At the midpoint in a week of nonstop campaigning 21 months before what is expected to be a tough reelection showdown, freshman Sen. Paula Hawkins (R-Fla.) was bouncing in a light plane over the sprawling new subdivisions of central Florida and reading a letter from a constituent.

"Look," she said, "they say I should support the president, but don't cut my Social Security, my Medicare, my veterans' benefits . . . . It washes off the president like water, it doesn't stick . . . . But what about Paula Hawkins? I'm concerned about that."

Hawkins is one of the raw nerve ends of American politics, one of 22 Republican senators facing reelection in 1986. She is part of the class of Republican senators swept into power with Ronald Reagan in 1980 and now the target of a Democratic drive to regain the majority in the Senate.

A few months ago, Democrats and Republicans alike predicted that the GOP would have trouble holding on to the Senate in 1986. With just 12 Democrats facing reelection, with many of the Republican "Class of 1980" lightly regarded, with the GOP majority 53 to 47, the numbers appeared to favor the Democrats.

The 1986 contests take on special significance because they are the Democrats' best chance to recapture the Senate from the GOP and begin a rebuilding process.

"They're playing for a decade," said Peter D. Hart, a Democratic pollster. "If the Democrats don't carry the Senate in 1986, they've definitely lost control for a decade -- 1980 to 1990 -- and possibly for 15 years."

Senate Majority Leader Robert J. Dole (R-Kan.), himself one of the 22 facing reelection next year, agreed. "This is the watershed year, in my view," he said. "If we are in fact the emerging majority and we can get through this one rough spot . . . then I think we're in good shape for a decade or more."

Despite the Democrats' numerical advantage in the 1986 elections, the outlook today has brightened for the GOP, and one reason is a skillful Republican political balancing.

For instance, when President Reagan sent his fiscal 1986 budget to Congress and gave his State of the Union address a few days later, two of the Senate GOP first-termers quickly scheduled news conferences in their home states.

Sen. Robert W. Kasten Jr. (R-Wis.) decided to take advantage of Reagan's strong endorsement of tax simplification by flying around snow-covered Wisconsin to promote his own sponsorship, with Rep. Jack Kemp (R-N.Y.), of one of the principal tax-revision bills in Congress.

Sen. Arlen Specter (R-Pa.), on the other hand, made a hasty trip to Pittsburgh the morning after Reagan's speech to denounce the administration's proposed budget cuts. "I told them, hold on, we're going to fight this," Specter said.

"It was a reasonable assumption in 1980 that we were a bunch of people who'd only be around for six years," said Sen. Dan Quayle (R-Ind.). "But we dug in, did our homework, raised a lot of money, scared off potential opponents . . . . It comes down to the economy. If the economy holds up, we're in good shape."

Said Kasten: "A lot of us feel more optimistic. I know I do."

Adding to this feeling is the belief that enough vulnerable Democrats are up for reelection in 1986 to offset minimal GOP losses, and the assumption that the Democratic Party's internal problems will make it harder for them to mount an effective campaign.

But before those elections arrive, anything could happen: rising interest rates, a declining economy, a surge of inflation, even a falloff in Reagan's popularity. The Republicans facing reelection see the pitfalls, especially in potentially unpopular votes on Reagan's budget. As such, they are putting distance between themselves and the least popular elements of Reagan's program, and paying close attention to their states. In Florida, a Tossup?

Hawkins is a good example.

Facing a probable challenge from Florida's popular two-term governor, Democrat Robert Graham, she is regarded by her colleagues and many political strategists as perhaps the most vulnerable of the 22.

But with Reagan's extraordinary popularity in Florida, the steady growth of the GOP in the once staunchly Democratic state and Hawkins' own feisty, maverick style, both Republican and Democratic strategists say the race ultimately could be a tossup.

In Washington, Hawkins is viewed as a conservative with a penchant for embarrassing missteps, such as holding a gourmet luncheon to launch a crackdown on food-stamp cheats.

But, in Florida, old-timers remember her as a hard-hitting consumer advocate as a state public service commissioner, and newcomers see her regularly on television championing causes ranging from drug control to missing children and organ transplants for infants.

She has returned to Florida almost every weekend since her election in 1980, and her schedule for last week's congressional recess read like a hectic campaign windup week, with days stretching to 15 hours or more.

Although she is expected to call Graham a liberal in conservative camouflage, Hawkins tends to leave her ideology behind when she is seeking legislative largess for Florida.

While extolling the proposed balanced-budget constitutional amendment, she has resisted some of the largest budget-reduction proposals and added a few spending proposals of her own, such as $60 million in matching funds to restore Florida's storm-ravaged east coast beaches. Although fond of saying the government "can't run a two-car funeral," she is pushing legislation to use government computers as a clearinghouse for organ donations.

And though she basks in the glow of Reagan's popularity, she stays a comfortable distance from the flames.

