Republicans retained control of the Senate last night, although Democrats scored impressive victories in many races and badly scared GOP incumbents in several others.

Democrats won more of the 33 seats at issue than did the Republicans. But they also had more seats at stake.

Ousted from control of the Senate after more than a quarter-century in 1980, the Democrats needed to pick up a total of five seats to regain Senate control, and as of early this morning had gained a net of one, although one currently Democratic seat was still in doubt.

New Jersey was one state that swtiched hands, as Democrat Frank R. Lautenberg defeated Rep. Millicent Fenwick (R) for the seat currently held by appointed Republican caretaker Nicholas F. Brady.

The Democrats also gained in New Mexico, where state Attorney General Jeff Bingaman (D) scored a major upset in ousting Sen. Harrison Schmitt (R).

But in Virginia Rep. Paul S. Trible (R) edged out Democratic Lt. Gov. Richard J. Davis for the seat being vacated by Harry F. Byrd Jr., who called himself an independent but voted with the Democrats on party control of the Senate.

The one race still in doubt involved a Democratic seat in Nevada, where Sen. Howard W. Cannon was narrowly trailing his Republican challenger, Chic Hecht, with votes from heavily Democratic areas still to be reported.

Regardless of the Nevada outcome, Republicans were assured of keeping control of the Senate.

The GOP saved a challenged seat in Connecticut, where Sen. Lowell P. Weicker Jr. turned back Rep. Toby Moffett (D) and won reelection.

Republicans rescued another hotly contested seat in Utah, where Sen. Orrin G. Hatch (R) won by an unexpectedly large margin over Salt Lake City Mayor Ted Wilson.

They also avoided possible losses in Rhode Island, Minnesota and Missouri.

In Rhode Island, Sen. John H. Chafee (R) turned back a tough challenge from former state attorney general Julius C. Michaelson (D).

In Minnesota, Sen. David Durenberger (R) won out over millionaire businessman Mark Dayton (D) in another hard-fought race.

In Missouri, Sen. John C. Danforth narrowly edged out Democratic state Sen. Harriett Woods.

And in California, San Diego Mayor Pete Wilson (R) defeated retiring Democratic Gov. Edmund G. (Jerry) Brown Jr. for the seat being vacated by Sen. S.I. Hayakawa (R), thereby keeping it in Republican hands.

In Maine, on the other hand, Sen. George J. Mitchell (D), who once appeared to be in big trouble, easily brushed back his challenger, Rep. David F. Emery (R).

Sen. John Melcher (Mont.), another Democrat who once appeared shaky, also won reelection.

Sen. Edward M. Kennedy (D-Mass.) was a big winner, by a margin large enough to boost his presidential ambitions for 1984.

Also cruising to an easy victory was Senate Minority Leader Robert C. Byrd (D-W.Va.), who had been an early mark for the Republicans.

There were no surprises among most of the other Democratic survivors, even though some of them had appeared vulnerable before the economy soured the GOP's prospects, especially in hard-hit industrial states.

Victorious Democratic incumbents included Sens. Lawton Chiles (Fla.), Paul S. Sarbanes (Md.), Donald W. Riegle Jr. (Mich.), John C. Stennis (Miss.), Daniel P. Moynihan (N.Y.), Howard M. Metzenbaum (Ohio), Jim Sasser (Tenn.), Dennis DeConcini (Ariz.), Quentin Burdick (N.D.), William Proxmire (D-Wis.) and Lloyd Bentsen (Tex.).

Democratic Sen. Edward Zorinsky (Neb.) was leading.

Among Republicans who won predicted victories were Sens. Richard G. Lugar (Ind.), John Heinz (Pa.), William V. Roth Jr. (Del.), Robert T. Stafford (Vt.) and Malcolm Wallop (Wyo.).

In Washington state and Hawaii, Sens. Henry M. Jackson and Spark M. Matsunaga jumped to commanding leads in early returns.

Democrats, who lost control of the Senate for the first time in a quarter century in the sweep for President Reagan two years ago, needed to pick up five seats to win back control.

The current Senate breakdown is 54 Republicans and 46 Democrats.

The hard-fought, expensive and sometimes downright nasty 1982 Senate campaign came only two years after Republicans, surprising even themselves, ended 26 years of Democratic control by picking up a dozen seats from Alaska to Florida.

