Can Reagan hold off stampede?

President Reagan and the Republicans are trying to forestall a last-minute, sudden lurch of the November election toward the Democrats.

This is the meaning of the drumbeat of White House emphasis on the ''plus'' side of the economic news, highlighted by President Reagan's address to the nation Wednesday night. That address was followed Thursday by a full-dress White House pitch on crime, the nation's second-ranked issue after the economy.

Reagan is trying to hold the center ground he captured in 1980, particularly among independent voters who traditionally are last to make up their minds. As late as the last weekend of an election, fully 25 percent of voters traditionally have not made up their minds, history shows.

Even more to the point, in the last two national elections the electorate as a whole broke late, fast, and in one direction. In those elections, the break went against the Democrats. In 1978 they lost six Democratic Senate seats. In 1980, pollsters for both sides noted a massive movement the last week of the campaign to Ronald Reagan and the Republicans.

''The 1980 election became a referendum on Jimmy Carter,'' notes Carter pollster Paul Maslin, vice-president of Cambridge Survey Research. ''This election is becoming more and more a referendum on Ronald Reagan, his economic program, and Republican philosophy. The White House is trying to keep the finger in the dike as long as possible. They're afraid of a psychological break.''

The White House has maintained a steady program of damage control, trying to keep public reaction to the recession in check by emphasizing gains against inflation and interest rates.

They have been cushioning the shock of bad news, nudging upward their own estimates of House seat losses - now put at 25 to 35 seats by Reagan White House pollster Richard Wirthlin's latest count - to take the bite off an expected Nov. 2 reversal.

Key public indicators watched by the Reaganites have started to cut against them. In one Southern state, for example, polls in early September showed a majority (53 percent to 36 percent) thought the country was headed the right direction. This week, that state's view had reversed: 37 percent said the country was headed the right direction, 49 percent the wrong.

In no state is the fight to hold the center ground against a sudden erosion more crucial to Reagan than in California.

Republican losses to Democrats Edmund G. Brown Jr. in the Senate race and Tom Bradley in the governorship - in Reagan's home state and base of the anti-tax conservative movement - so soon after 1980 would, at best, be deeply embarrassing.

But California's voters are traditionally among the nation's most independent , and no one is confidently calling these races as yet.

''This state's largely the same since Reagan came on the scene in 1966,'' says Bill Roberts, a California GOP strategist. ''The voters pick and choose. They go for individuals.''

Peter Kelley, Democratic state chairman for southern California, agrees.

''Statewide races are decided by about 1 million Democrats who go either way, '' he says. ''Either side starts out the same, going for the middle. That hasn't changed in almost 20 years. In 1970, Ronald Reagan was defeating Jess Unruh for governor while (Democrat) John Tunney was defeating Sen. George Murphy for the Senate. In '68, while Humphrey was losing to Nixon, Alan Cranston was beating Max Rafferty. In '80, at the same time Ronald Reagan was winning by a landslide, Alan Cranston's race wasn't even close.

''I voted for Reagan last time,'' says Alan Harper, a retired Orange County business executive. ''If he ran now I'd vote for him against any of the likely Democrats.''

But Mr. Harper, showing a characteristic of California voters, doesn't let his feelings for Reagan keep him from picking and choosing from both parties' candidate lists this fall.

''I seldom vote a straight ticket,'' Harper says. ''I will not be voting for Brown - I'll vote for Pete Wilson, I think. And probably for Bradley. I think Bradley's a little more reliable than Deukmejian.''

''Reagan was a wonderful governor, we thought,'' says William Lynch of Santa Barbara. ''He was for cutting taxes and all that. Now he seems like he's all for the rich people. We've always liked him.''

Mr. Lynch likes Democrat Brown in this election. ''He's a good politician,'' says Lynch. ''On occasion I'd be mad at Brown. But I believe in forgiving and forgetting.''

Republican Wilson's stand on social security made it easier for Lynch to forgive Brown. ''Wilson said he would make social security voluntary,'' Lynch says. ''That knocked him right out of the box as far as a lot of Californians are concerned. We don't need someone like that in Washington.''

''Reagan's still personally popular in California,'' says Harvey Hukari, Republican National Committee coordinator for California's 45 congressional races. ''California is his home state. People are comfortable with him. But it's hard for any outsider to play a decisive role - to add anything but a kind of credibility to a candidate, or enhance his visibility.''

Though his personal influence appears limited, Reagan's policies are framing the debate in California, in the Senate race particularly. And the mixed results of his economic program have made the Democrats' prospects competitive.

''Yes, it's still competitive for the Democrats,'' says Kelley, southern California Democratic chairman. ''But if Reagan had instituted his program without the third-year tax cut, if he'd reduced the defense buildup, he would have kept interest rates down and the budget would be nearer balanced. Then, the Republicans would be running through America with a steam roller.''

As it is, the Republicans are not even running through Reagan's home state in 1982 with a steam roller. And the US political center appears as much up for grabs as ever.