Ronald Wilson Reagan won a landslide reelection victory as president of the United States yesterday, gaining a mandate for continuation of his conservative policies from voters all across America.

The 73-year-old incumbent, who promised in his campaign to seek further tax rate reductions and curbs on the growth of the federal government, defeated former Vice President Walter F. Mondale (D), carrying at least 49 states and winning the largest electoral college total in history.

By early this morning, Reagan had won every state but Minnesota and the District of Columbia, carrying 525 electoral votes and about 59 percent of the popular vote in what may have been a record voter turnout. In his 1980 race with President Jimmy Carter, Reagan lost six states and the District and had 51 percent of the votes and a 10-point margin in a three-way race involving independent candidate John B. Anderson.

Republicans lost two Senate seats in Iowa and Tennessee, while the races in Illinois and Kentucky appeared to be going against the incumbents, Sens. Charles H. Percy (R-Ill.) and Walter D. Huddleston (D-Ky.). GOP gains in the House appeared likely to fall somewhat short of recouping their 26-seat loss of 1982.

In giving the Republican Party its biggest back-to-back presidential victories since Dwight D. Eisenhower and Richard M. Nixon, Reagan assured Republican occupancy of the White House for 16 of the 20 years between 1969 and 1988.

The former California governor told home state supporters that the "prairie fire" of conservatism which they launched almost 20 years ago had gained new strength from the voters' overwhelming verdict, and he pledged to continue his economic policies until "the recovery is complete for everyone."

Savoring his last election victory, Reagan was greeted by the chant, "Four more years," which had been heard at every rally this fall.

"I think," he quipped, "that's just been arranged."

Earlier,Mondale told his supporters in the St. Paul, Minn., civic center and told them he had phoned Reagan with his congratulations.

Speaking of himself and his running-mate, Rep. Geraldine A. Ferraro (D-N.Y.), the first woman on a major-party ticket, Mondale said, "We didn't win, but we did make history."

The longtime battler for liberal causes told young people in the audience that "in every defeat are to be found the seeds of victory," and he urged them, as he pledged himself, to "fight on . . . for an America that is just and fair."

"I am," he said, "at peace with the knowledge that I gave it everything I've got. I'm confident history will judge us honorably."

Riding Reagan's popularity, Republicans assured themselves of continued, though diminished, control of the Senate for another two years. Their 1981-87 Senate majorities measure the GOP's longest span of dominance in that chamber since the party's heyday of the 1920s.

Sen. Jesse Helms (R-N.C.) won his record-spending fight for reelection over Gov. James B. Hunt Jr. (D), but a fellow-conservative, Sen. Roger Jepsen (R-Iowa), was defeated by Rep. Tom Harkin (D-Iowa.). The Democrats gained another seat when Rep. Albert Gore Jr. (D-Tenn.), as expected, claimed the seat left open by the retirement of Senate Majority Leader Howard H. Baker Jr. (R-Tenn.). In Kentucky, Jefferson County (Louisville) Judge Mitch McConnell (R) had a 4,000-vote lead over Huddleston. In Illinois, Percy trailed Rep. Paul Simon (D) by 4 percentage points with about three-fourths of the vote counted.

In the House, experts of both parties estimated Republican gains would total between 15 and 20 seats by the time the count is completed. The latter figure would bring them close to the point of restoring the conservative coalition majority of Republicans and southern Democrats that gave Reagan his landmark tax and budget victories in 1981.

The GOP also strengthened its minority position in the state capitols, gaining governorships in North Carolina, Rhode Island, Utah and West Virginia. Democrats gained governorships in North Dakota and Washington, with the Vermont race still undecided.

Sen. Paul Laxalt (R-Nev.), chairman of the Republican campaign, said, "We have got at least a reasonable chance to have the most historic landslide in all American history."

Democratic National Chairman Charles T. Manatt countered, "We are alive and well and coming back very soon."

The key to the victory for Reagan and the Republicans, according to an analysis of network exit polls, was the massive support they received from white, middle-class voters, who said they admired Reagan's personal leadership qualities and, equally, believed that a continuation of his economic policies will benefit them and the country.

