After six years of jollity and song, some Republican senators feel their sense of well-being withering at the roots. But the Democratic Party is going through one of those patches during which even silver clouds have lead linings, and so in the November elections the party may suffer a crippling victory.

A Democrat, anticipating 1988, recently said: ''If we can't beat George Bush, the Democratic Party should pick another country.'' Of course, Democrats talked that way about Ronald Reagan in 1979. But that also is how many Democrats feel about the challenge of recapturing control of the Senate this November. Those feelings rest on theories that may, or may not, be sand.

Peter Hart and Geoffrey Garin, both Democrats, have asked ''focus groups'' of voters to describe a situation in which they would like to be a ''fly on the wall'' to observe and judge a candidate. Two answers have recurred. The voters would like to observe a private meeting about political strategy, and the candidate at the family dining table. The latter, according to Hart and Garin, in their introduction to ''The Democratic Fact Book,'' supports their theory that today ''The messenger is the message.'' That is, voters are unusually interested in the candidate's character.

Most voters agree about what problems are important -- the budget deficit, the Soviet Union -- but there is no widespread confidence in particular solutions. Therefore voters say: ''I am unsure which policy choice should be made, so I will concentrate on being sure about the people making the choices.''

Hart and Garin also analyze mid-term elections with reference to persons who, as heroes or villains, embody issues. In 1974, the heroes were Sen. Sam Ervin and Rep. Peter Rodino; the villains were Richard Nixon and Spiro Agnew. In 1978, the hero was Howard Jarvis, the flint who sparked California's Proposition 13 tax revolt. In 1982, James Watt was a symbol of Reaganite excess. Hart and Garin say that in 1986 the hero is Democratic Sen. Bill Bradley, architect of tax reform that puts special interests in their place, and the villain is Mike Deaver, Republican lobbyist.

William Schneider of the American Enterprise Institute, writing in The New Republic, sees Democratic success for three other reasons. First, presidential elections play to Republican strength, the direction of national policy. Current polls show the public thinks the GOP is best equipped to perform that task. Other elections are about benefits and services, the Democrats' perceived strength in our political division of labor.

Second, television coverage is now coming more easily to candidates challenging incumbents, making them more competitive. More Republican than Democratic incumbents are seeking reelection in 1986. The third reason Schneider expects Democratic success is turnout. In most midterm elections there is a drop-off in turnout, a drop-off detrimental to the president's party. If 1986 is like 1982, turnout will be 20 percent smaller and substantially more Democratic than in 1980.

The voters who stay home in midterm elections are ''Zeitgeist voters,'' casual voters who register less their passionate convictions than the national mood. In 1980, when such voters voted they reflected the anti-Democratic mood, and 16 freshman Republicans were elected. But even in those favorable conditions, the 16 won with an average of only 53 percent, and five won with 50 percent or less. Furthermore, many of the Democratic incumbents who lost were vulnerable because they had last run in the unnaturally hospitable climate of the 1974 Watergate election.

Schneider says big Democratic gains in the Senate could be injurious to Democrats because they may think they can stop rethinking. Actually, he says, big gains in 1986 ''will mean as much for 1988 as the big Democratic gains of 1982 meant for 1984: precisely nothing.''

Many Republicans supporting presidential candidates other than George Bush or Bob Dole hope for a Senate split 50-50. Bush often would be nailed to a chair, presiding over the Senate to provide tie-breaking votes, and Dole would find the duties of Republican leader even more demanding. If, however, Democrats control the Senate, Dole's diminished duties as minority leader will leave him more free to campaign. So Democratic success in 1986 could strengthen the Republican candidate who a growing number of Republicans consider best equipped to defeat Democratic designs in 1988.

Furthermore, if Democrats control both houses of Congress, 1987-88, Republicans will run against them as the tormentors of the Gipper and the thwarters of his mandate. Republicans, having had custody of most of the government for eight years, would nevertheless be able to do what they do best, which is run against government. This would reinforce the Republicans' most obnoxious habit, and that habit might produce another victory.