For nearly five years the Democrats have been licking their chops, looking forward to recapturing control of the Senate in the 1986 elections.

All those weak Republicans swept in by the 1980 Reagan landslide, they figured, would be done in when they had to run their own. They would be running at a time -- six years after their party came to power -- when parties have been most vulnerable, as the Democrats were in 1938 and 1966 and the Republicans in 1958 and 1974.

This "six-year itch," the weakness of the incumbents, the possibility of another Reagan recession -- all these would conspire to bring back the normality of a Democratic Senate.

Maybe, but the chances are looking slimmer every day. We may still have a recession for all know, but it's not clear otherwise that voters will be feeling a six-year itch. As for incumbents, suddenly the Democrats, with only 12 seats up seem to have as many vulnerable seats as the Republicans with their 22 seats theoretically at risk. Every week the news for Democrats gets worse.

Item: Thomas Eagleton of Missouri says he'll retire. The favorite is Republican ex-governor Christopher Bond, the state's best-known politican for more than a dozen years. The Democrats have a good candidate in Lt. Gov. Harriett Woods, but Republicans have been winning low-level as well as high-visibility offices in this bellwether state.

Item: Russell Long says he'll retire after spending almost all of his adult life in the Senate. The favorite is Republican Rep. Henson Moore, who, thanks to his conservative views and seat on the Ways and Means Committee, has more than a half-million dollars sitting in his campaign treasury.

Item: Gov. Bruce Babbitt of Arizona announces he won't run for Barry Goldwater's Senate seat (or for reelection either). He's one of the Democrats' brainiest young officeholders and was easily the party's best chance to gain this seat. The Republicans have a crackerjack candidate in Rep. John McGain.

Item: Georgia Sen. Sam Nunn announces he won't campaign publicly against his Republican colleague, Mack Mattingly. This is senatorial courtesy, certainly -- Mattingly endorsed Nunn in 1984. But Nunn is one of the Senate's safest Democrats and Mattingly one of its politically weakest Republicans, in a state where Walter Mondale didn't do at all badly.

Item: Geraldine Ferraro makes an ad for Diet Pepsi for $500,000. Whatever one thinks of this, it seems unlikely to make her a stronger candidate for the seat of Republican incumbent Alfonse D'Amato in 1986.

As a result, by my calculations, the Republicans are in a good position to challenge seriously for seven Democratic seats (and are already the favorites in two). The Democrats are in a position to challenge seriously for 10 Republican seats, half of which are in the South, where the national party's image is a liability.

Given a random distribution of luck, and ignoring the financial and technical edges that have enabled Republicans to win 15 of 18 seats decided by 2 percent or less in the last three elections, that means neither party is likely to gain much unless there is an overwhelming tide of opinion in its favor. It will be hard for the Democrats to make the net gain of four seats they need for control.

Is it just an accident that the Democrats' advantages for 1986 seem to be evaporating? I think not. Consider the retirement decisions of Eagleton and Long: Both, whatever else you may say about them, are active, intelligent legislators who have been languishing without much work to do since the Republicans took control of the Senate. As they looked ahead, other work seemed more attractive to them. Even if the Democrats were to regain control, they have no clear legislative agenda (as Democrats did in 1958, for example). There is little evidence that voters want now or will want in 1986 significant changes in the level of taxation, the balance between public and private sectors, the operation of most government programs. Maintaining the status quo, for men as competent as Eagleton and Long, can get pretty dull.

In the meantime, each bit of news that helps Republicans' chances of maintaining control in 1986 tends to become a self-fulfilling prophecy. If voters come to believe that Republicans will stay in control, that helps Republican incumbents argue that their committee positions will aid their states (as Democrats did for so many years); if campaign contributors come to believe Republicans will retain control, they'll be more likely to give money to Republican incumbents and less interested in backing Democratic challengers.

For there is no necessary reason for the "six-year itch." If you go back far enough in history, you will find there was no "six-year itch" in 1926 or 1902 or, for that matter, in 1866. The reason: voters were satisfied with the policies produced by the congresses and presidents in those years and saw no reason for change.

Today, apart from the farm revolt, which could help Democrats in one or two Senate races, there is not much evidence that the voters want a major change from the equi Republican administration and Senate and a Democratic House. Until they do, it's going to be hard for the Democrats to win the victory they've been counting on for nearly five years now.