For Republicans, this year has no theme.

At a critical moment, when their control of the Senate and hope of attaining majority status are at risk, Republican strategists have been unable to manufacture an idea or even a slogan to run on in November.

Since 1978, when the Republican ascendancy began, the party has approached every election by projecting a national theme that has crystalized its sense of purpose.

In 1978, still trying to remove the taint of Watergate, the Republicans advertised their good intentions by "working to make life better." Late in the campaign, they illustrated their message by attaching themselves to the national tax revolt. By 1980, with Ronald Reagan as standard-bearer, the GOP had dispensed with the defensive tone; it was time to "vote Republican for a change." In 1982, during the worst recession since the Great Depression of the 1930s, voters were beseeched to "stay the course." Two years later, the recession had lifted: "morning again in America."

"Inherent in this series of good years, and even in holding on in 1982 and not getting beat worse than we did, has been an underlying theme," said Robert Teeter, a Republican pollster.

Some Republican strategists believe that underlying the 1986 campaign is the public's ardent desire for continuity, a desire to extend the happy days of 1984. And buoying the optimistic mood is a buoyant economy. "The emphasis is mainly on keeping what you've got," said William Greener, the Republican National Committee's director of political operations.

"The only national theme, if it's that, is the mood of the country, which is very good," said Thomas Griscom, the executive director of the National Republican Senatorial Committee. "You don't see an overriding national message . . . . But prosperity is always a good issue."

"We're in an era of good feelings," said Lee Atwater, a Republican consultant. "What happens in an era of good feelings is that incumbents get reelected. When an election nationalizes in an off year, the rule is that it nationalizes against the party in power. Due to the era of good feelings, you're not going to have a nationalized election against the Republicans." By Atwater's reasoning, the mood has supplanted the theme -- a good sign, particularly for the 22 Republican senators running for reelection.

To Republicans, continuity means continuing the Reagan magic. It demands of voters that they regard the present campaign as the latest episode in the epic that began in 1980, not as an isolated decision about individual candidates. "We had a clear vision in 1980," said Greener. "That vision resulted in very positive changes. There are still problems that need to be solved. The Republican Party continues to offer the solution." It may be afternoon in America, but it's still sunny.

"The most important thing this year," said Teeter, "is the fundamental idea of what the Republican Party stands for. Sometimes you use a persona, sometimes you use other examples."

The persona that completely dominates the Republican mind is, of course, Ronald Reagan. "Ronald Reagan is the Republican Party," said Greener. The president's persona may be a unifying force, but many of the conditions surrounding his administration are not. The burdens of governing have already led to Republican splits, especially over economic policy, and those splits have thwarted the emergence of a coherent theme.

Controversy over the federal budget deficit, for example, has highlighted the ideological schism between the traditionalist Republicans and the supply-siders. Many Senate Republicans want a tax increase, which the president is vehemently resisting. The prospect of deep budget cuts, meanwhile, has created scores of local issues (aid to Amtrak in New Jersey, charges for grazing lands in Colorado, etc.), diminishing the possibility of developing a national message. The intra-party combat, in this stage of the Reagan presidency, has begun to look like the war for the post-Reagan succession. "You have Senate Majority Leader Robert J. Dole barking up everyone's tree about the deficit and taxes, Rep. Jack Kemp talking about the absurdity of austerity, and Reagan and Vice President Bush battling tax increases and stuck in a variety of themes of the week," said a Republican Party official.

Historically, the second midterm election is always perilous for the party in power. By now, most of its ideas have been subjected to the reality principle. Except for the special case of 1902, after Theodore Roosevelt succeeded the assassinated William McKinley as president, the incumbent party has invariably lost House and Senate seats after six years in office.

The best parallel to 1986 may be 1958, when a popular Dwight D. Eisenhower, viewed favorably by 58 percent of the public, presided over the Republican loss of 48 House seats and 13 Senate seats. Yet that election took place during a nasty recession, not a boom.

