Democrats approach the 1988 elections as early favorites to retain control of the Senate while Republicans scramble for candidates, money and a way to recapture their momentum of the early 1980s.

But because of a series of breaks for the GOP last week in states ranging from Wisconsin to Florida and New York, coupled with glimmers of a fund-raising recovery, Republicans are talking more hopefully of picking up the four or five seats necessary to reclaim control of the Senate.

"Without question . . . it's now doable," said Jann L. Olsten, executive director of the National Republican Senatorial Committee,

But the odds still favor the Democrats, who go into the campaign season with the prospect of fewer vulnerable incumbents and more big-name challengers, along with a renewed sense of confidence sparked by their recapture of the Senate last year and an upsurge in fund-raising this year.

While conceding that the Republicans had a good week -- including the decision by Sen. William Proxmire (D-Wis.), a 30-year veteran, to retire next year -- Robert A. Chlopak, executive director of the Democratic Senatorial Campaign Committee, said last Friday that Democrats retain an "excellent chance of maintaining control and some chance, possibly, of gaining."

With 14 months to go before the Senate elections -- which could be influenced significantly in the meantime by the economy, the presidential campaign and other forces -- it is far too early to predict how the 33 contests will turn out.

Uncertainty is heightened by the fact that Democrats currently control the Senate by a margin of only 54 to 46 and will be defending 18 seats, three more than the Republicans. It would take a gain of four GOP seats to win control if the vice president, who breaks tie votes in the Senate, remains a Republican and a gain of five seats if the vice president is a Democrat.

But unless there is a major upheaval in the economy or in the national political balance, some strategists believe that this Senate contest -- more than others that have been decisively influenced by national trends -- could be shaped significantly by the current search for candidates.

It is in the area of individual candidate strength that Democrats appear to have gained an early edge, both in terms of incumbents and challengers. Not only do the Republicans appear to have more vulnerable incumbents than the Democrats, but some of the strongest prospective Democratic challengers would be pitted against the most vulnerable Republicans.

A leading example of the Democrats' opportunities is in Nevada, where first-term Sen. Chic Hecht (R-Nev.), probably the GOP's shakiest incumbent, faces a likely challenge by recently reelected Democratic Gov. Richard H. Bryan, only two years after Democrats captured the state's other Senate seat.

Another is in Nebraska, where former Democratic governor Bob Kerrey may run against the survivor of a GOP primary fight between Sen. David Karnes and Rep. Hal Daub. Karnes, who had little previous political experience, was appointed last spring to succeed the late Sen. Edward Zorinsky (D).

In Minnesota, Sen. David F. Durenberger (R) faces trouble from State Attorney General Hubert H. (Skip) Humphrey III. In California, Sen. Pete Wilson (R) faces a strong Democratic field led by Lt. Gov. Leo McCarthy and Secretary of State March Fong Eu. In Rhode Island, Sen. John H. Chafee (R) could face the challenge of his career from Lt. Gov. Richard Licht.

Republicans face less severe difficulties in several other states.

Sen. Paul S. Trible (R-Va.) will probably be in deep trouble if former governor Charles S. Robb (D) decides to run against him, although Trible's fortunes would improve markedly if Robb decides not to run. Sen. Malcolm Wallop (R-Wyo.) may be in similar difficulty if former governor Ed Herschler decides to challenge him. Sen. William V. Roth Jr. (R-Del.) could face problems if Rep. Thomas R. Carper (D) opposes him.

And the GOP's hold on Washington State may well depend on whether Sen. Daniel J. Evans (R) decides to run again. Evans has not made his intentions known. Nearly all the state's Democratic House members are eyeing the race.

Republicans' hopes soared last week when Proxmire, considered a shoo-in for a sixth term, announced he will retire next year, paving the way for a strong bid by any one of several Republican officeholders. They were also boosted when Rep. Connie Mack (R-Fla.) let it be known he has changed his mind and probably will challenge Sen. Lawton Chiles (D-Fla.) and when Rudolph W. Giuliani, Manhattan's corruption-busting U.S. attorney, confirmed he is considering a race against Sen. Daniel Patrick Moynihan (D-N.Y.).

Democrats may still be favored in all three states, but Republicans now at least have the prospect of major-league candidates. "It only takes a couple of these {breakthroughs} to create some real momentum," said Olsten.

Republicans also cling to some hope that former Republican governor Lamar Alexander will run against Sen. James R. Sasser (D-Tenn.) but have little if any backup strength if, as appears likely, Alexander does not run.

But they have no proven vote-getters to oppose Sen. Jeff Bingaman (D-N.M.), whom they describe as vulnerable. Republicans are still trying to pin down candidates to oppose Sen. John Melcher (D-Mont.), another prime target. Prominent Republicans are shying away from challenging Sens. Dennis DeConcini (D-Ariz.) and several others, including Sen. George J. Mitchell (D-Maine), who faced possible trouble from Rep. Olympia Snowe (R) until she decided not to challenge him. No well-known challengers are in sight to oppose other Democratic leaders, including Majority Leader Robert C. Byrd (W.Va.), Finance Committee Chairman Lloyd Bentsen (Tex.) and Sen. Edward M. Kennedy (Mass.).

Sen. Quentin Burdick (D-N.D.), 79, one of the Senate's most low-profile members, has problems, but they come mainly from Rep. Byron L. Dorgan, his expected challenger in the Democratic primary. Republicans have no big names on line for the race.

Republicans face a different problem with another Senate Democratic elder: Sen. John C. Stennis (Miss.), who is 86 and in frail health. House Minority Whip Trent Lott (R) could run a strong race but will not challenge Stennis, and Stennis has given every indication of running again.

The GOP does have a formidable challenger for a vulnerable incumbent in New Jersey. Pete Dawkins -- football hero, Rhodes scholar, retired Army general and investment banker -- is expected to run against Sen. Frank R. Lautenberg (D) in what is probably the party's best prospect for next year.

The Republicans have two strong contenders against Sen. Howard M. Metzenbaum (D-Ohio), Cleveland Mayor George Voinovich and Rep. Bob McEwen. But a GOP campaign memo linking Metzenbaum with "communist causes" backfired in a barrage of negative publicity, triggered disavowals from party leaders and inspired a Metzenbaum fund-raising blitz with the "smear" as its centerpiece.

With Rep. James M. Jeffords (R-Vt.), Republicans are favored to retain the seat of retiring Sen. Robert T. Stafford (R-Vt.), especially now that Gov. Madeleine M. Kunin has decided against running. Sen. Orrin G. Hatch (R-Utah) can breathe easier now that former governor Scott Matheson (D) made the same decision. Sens. Lowell P. Weicker Jr. (R-Conn.) and John C. Danforth (R-Mo.), both of whom had tough races six years ago, will have Democratic opposition, although its seriousness is not yet clear.

Although the GOP's Olsten said fund-raising improved during the summer, his committee raised only half as much in the first six months of 1987 as it did two years ago, hampered in part by embarrassing revelations of huge staff bonuses paid out after the party took a drubbing in the 1986 elections.

By contrast, Democratic contributions have picked up by 50 percent, and some Democrats are rolling in reelection funds. Bentsen has helped fend off opposition by raising more than $5 million so far. Even Chiles, who has limited contributions to $100 and banned gifts from political action committees (PACs), has raised nearly $1 million.

But the Republicans still retain a huge financial edge: $12 million compared with $2.7 million for the Democrats. And, if they keep getting breaks like last week's, they could also reverse the odds for Election Day.