"I think it's crazy," Hawkins said of the president's proposals for cuts in Medicare and veterans' health care and his willingness to look at proposals for freezing Social Security benefits. "He won't get this senator . . . . I'm voting for my constituents."

She worries that Senate Republican leaders are trying to outdo Reagan on spending cuts. "You've got to worry when the Democrats start saying you're acting responsibly," she said. Having It Both Ways

There are echoes of Hawkins' lament all across the country among the "Gang of 22," from complaints of Sen. Alfonse M. D'Amato (R-N.Y.) about elimination of mass transit subsidies to protests of Sen. James Abdnor (R-S.D.) about severe cutbacks in farm programs.

Abdnor, whose state is reeling under a farm credit crisis, was especially bitter. "A proposed 13 percent defense increase and a drastic farm and rural decrease is unthinkable," he said. "I'm against it."

Quayle may have summed it up best. "Everyone wants to have it both ways: to support Reagan and yet to distance themselves in case everything goes wrong . . . criticize some things and let it be widely known that you're doing so back home," he said.

Sen. Don Nickles (R-Okla.) also has applied this lesson to tax-revision proposals. Although one of the strongest Reagan loyalists in the Senate, Nickles has served notice at home that he will filibuster repeal of tax breaks for the oil and gas industry.

Quayle and some others fear that a restive, fractious Senate and an uncompromising lame-duck White House will lead to serious conflict between the two Republican centers of power in Washington, possibly even to stalemate.

But other first-termers, such as Sen. Slade Gorton (R-Wash.), figure that many of Reagan's proposals are so drastic that anything the Republican Senate majority does will look moderate by comparison.

"We'll be heroes if we get them half of what they ask in 1985," said Gorton, speaking of the constituency groups that will be fighting for more than Reagan is willing to give them.

"Anything will be greatly appreciated. What Reagan has done is hand us a ready-made issue to establish our own independence."

All of this worries Dole, who, as majority leader, is trying to assemble a deficit-reduction package of delicately balanced spending cuts that could easily crumble if one piece is pulled out.

But Dole also contends that a budget package that achieves his goal of halving deficits to $100 billion by 1988 will begin to pay off quickly in political terms in lower interest rates and other boons to the economy, an argument he uses in part to keep his troops in line.

Only if the effort fails will Republicans suffer a political fallout, he contends.

Although there is some dissension over the hoops that Dole is asking his flock to jump through, many of those running in 1986 characterize his aggressive, independent style of leadership as an asset -- an "extra insurance policy," as one said.

"If Dole keeps on running a good shop, it's bound to help us in '86," said Sen. Charles E. Grassley (R-Iowa), one of the more independent-minded members of the class. No Longer a Long Shot

The twin-engine plane has just landed in Eau Claire, Wis., after hopping from airport to airport across the sunny, frozen landscape of the Midwest. Out hops Kasten, who six years ago was gamely trying to make a political comback in a long-shot challenge against incumbent Sen. Gaylord Nelson (D-Wis.).

Few Democrats took Kasten seriously at the time, and even after he defeated Nelson, he was immediately put on the target list for 1986. Today, a prominent Wisconsin Democrat said, "It's going to be very, very tough for the Democrats to beat him."

"He's doing the politically correct things," the Democratic leader added. "When he should take off after Ronnie, he does. He's taking off after Agriculture Secretary John Block now . . . . Any time a contract is let for the state, his name is in the headlines."

Eau Claire is Kasten's fourth stop of the day, each a brief airport news conference to promote tax revision and his role in it, attack the Pentagon for its budget request and, where appropriate, beat up on Office of Management and Budget Director David A. Stockman on the farm credit issue.

On tax revision, Kasten sounds a populist theme, warning that "the special interests are digging in in Washington."

He calls for the public to "stand up and make their voices heard -- just as they did on 10 percent withholding a year ago," a reference to his leadership in the effort to repeal the automatic withholding of taxes on interest in savings accounts.

Kasten has helped bring money to Democratic-controlled Milwaukee, and as chairman of the Senate Appropriations subcommittee on foreign operations has been an ardent supporter of Israel.

Most recently, he became a co-sponsor with Sen. Edward M. Kennedy (D-Mass.) of a famine relief bill.

Kasten still may face a difficult campaign, especially if Gov. Anthony S. Earl (D) decides to run against him.

But, despite a period of bad news -- a former business partner of Kasten's went to jail -- and earlier Democratic assumptions that he was an easy target, the junior senator from Wisconsin looks more formidable than once thought. Democrats Reassess Odds

The same might be said for other Republicans facing reelection in 1986.

"I still think we've got a good chance of picking up the Senate, but it ain't a cakewalk," Democratic pollster William Hamilton said.

Said pollster Hart: "I think a lot of the easy seats the Democrats hoped to pick up aren't going to be that easy. A lot of these 'coattail' winners of 1980 have done their homework and are not as vulnerable in their home states as they look from Washington."

