Los Angeles — The only time during Ronald Reagan's reelection campaign when he felt he had lost control of the public agenda, says Stuart Spencer, ''was during the flap over taxes'' - those few days in August when Vice-President George Bush was admitting that a tax hike might be necessary and the President was denying it.
Mr. Spencer and James Baker III, the White House chief of staff, resolved the contradiction by having the President say he would accept a tax increase ''only as a last resort.'' This provided the Republican ticket with ''wiggle room'' on the issue.
In the wake of an election triumph, President Reagan's top political strategist, back home in southern California after a post-campaign holiday, looked back and ahead at a breakfast with reporters this week.
''I don't think you're going to see another Ronald Reagan in our lifetime,'' said Spencer, who has masterminded campaigns for Reagan since 1966 - and one against him (for Gerald Ford) in 1976. ''He's unique. He's got an appeal. . . . I don't see anyone else in either party that's got it.''
But the 1984 campaign, he said, was not a runaway from the start. In early August, the Reagan campaign's polls showed the President only two percentage points ahead of Mondale.
Even accounting for the four-point ''bounce'' the Republicans were destined to get from their own convention, this was close enough to call it a ''horse race,'' Spencer explained.
But it did not work out that way. ''I had the sense that the Mondale campaign never had its act together until well after Labor Day,'' Spencer recalled.
The choosing of Geraldine Ferraro as running mate was a ''wash'' for the Democrats, according to Spencer. After the blush of excitement over Ms. Ferraro had worn off, he says, the Democrats were left with ''a liberal-liberal ticket with a woman on it.''
The South, where whites gave Reagan a handy margin, would have been much more competitive if the Mondale ticket had a Southerner like Sam Nunn in the second slot, he adds. ''We'd have been fighting for the South all the way.''
Spencer says he is not convinced that Reagan's landslide win signals a long-term shift among vot-ers to the Republican Party. Rather, he sees a further weakening of ties to either party, especially among the baby-boom generation.
Although this group - economically conservative, so-cially liberal - went solidly for Reagan this year, their political loyalties were not well tested, in Spencer's view.
''Reagan wasn't liberal on the social issues, but for some reason they overlooked that. They said, 'Well, he doesn't understand.' ''
As vice-president, George Bush has to be considered the leading prospect to be the next Republican banner carrier, Spencer says.
Mr. Bush has won over allies among the Reaganite conservatives who control many of the GOP leadership posts around the country, according to Spencer. ''He's done his homework as vice-president.''
''But there's a lot of room,'' he adds. Of the other potential candidates, Spencer rates New York Congress-man Jack Kemp high on having the kind of appeal that carries on evening-news television.
Tennessee Sen. Howard Baker, he says, makes a good appearance on talk shows and interview programs, where his mastery of the issues comes across, ''but the nightly news is more important, and he doesn't shine there.''
Kansas Sen. Robert Dole, with his acid wit, comes through distinctively on television, but he is ''sharp, biting.''
''They're all going to be compared to Ronald Reagan,'' Spencer says. And they will all, he implies, suffer by the comparison.
Spencer mentions several prominent prospects on the Democratic side, including New Jersey Sen. Bill Bradley, Maryland Sen. Joseph Biden, New York Gov. Mario Cuomo, Colorado Sen. Gary Hart, and Massachusetts Sen. Edward Kennedy.
This year's election would have been tougher for Reagan to win against Hart than against Mondale, he says.
Of the New York governor, who drew rave reviews after his keynote speech at the Democratic convention next summer, Spencer says: ''Cuomo's got a style, but I don't think it will play on national television. It's too New York.''
He drew guffaws from reporters with another speculation:
''I don't think we've seen the last of Jerry Brown.'' The former California governor has got the tenacity and the political ''smarts,'' he says, to find his way back to political favor.