George McGovern (S.D.), Warren G. Magnuson (Wash.), John C. Culver (Iowa) and Birch Bayh (Ind.) last night and threatened enough other liberal Democrats to take control of the Senate for the first time in 26 years.

Democrats currently control 59 of the 100 Senate seats, and Republicans were winning or leading in races for seats now held by 8 Democrats, including 7 where incumbents were seeking reelection.

A loss of nine seats would create a tie that could be broken by Republican Vice President-elect George Bush, who will preside over the Senate. Also Sen. Harry F. Byrd Jr. (Ind.-Va), who has caucused with the Democrats but supported Ronald Reagan, could switch to the GOP.

In a major victory for conservative Republicans, Rep. James Abdnor beat McGovern, the Democratic presidential nominee in 1972, and Rep. Dan Quayle trounced Bayh, chairman of the Senate Intelligence Committee. Washington state Attorney General Slade Gorton, a moderate, unseated Magnuson, the senior Democrat in the next Senate and its president pro tempore as well as chairman of the Appropriations Committee. Rep. Charles Grassley (R-Iowa), a conservative, knocked out Culver.

In Florida, Republican Paula Hawkins defeated Democrat Bill Gunter in the race to succeed Sen. Richard Stone (D-Fla.), whom Gunter unseated in the state's Democratic primary. Hawkins will become the second woman in the Senate, joining Nancy Kassebaum (R-Kan.).

Republicans, mostly conservative, were also leading by varying margins in races against Sens. Frank Church (D-Idaho), the chairman of the Senate Foreign Relations Committee, and John Durkin (D-N.H.). Sen. Gary Hart (D-Colo.) was running narrowly ahead, as was Sen. Patrick J. Leahy (D-Vt.). Caught in tight races were Sens. Robert MORGAN (D-N.C.) and Gaylord Nelson (D-Wis.).

Republican Jeremiah Denton, a retired admiral and former Vietnam prisoner of war, defeated Democrat Jim Folsom to take the Democratic seat now held by Sen. Donald Stewart (D-Ala.), who lost in a primary. Denton was one of several GOP winners who had the support of the Moral Majority and other groups from the New Right.

Republicans appeared to be fending off challenges by Democrats in New York -- where Rep. Elizabeth Holtzman was trailing Republican Alfonse D'Amato -- and in Oklahoma and Pennsylvania.

The only Republican incumbent who appeared to be in trouble was Arizona Sen. Barry Goldwater, the GOP's 1964 presidential nominee, who was running slightly behind his challenger, Bill Schulz, a moderately conservative Democrat.

One Democratic liberal who was doing well was Senate Majority Whip Alan Cranston (D-Calif.), who was projected as the winner in his race by NBC. Also running ahead was Thomas F. Eagleton (D-Mo.).

Still to be heard from Alaska, where Democrat Clark Gruening and Republican Frank Murkowski were expected to run close in their battle to succeed Sen. Mike Gravel (d), who was beaten by Gruening in the Democratic primary.

Most moderate-to-conservative Democrats appeared to be holding onto their seats based on initial returns from yesterday's voting for Senate seats in 34 states, but the big losses from the party's liberal wing gave the GOP a better chance than the Republicans themselves predicted only a few days ago.

Among Democrats, Sen. Wendell Ford (D-Ky.) and John Glenn (D-Ohio) won reelection handily. Senate Budget Committee Chairman Ernest F. Hollings (D-S.C.) captured a thrid term. Agricultural Committee Chairman Herman E. Talmadge (D-Ga.), reprimanded by the Senate for financial irregularities a year ago, was running well ahead. So was Dale Bumpers (D-Ark.).

Rep. Christopher Dodd (D-Conn.) won the race to succeed retiring Sen. Abraham Ribicoff (D-Conn.). Illinois Secretary of State Alan Dixon (d) led Lt. Gov. David O'Neal in the fight to take over the seat being vacated by Sen. Adlai E. Stevenson (D-Ill.).

Among Republicans, Sen. Charles Mathias (R-Md.) won a third term, as did Sen. Bob Dole (R-Kan.). For the seat being vacated by Sen. Henry Bellmon (R-Okla.), Republican state Sen. Don Nickles defeated Democrat Andy Coats. For the seat of retiring Sen. Richard Schweiker (R-Pa.), former Philadelphia district attorney Arlen Specter (R) was projected the victor over former Pittsburgh Mayor Pete Flaherty.

In North Dakota, Rep. Mark Andrews (R) took the seat being relinquished by Sen. Milton R. Young (R).

Whatever the party-line totals, it appeared that the Senate would take on a considerably more conservative outlook because many of the leading Republican contenders are strong conservatives and the imperiled Democrats come primarily from the liberal wing of their party.

Republicans had a big advantage from the start because they were defending only 10 of the 34 seats at stake in the 1980 senatorial elections, mostly from staunchly GOP states.

The Democrats, on the other hand, were forced to defend 24 seats, many of them from swing or even Republican-leaning states that had gone Democratic in 1974 in the reaction to Watergate. Several of the states, including Iowa, New Hampshire and Colorado, ousted other Democratic senators just two years ago.

As a party, the Republicans outspent the Democrats by about 6 to 1 -- $6 million as opposed to less than $1 million -- leaving the Democratic candidates largely to forage for themselves, although many of them, aided by organized labor and their own devices, did so with lucrative results.

A half-dozen of the most conservative Republican challengers also were helped, at least at the start, by a barrage of strictly negative advertising against their incumbent foes by the National Conservative Political Action Committee, anti-abortion groups and other components of the so-called New Right.

Chief among the New Right's targets were McGovern, Church, Bayh and freshman Culver -- all of them liberals who were considered among the most vulnerable incumbents as the campaign came to a close.