?
* Where could serious battles break out in today's platform session?
* What does the Mondale draft platform attempt to achieve in terms of the fall campaign?
Each of those questions merits an answer in some detail.
First, the Manatt miscue.
Mr. Strauss, speaking with a number of reporters at breakfast here, said this is just the kind of mistake any campaign can make when events are moving fast, as they are now. This decision, he suggested, was probably taken by just a couple of people, without consultation, inside the Mondale organization.
One Georgia source said that when Bert Lance learned that Mondale intended to make him the new chairman, he asked permission to call a few top party leaders. Lance, still a controversial figure in some quarters, wanted to smooth the way for his appointment, and perhaps head off criticism from certain party officials.
Many Democrats are quick to recall that Lance resigned under fire as White House budget director. He was later cleared of all charges, but still carries ''considerable baggage,'' some Democrats feel, by being tied so closely to Jimmy Carter.
Lance's request to make the telephone calls, however, was turned down. Mondale aides felt the news would leak if he began letting others know.
Even so, some leading Democrats, including House majority whip Jim Wright of Texas, got wind of what was happening. Mr. Wright tried to contact Mondale (with whom he frequently consults on the telephone). But he couldn't reach the candidate in time. The story about Manatt was in the newspapers before Wright could sound a warning.
Mondale apparently wanted to oust Manatt for two main reasons.
First, he hoped appointing Lance would assuage Southern feelings that might have been ruffled by his failure to select a Southern running mate. If Mondale doesn't win at least three or four Southern states, he probably has little chance of election.
Second, Mondale also was unhappy about the unsatisfactory outcome of Manatt's discussions with Jesse Jackson over complaints about the party rules. Lance gets along swimmingly with the Rev. Mr. Jackson.
Ironically, Jackson chastised Mondale for trying to get rid of Manatt, saying that one doesn't change players in the middle of the World Series.
All of this maneuvering infuriated Western delegates who support Manatt. There even was a threat that some would boycott Mondale on the first ballot.
The flap is expected to fade. But the party finds itself in an awkward spot. It has a chairman who knows that the nominee would prefer he weren't there. And some dismayed delegates ask whether Mondale has enough clout to run even his own party.
The convention's attention turns next to the platform. Five possible changes - one from Hart, four from Jackson - will be at stake today.
Sources close to Jackson say he doesn't want to cause major trouble at the convention, but that he will stand firmly for what he believes. That attitude is ambiguous enough that Mondale's team will watch events warily.
Briefly, Jackson's four substitute planks call for:
1. Strengthening affirmative-action programs. (This is especially unpopular with many Jewish voters.)
2. Attacking voting laws that include runoff primaries. (This is unpopular with many Southern whites.)
3. Pledging ''no first use'' of nuclear weapons in Europe. (This concept has been opposed by every modern president.)
4. Cutting defense spending. (This is opposed by both Mondale and Hart.)
Senator Hart's plank essentially requests that the US move swiftly to make itself independent of high-risk Mideast oil. He would also make it much harder for the US to get bogged down in unilateral military efforts to protect that oil. Mondale disagrees.
No major trouble is expected on these planks. But the unexpected often happens under the heat of TV lights and political rhetoric.
Finally, and in some ways most important, what does the Mondale-Ferraro team hope to achieve with the platform being adopted tonight?
Democratic platform writers - led by Ms. Ferraro as their chairwoman - had a dual mission this year. They wanted to keep the platform thematic, rather than specific, to avoid convention fights over controversial proposals. They also wanted to find a way to attack a sitting president at a time when the economy is moving up and the nation is at peace.
In writing the platform, Ms. Ferraro pulled out the Democrats' crystal ball and predicted a grim future if the Republicans are kept in power.
The Mondale-Ferraro platform tells voters that with another four years of a Reagan White House, the nation must be fearful of war, of recession, and a host of other threats.
This theme is expected to be central to the Mondale-Ferraro campaign. The strategy, straightforward and blunt-edged, is intended to scare voters away from the President.