Riding in part on Reagan's coattails, they swept out of office many of the Senate's most prominent Democratic liberals, including South Dakota's George McGovern, Idaho's Frank Church and Indiana's Birch Bayh.

In addition to those 12 seats that changed hands, Republicans picked up another when Sen. Harrison Williams (D-N.J.) resigned early this year to avert expulsion following a bribery conviction, giving the GOP 54 of the Senate's 100 seats as the 1982 campaign began.

Republicans had a further advantage stemming from the fact that 20 of the 33 Senate seats at stake in the 1982 elections were held by Democrats, including Virginia's Harry Byrd, who called himself an independent but voted with the Democrats on questions involving party control of the Senate.

Moreover, many of the 13 Republican-held seats were presumed safe, because of the popularity of the incumbents or the historical GOP strength in the states from which they came.

Republicans have regarded this election all along not just as important in its own right but as crucial if they are to retain Senate control over the next four years.

The reason is that, like the Democrats in 1980 and 1982, the Republicans in 1984 and 1986 will be defending more seats than they are challenging, thereby increasing their chances of slippage.

In 1984, Republicans will be defending 19 of 33 seats; in 1986, they will be trying to hold onto 22 of 34.

In the months just after their 1980 victory, the exultant Republicans had great hopes of gaining additional ground this year.

Those hopes faded, however, as the economy soured and Reagan's popularity slipped, turning many Democratic liberals into sure winners and putting some once-secure Republicans from economically hard-hit states onto the endangered list.

In race after race, Democrats pounded on the unemployment rate, which by early October had hit 10.1 percent nationally and was considerably higher in some battleground states.

Especially in states where Reagan and his economic program appeared to be political liabilities for Republicans, they attempted to tie reluctant GOP incumbents tightly to the president.

Social Security was also a heavily exploited issue, with Democrats pouncing on a half-dozen Senate votes in which most Republicans had gone along with efforts to squeeze some budget savings out of the huge retirement system.

By mid-October, polls indicated that the Democrats had narrowed the gap in enough states to put them, theoretically at least, within reach of recapturing control of the Senate if everything went their way in the final days of the campaign.

At this point, many of the most vulnerable Republicans, as well as some seemingly safe ones who were leaving nothing to chance, started fighting back with sharpened attacks on their challengers -- a counteroffensive that was made possible in part by a campaign financing edge shared by incumbents regardless of party.

As of mid-October, incumbent senators were outspending their challengers, $40.5 million to $25.1 million, according to Common Cause.

Democratic candidates, including challengers as well as incumbents, were outspending their GOP counterparts, $43.6 million to $38.5 million, although Republicans ended up with a narrow money edge when political party expenditures were included, the lobbying group reported last weekend.

In one of the ironies of the campaign, Reagan's impressive tax and budget victories in Congress, coupled with the extraordinary unity of Senate Republicans in turning back relentless Democratic efforts to modify the Reagan program, gave insurgent Democratic candidates a lot to shoot at.

While they had lost on nearly every economic issue in Congress, Democrats had forced the Republicans in vote after vote to support cuts in popular programs, from child immunization to veterans benefits and Social Security.

In many cases, Republicans dealt with the Democratic campaign barrage on these issues by stressing occasions when they had departed from the Reagan script and by emphasizing their independence over their loyalty.

Several embattled Republican incumbents from the East and Midwest made it known they would appreciate being skipped when the president's campaign schedule was made up.

Republicans also countered with a contention that, bad as economic conditions might be, they would be made worse by a return to what they called the high-spending, high-taxing, big-government policies of the Democratic past.

Few Democrats argued for a return to pre-1980 tax and spending policies, however. Mainly, they criticized the Republican record of the last two years.

When the Democrats did offer alternatives, they were usually in the nature of mid-course corrections, varying from modest to drastic, but still stopping short of massive government intervention to bail out the economy.

Adopting a theme that was once the almost exclusive preserve of Republicans, Democrats pounded on Reagan's big budget deficits and called for economies, mainly in defense.

Republicans were in the theme-borrowing business, too, as they joined Democrats in calling for some moderation in Reagan's expensive military buildup in order to help get budget deficits down.

Partly because some senior Democrats were caught off guard two years ago, even some of the presumably safest Republican incumbents campaigned as though their races were closer than the polls showed them to be.

Negative campaigning, usually employed by challengers to tear down an incumbent's record, became almost universal.