This year, to confound the historic pattern, the Republicans plan to exploit economic good times and "try to ride it out on personality and local elections," said Lance Tarrance, a Republican pollster. "You don't have what I call the 'faith psychology' that you have when you go through a presidential race." The evangelical fervor of 1980 and 1984, directed upward toward the charismatic standard-bearer, is now dispersed race by race, state by state.

Today's Republicans pose proudly next to the economy. But in a midterm election, local numbers may be a more relevant political guide than national statistics. And figures provided by the Bureau of Labor Statistics show that 25 states clustered in the Midwest, South and West have unemployment rates above the national average. Prosperity is uneven.

Nor is the perception of prosperity shared by all classes. Working-class voters making between $20,000 and $30,000 a year -- a crucial swing group constituting one fifth of the electorate -- have become increasingly gloomy about their economic prospects since Reagan's second inauguration. In February 1985, 47 percent of this group believed that the economy was getting better, according to a Washington Post/ABC News poll. But 12 months later only 25 percent were optimistic.

For more than a year, top Republican strategists have been privately attempting to devise a theme that would maintain the contentment, but without success. "An overarching theme likely will not emerge in 1986," Greener said.

The problem is compounded by disunity at the top, fostered by competing 1988 presidential candidates. Their jockeying, which has already compelled many Republicans to choose sides, threatens to become this year's theme. To avoid being overwhelmed by the image of internal strife, RNC chairman Frank J. Fahrenkopf, on Jan. 15, brought together representatives of Howard H. Baker Jr., Jack Kemp, George Bush, Pierre S. du Pont IV, Robert J. Dole and Pat Robertson, and lectured them on the need to suppress open hostilities, at least through this year.

The Democrats, reading the signs of things to come, or more precisely, the nonappearance of signs, are sounding optimistic. "Having a theme is a tremendous asset," said Sen. George J. Mitchell of Maine, chairman of the Democratic Senatorial Campaign Committee. "It creates the impression of vitality and organization that is enormously beneficial. The perceived presence of it among the Republicans and the absence of it among Democrats has plagued us." This themeless Republican year, he said, "augurs well for us. They've reached their high-water mark. I think we'll take back the Senate."

Mitchell admits that Democrats have no theme either. "We confront the same problem," he said. But the Democrats' vagueness may actually make it harder for the Republicans to formulate their own theme.

Over the years, the Republicans' thematic coherence has depended upon a clear picture of the enemy. Certain Democratic leaders, depicted in an unflattering light, have provided convenient targets against which Republicans could comfortably position themselves. Yet Jimmy Carter has faded away, Thomas P. (Tip) O'Neill is retiring, Walter F. Mondale is nowhere to be seen, and Edward M. Kennedy has withdrawn from presidential contention. "We're missing the image of the bad guys," said Eddie Mahe, a Republican consultant. "We need bugaboos," said a Republican official. "We always have. What's hard for the Republicans is to have no one to attack."

But they do have someone to love -- the most popular president in modern times. Can Reagan's popularity help Republican candidates this year? Or does Reagan's magnetism work only for Reagan?

"The president's popularity," Mitchell said, "is uniquely personal and does not extend to his own policies, much less his own party or other political figures . . . . In 1984, we gained two seats in the Senate. He doesn't have any coattails. He didn't have any when he was running. He'll have less when he isn't running."

Reagan's appeal is constructed partly of anti-Washington, antigovernment themes that are becoming awkward for other Republicans to handle. "Reagan still does that well and gets away with it," Mahe said. "We still have people who tend to run against the government. But if the government is perceived as being run by us, you're hitting yourself in the jaw every time you do that. It has become more difficult to be elected as a Republican by saying 'government is bad, elect me and I'll fix it.' "

Thus, six years after Reagan assumed power, the GOP finds itself in the seemingly paradoxical position of offering itself as both the party of change and the party of the status quo, the party of outsiders and the party of insiders.