Republican pollster Robert Teeter said the perception of the Class of 1980 as little more than Reagan clones never was correct. "A lot of these guys laid away some pretty good senators," he said. "They didn't win on a fluke. It wasn't that they had an open seat and drew a weak opponent."

Kasten beat Nelson, Grassley defeated John C. Culver, Abdnor beat George McGovern, Sen. Steve Symms (R-Idaho) upset Frank Church, Sen. Mack Mattingly (R-Ga.) squeaked past Herman E. Talmadge, and D'Amato defeated Jacob K. Javits in the Republican primary.

Right now, the Democratic hit lists include Sen. Jeremiah Denton (R-Ala.), Hawkins, Mattingly, Symms, Sen. John P. East (R-N.C.), Nickles, Specter, Abdnor, Kasten and the open seat in Arizona that will be created by the retirement of Sen. Barry Goldwater (R-Ariz.).

Other potential targets are D'Amato, Sen. Charles McC. Mathias Jr. (R-Md.) and Sen. Mark Andrews (R-N.D.). If Sen. Paul Laxalt (R-Nev.) were to retire, Democrats say they could take his seat.

"The Class of 1986 is a strong class," said Sen. John Heinz (R-Pa.), chairman of the National Republican Senatorial Committee, "and, while it's possible one or two might lose, I don't see anything like the losses or vulnerabilities that get talked about. What I don't see talked about is the remarkably good opportunity to pick up seats."

Heinz and other Republicans include on that list Sen. Alan Cranston (D-Calif.), whose unsuccessful presidential campaign in 1984 may have weakened him at home, Sen. Patrick J. Leahy (D-Vt.) and Missouri, where Sen. Thomas F. Eagleton (D-Mo.), is retiring and the Republican candidate is expected to be ex-governor Christopher S. (Kit) Bond.

In addition, Republicans hope to capture the Senate seat in Colorado if Sen. Gary Hart (D-Colo.) decides to forgo reelection for a 1988 presidential campaign.

Democrats argue that the numbers and history are on their side.

"The numbers are with us," said Sen. George J. Mitchell (D-Maine), chairman of the Democratic Senatorial Campaign Committee. "Of 34 seats up in 1986, 22 are Republican. Of those 22, 16 are completing their first term and a different 16 won in 1980 with less than 55 percent of the vote."

Not in more than half a century have there been as many Republican senators facing reelection.

Democrats also hope that history will repeat -- in their favor.

Historically, the party holding the White House suffers losses in midterm elections, and especially so six years into a two-term presidency.

GOP officials say that another trend may be working in their favor: the tendency of voters to stick with Senate incumbents. In the late 1970s, incumbency often proved a handicap. In 1982 and 1984, almost 90 percent of incumbent senators won reelection bids. Minding the Store

"Don't drive away," D'Amato yelled to his driver. "Don't, don't, don't."

The junior senator from New York was locked in traffic in midtown Manhattan, and a free-lance windshield washer approached the car.

D'Amato's driver tried to avoid the spirit of entrepreneurship, but the senator, as has marked his tenure in office, preferred to reach out.

"Give him a quarter," D'Amato ordered. "Don't drive away."

Heinz calls D'Amato "arguably our strongest senator," a characterization that many Democrats dispute. Still, they grudgingly praise him for the job he has done.

Critics have labeled D'Amato a right-wing ideologue, a demagogue, an opportunist and a politician willing to do almost anything for publicity. But even Democrats say he has been skillful in creating an image as a fighter for New York.

He also has been skillful in expanding beyond his narrow Conservative Party and Republican Party base in New York by appealing to blue-collar voters on crime and drug abuse and by his tireless courtship of traditionally Democratic Jewish voters.

He faces a potentially tough campaign, with possible opponents including former representatives Geraldine A. Ferraro and Elizabeth Holtzman, whom he defeated in 1980, but even Democrats talk pessimistically about their chances.

Many of his Republican colleagues have tried to secure themselves politically by acting almost like House members, with regular visits home, studious constituent service and grantsmanship for their states. D'Amato has taken it a step further.

"You know what he is?" a New York Democrat said. "He's an assemblyman."

Last week, D'Amato held a news conference to announce a federal grant to fix faulty subway cars. Earlier he had flown to upstate New York for ceremonies marking the arrival of an Army division -- and new jobs -- in the state.

Later, he attended a Lincoln Day dinner at the Waldorf-Astoria Hotel, then caught a late plane to Los Angeles for a day of fund-raising and a news conference to introduce a witness who said he saw alleged Nazi war criminal Josef Mengele in a U.S. POW camp after World War II.

D'Amato's next stop was a radio talk show. Returning from a commercial and preparing to take calls from listeners, the show's host said of his guest: "Alfonse D'Amato. Lights up the switchboards before the number is even given out. Let the pollsters and others put that into their campaign cheat sheets.''