Mondale's support was concentrated among low-income voters and racial and religious minorities -- notably blacks and Jews. His precedent-shattering selection of Ferraro as his running/mate for vice president was cited as an important voting factor by a significant slice of the voters interviewed in the exit polls, but the evidence was that she"may have cost the ticket more votes than she gained it.

Nonetheless, Ferraro told supporters in a New York City hotel that Mondale deserved praise for picking her.

"For more than two decades, Walter Mondale has forsaken personal gain for the public good," she said. "He stood up for people who need help and supported every decent cause in 25 years. For two centuries men have run for president but not one asked a woman to be his running mate until Walter Mondale. He opened a door that will never be closed again . . . . American women will never again be second class citizens."

Meantime, the Republicans continued to make progress in erasing their 50-year-old status as the second-place party in American politics.

ABC News exit polls showed that Tuesday's electorate -- swollen by the year-long voter registration efforts of both parties -- included 38 percent self-identified Democrats, 33 percent Republicans and 26 percent Independents. That is essentially identical to Election Day in 1980, but a significant shift from the 1960s cnd 1970s, when there were three self-identified Democrats for every two Republicans. It is explained, at least in part, by the support Reagan and the Republicans won from the youngest voters -- who shifted sharply in the president's direction this year.

An even stronger hint of a possible underlying alteration in the position of the two parties was found in responses to a question on which party the voters trust to do a better job in dealing with the main problems the country faces over the next few years. Republicans won that test by a 57-43 percent ratio, far and away their best showing in modern times.

That result is somewhat below Reagan's margin over Mondale, the difference probably reflecting the personal appeal of the incumbent and the relative weakness of his challenger.

In an interview with The Washington Post while awaiting the results in Los Angeles, the president said he and Vice President George Bush would give top priority in their second term to pressing arms control talks with the Soviet Union and securing two constitutional amendments aimed at reducing federal deficits -- one requiring a balanced budget and the other giving the president an "item-veto" power to reject specific appropriations items.

But Reagan gave no more hints than he had during his campaign about how he intends to address the deficit in the budget he sends Congress in January.

The vagueness of the mandate -- as compared to 1980, when Reagan was specific about the tax cuts and defense hikes he would seek -- did not bother the voters. Their verdict was clear and from all appearances came early.

ABC News said the late-deciders who made up their minds in the final week of the campaign went for Mondale by a 6-4 ratio. But they constituted only 10 percent of the electorate, and the others were equally strong for Reagan.

There was evidence of a gender gap, with women giving Reagan 7 percentage points less than men did. But he won majorities among both sexes, as he did among all age groups, all occupations, and all but a few economic and demographic groups.

Mondale carried voters from very-low-income families (under $10,000 a year), the unemployed, union households, Jews and -- overwhelmingly -- blacks.

Reagan's voters cited both his strength of leadership and their confidence in his economic and defense policies as important reasons for their votes. Mondale supporters were influenced by the "fairness" issue -- saying that they believed Reagan favored the rich while Mondale would be sympathetic to the problems of the poor.

Overall, foreign policy questions seemed to be of lesser importance to the voters, but one in five Mondale supporters cited his endorsement of the nuclear weapons freeze proposal as important to them.

In the Senate contests, Democrats were reelected with relative ease in Alabama, Arkansas, Delaware, Georgia, Louisiana, Michigan, Montana, Nebraska, New Jersey, Oklahoma and Rhode Island, while Republican incumbents coasted to another term in Alaska, Colorado, Idaho, Kansas, Maine, Minnesota, Mississippi, New Hampshire, New Mexico, Oregon, South Carolina, South Dakota, Virginia and Wyoming.

In Massachusetts, Lt. Gov. John F. Kerry (D) defeated conservative businessman Raymond Shamie (R) and gained the seat relinquished by retiring Sen. Paul E. Tsongas (D). In West Virginia, Gov. John D. (Jay) Rockefeller IV, (D) had more of a struggle than he expected before pulling away from businessman John R. Raese (R) for the seat of retiring Sen. Jennings Randolph (D).

In Texas, Rep. Phil Gramm (R), a recent convert from the Democratic Party, defeated state Sen. Lloyd Doggett (D) for the seat of retiring Sen. John G. Tower (R).

