LAST WINTER, when Sens. Russell Long (D- La.) and Thomas Eagleton (D-Mo.) announced they would retire in 1986, the odds on the Senate elections shifted toward the Republicans. Now, with the retirement announcements of Sens. Paul Laxalt (R-Nev.) and Charles Mathias (R- Md.), the odds have shifted the other way. Mr. Long and Mr. Eagleton were counted as strong candidates for reelection; but as soon as they left their races, Republicans -- Rep. Henson Moore and former governor Christopher Bond -- were installed as favorites. Mr. Laxalt and Mr. Mathias would have been strong, quite possibly unbeatable candidates as well. Now Democrats may be the favorites to succeed them in states that have elected Democrats to the Senate more often than Republicans over the past several decades.

That leaves the battle for the control of the Senate very close to even. The numbers work for the Democrats: they have 22 Republican seats to attack; the Republicans have only 12 Democratic targets. Money works for the Republicans: their campaign committee will have three or five or 10 times -- some significant multiple -- as much money as its Democratic counterpart, and the Republicans have shown in the past that they know how to use their money to the best advantage. Their intensive tracking polling in every race, for example, has enabled them to spot candidates in trouble before anyone else and to take appropriate action in response. As a result, Republicans won the lion's share of close races in 1980, 1982 and 1984. That alone accounts for most of the difference between the narrow Republican majority in the Senate today and the considerably larger margins the Democrats won from 1958 to 1978.

The Senate Republicans have one other advantage, which they have squandered -- or which has been squandered for them. As a party, they have stood for a unified, responsible approach to major macroeconomic issues before the nation. No other group of politicians has done as much to attack the deficit. Yet their budget initiative this year, though first accepted, was then squelched by President Reagan. That left many Senate Republicans vulnerable on the points of the package that are less than universally popular and without credit for a total package that would be rather palatable. At the same time the administration, by concentrating on the not-very-popular tax issue, has left Republican candidates for Senate seats without a strong Reagan platform to stand on.

In these circumstances it will not be surprising if most Republican candidates follow the lead pioneered for this term by Sen. Charles Grassley (R- Iowa). Mr. Grassley came to the Senate after six years in the House with the conservative label firmly attached. But he has proved not to be the Reagan White House's kind of conservative. His brand of frugality got him looking into the corners of the Pentagon and summoning administration critic Chuck Spinney to testify. His agrarian bent got him criticizing the administration long and hard for callousness on farm programs. Mr. Grassley practices -- out of conviction as well as ambition, it should be said -- the politics of camouflage played so well by so many Democratic senators for years: he adapts to the local terrain. In the process he has become the most popular politician in tightfisted, dovish, farm-filled Iowa.

Undoubtedly other Republicans will follow or are following such a strategy. It may serve them well in 1986 and may preserve their majority. But majority for what? If the Republicans become, as the Democrats for many years were, a party of everyone for himself, they will have paid a high price for political success.