These are prayerful times for Republicans and Democrats who plot or ponder their party's chances to win the White House.

Republicans pray for the nation's contin Republicans pray for the nation's continued economic health, at least until Nov. 9 -- the day after Americans elect their next president.

Democrats pray for a sign of impending bad times. And while few, if any, wish for economic collapse, Democrats desperately need more bad business news to legitimize their continuing indictments of Reagan prosperity.

The reason: Voter perceptions of the nation's economic health play a major though sometimes overstated role in shaping voter preferences. And while it's not always the case, the bromide that a presidential party will be ousted in economic bad times but maintained in prosperity is about as true as anything in politics.

Entering 1988, it appears that both prayers have been answered. Even after more than five years of growth, the economy continues to perform well. But Black Monday raised enough anxieties to open a crack in the Republican's economy-based defense of the White House. And news about the federal deficit, an issue that puts many voters to sleep and keeps economists awake, remains generally bad.

"There's such mixed feelings out there," said Thomas E. Mann, a political scientist and director of the government-studies program at the Brookings Institution. "There is a general feeling things are going reasonably well, combined with certain anxiety about the future. Under those circumstances, the partisan impact is less clear. It's easy when times are bad, and it's easy when times are good."

Recent Washington Post-ABC News polls document high anxiety over the economy following the Oct. 19 market collapse. Nearly half -- 47 percent -- of those surveyed in December said they thought the economy is getting worse and only 15 percent said it was getting better. That represents almost a total reversal from December 1983, when 46 percent said the economy was getting better and 20 percent said it was getting worse.

The most recent poll numbers would appear to be just what the Democrats needed. "What it did is it gave the Democrats an opening," Mann said. "It gave some substance to Democrats' charges that serious economic problems await us. So the stock market crash gave Democrats a credible basis for talking about the economy."

In fact, the most recent polls actually may be bad news for Democrats, since they appear to reflect the first signs of a restoration in public confidence in the economy following a period of uneasiness precipitated by Black Monday.

"The stock market swings have been discounted by the experience of Black Monday, when it went way down, and now has partially recovered," Mann said. "Now we get a swing, and its 'ho hum.' We're only moved by triple-digit swings."

Gallup polls substantiate the importance of the economy in electing a president. For more than three decades, Gallup has asked this question of the American public: "Looking ahead for the next few years, which political party -- the Republicans or the Democrats -- do you think will do the better job of keeping the country prosperous?

Through the 1970s and '80s, The candidate from the party that the public identified as most likely to deliver prosperity generally has captured the White House.

The one exception was 1980, when an October Gallup poll showed Republicans and Democrats tied at 35 percent, with the remainder saying it made no difference. However, that tie represented a clear shift toward the GOP. As late as March of 1978, the Democrats were the choice of 42 percent of those surveyed, while Republicans were selected by 23 percent.

In December of this year, 41 percent of those surveyed by Gallup said the Republicans were likely to do a better job of keeping the country prosperous, while 38 percent said the Democrats.

Such a shorthand analysis only hints but does not establish the impact of the economy on presidential preference. More elaborate models developed by political scientists generally show that the economy is an important factor but not the only factor in voter preference, and sometimes even a minor factor in some elections.

In fact, some Republican strategists have come away from recent reexaminations of old polling data convinced that Reagan's commitment to a strong national defense and his get-tough policy with the Soviets were more important in his 1980 victory than they first believed.

Increasingly, pollsters like Daniel Yankelovich are noting that all roads in 1988 seem to lead to the economy. In Iowa, the issue of the trade deficit, the problems in agriculture and the challenge of Japan have been linked by many of the candidates, most notably -- and apparently most successfully, according to the most recent polls -- by Democratic hopeful Rep. Richard Gephardt (D-Mo.).

So how should each party play the economy issue?

"The Republican message has to depend on which candidate you're talking about," said political scientist Michael Hawthorne, a specialist on presidential politics.

"If I were one of the front-runners, the approach I would take now is that the place we're at is pretty good, but I'm going to pick up those few areas where we aren't doing well and we'll do better. If you're farther back, you may want to highlight some of the problem areas, such as du Pont {former Delaware governor Pierre S. (Pete) du Pont IV} is doing now with Social Security."

Mann of the Brookings Institution agreed. "What you're going on is five years of economic recovery," Mann said. "No major changes. This is a time for capable management to follow up on the Reagan years. It's very hard to argue against that. Democrats have sounded very lame when they try."

The winning strategy for Democrats: "Pray for a downturn," said Mann. "Pray for bad news. Frankly, I think they're going to have a hard time making headway on the economy issue.

"They should acknowledge what's worked, but try to paint a picture of the future that poses challenges, but that can be managed with competent, thoughtful leadership. It seems to be that's the only story that's credible to tell.

"If they keep talking about the utter failures of the Reagan administration, I'm skeptical that the voters will accept it."