In the governors' races, Republicans gained a seat in North Carolina, where Rep. James G. Martin (R) defeated state attorney general Rufus Edmisten (D) for the post Hunt had held for"the past eight years. They picked up a second state house in Rhode Island, where Cranston Mayor Edward D. DiPrete (R) beat state Treasurer Anthony J. Solomon (D) and ended the Democrats' 16-year reign.

A third statehouse fell to the Republicans in West Virginia, where ex-governor Arch A. Moore Jr. (R) defeated state house speaker Clyde M. See Jr. (D) for the post that Rockefeller vacated. The fourth GOP pickup was in Utah, where state house speaker Norman H. Bangerter (R) defeated ex-Rep. Wayne Owens (D). That governorship had been in Democratic hands for 16 years.

Democratic pickups came in North Dakota, where state Rep. George Sinner (D) defeated Gov. Allen I. Olson (R) and in Washington, where Pierce County Executive Booth Gardner (D) beat Gov. John Spellman (R).

In Indiana, Gov. Robert D. Orr (R) won a second term, and New Hampshire Gov. John H. Sununu (R) also was reelected.

In Missouri, attorney general John Ashcroft (R) succeeded two-term incumbent Christopher (Kit) Bond (R).

In Delaware, Lt. Gov. Michael N. Castle (R) moved up to succeed two-term incumbent Pierre S. (Pete) duPont IV (R). DuPont's wife, Elise, lost her bid for a House seat to Rep. Thomas R. Carper (D).

Arkansas Gov. Bill Clinton (D) and Montana Gov. Ted Schwinden (D) kept their states in the Democratic column.

In early counting on the House races, five seats seemed possible Reublican pickups but two others now in the GOP column were trending the other way.

Although Mondale was even with Reagan in last January's Gallup Poll, the challenger had an uphill battle on his hands almost all the rest of the way.

Reagan became the first incumbent president to avoid a challenge to renomination within his party since Dwight D. Eisenhower in 1956. Mondale, on the other hand, started out as one of eight aspirants for the Democratic nomination and had a long, hard struggle to nail down the prize.

Mondale was the beneficiary of an unprecedented pre-primary endorsement by the AFL-CIO, gained the backing of the National Education Association, the National Organization for Women and other constituency groups and was the overwhelming choice of Democratic members of Congress. Still, he was almost knocked out of the fight in the early rounds.

Sen. Gary Hart (D-Colo.) upset Mondale in the Feb. 28 New Hampshire primary and the March 4 Maine caucuses, gaining immense publicity and momentum for the March 13 set of 11 primaries and caucuses. Although Hart won most of the races and delegates that day, too, Mondale salvaged primary victories in Alabama and Georgia -- just enough to keep him in the race.

As the fall campaign opened on Labor Day, Reagan found himself far enough ahead that he could adopt a no-risk strategy of making generalized, upbeat speeches at carefully staged rallies of the Republican faithful.

Mondale, who had made the politically risky proposal of a 1985 tax increase in his acceptance speech, in part an effort to bait Reagan into outlining how the Republicans planned to cut the deficit, carried on the high-risk strategy in the early days of the fall campaign.

He and Ferraro also campaigned heavily in Reagan's home state of California and in the South, despite polls showing that the incumbent was far in front in most of the Sun Belt states.

On the eve of the first televised presidential debate in Louisville on Oct. 7, Reagan had blunted every tactic Mondale had tried, and the Democrat clearly needed a major break to make it a race. That night, he seemed to get it. He put on a performance even more polished than the ones that had helped him in critical debate tests with Hart and Jackson. In contrast, Reagan appeared more fumbling, uncertain and ill-at-ease than most voters had ever seen him.

So surprising was Reagan's appearance that the issue of his age and mental acuity -- which had never surfaced before -- became a matter of public discussion. But the president rallied quickly in his campaign appearances and the polls indicated that while Mondale had gained perhaps 6 points, he was still 12 or more behind.

The two met for a last debate on Oct. 21, an encounter where Reagan appeared to hold his own. After that, his victory was secure.