More than ever, the candidates in this year's election campaigns have presented themselves by putting their best fist forward.

The rule of political etiquette this year: if you can't say something nice about your opponent, say something scurrilous:

* In Tennessee, GOP Senate candidate Robin Beard tried to depict Sen. Jim Sasser as soft on Castro by showing an actor made up to look like Castro light his cigar with an American bill and sneeringly say, "Muchas gracias, Senor Sasser."

In Massachusetts, Republican Rep. Margaret Heckler attacked Rep. Barney Frank, her opponent in a hard-fought House race, by running an ad that showed a picture of Frank with the words "Prostitution" and "Pornography" emblazoned across his face.

* In California, Gov. Edmund G. (Jerry) Brown Jr., the Democratic candidate for the Senate, attempted to portray his Republican opponent, San Diego Mayor Pete Wilson, as opposed to a freeze of nuclear weapons by juxtaposing a film clip of a nuclear explosion with shots of a child saying, "I want to go on living."

Negative political advertising is the standard fare nation-wide. The word from national Republican and Democratic campaign strategists went out early to the candidates: attack. Negative ads work. This is partly because they match the national mood.

"The country is basically down," said New York media expert David Garth. "People are mad at the politicians, and taxes, and the budget, and unemployment. But the question for all of us is how you use the negative spots--and when do you go into overkill?"

A memo distributed last June by national Republican officials to GOP candidates advised: "The candidate who attacks generates a positive vote intention on the electorate to vote for the attacking candidate." Democrats got similar counsel from their party strategists.

"One of the very significant things this year is that we have seen much more negative advertising by incumbents who were already running ahead of their opponents," said Jay Bryant, of Bishop, Bryant & Associates, co-author of the memo with pollster Lance Tarrance. They called it "inoculation advertising" in their memo.

"This had always been considered bad strategy before. Now I think that has been changed forever."

Adds Democratic pollster Patrick Caddell: "They the Republicans used negative advertising against us a bit in 1978--and it worked. And then we used it against them a bit more in 1980 Jimmy Carter vs. Edward M. Kennedy -- and it worked. And now everybody is using it in 1982."

Now that the floodgates are open, the pros are watching to see who survives because of negative advertising and who sinks because of it. So far the results are mixed.

The moderate and proper Heckler set out to have Frank banned in Boston. One of her television ads shows a picture of Frank and a voice says he voted to allow legalized prostitution and pornography in every town in the state. Across his face are the words "Prostitution" and "Pornography." Frank backed a bill that sought to keep prostitution and pornography from spreading into residential neighborhoods in every town by limiting adult entertainment operations to a single "combat zone."

Frank counterattacked with a television ad that also featured his picture. A voice says: "Barney Frank knows that your image gets messed up at election time . . ." -- and a hand pencils a mustache and goatee on him -- ". . . but this is going too far." Soon the face is a mess of red magic marker.

"There is a word for that kind of tactic," the voice continues. The screen finally was filled with the single word: "Smear."

Heckler trails Frank by 11 percentage points in the latest Boston Globe poll.

In California, Brown's controversial ad sought to depict his Senate race against Wilson, a moderate Republican, as a matter of life or death with Brown being on the side of life.

It ends with the voice-over: "Pete Wilson opposes the nuclear arms freeze. Jerry Brown supports it. Vote for your life. Elect Jerry Brown to the U.S. Senate."

In 1964, Lyndon B. Johnson ignited an explosion of outrage with a similar television ad depicting a girl picking daisies as a countdown ticks off and culminates in a nuclear mushroom cloud. It was intended to show what Johnson's Republican opponent, Barry Goldwater, might do as president. The ad was yanked after one showing. Brown's ad lasted about a week.

In a number of states, the tone of negative ads was set not by Republicans or Democrats, but by such conservative ideological organizations as the National Conservative Poltical Action Committee (NCPAC).

In Montana, NCPAC aired 1,000 commercials charging that Sen. John Melcher, a moderate Democrat who voted with Reagan on his tax and budget cuts, is "out of step with Montana."

Melcher lost a 27-point lead and trailed Republican Larry Williams until he cut an ad that had two cows do the talking for him. The cows tell the voters that outsiders have been coming into the state trying to influence the race, but that they have been "stepping in what they have been trying to sell." Melcher's talking cows helped carry him back into the lead.

Democrats have made Social Security, Reaganomics and double-digit unemployment their major themes. An ad circulated to Democratic candidates by the national committee shows a scissors snipping a Social Security card into tiny pieces, suggesting that Republicans are trying to destroy the program.

In Ohio, Rep. Clarence J. Brown, the Republican gubernatorial candidate, sought to deflect these issues by running ads accusing Democrat Richard Celeste of having voted, as a state legislator, for handgun control and advocating policies that would have doubled the state income tax.

Celeste, who retains a significant lead, responded with an ad saying that Brown showed up to vote on only 20 percent of the roll calls in Congress this year. Only eight congressmen in history have had so poor a record, the ad says -- "and seven of the eight were sick, dead, or indicted."

The Republicans' favorite theme is to try to tar their opponents with the traditional charge that Democrats are big spenders. Many have been effective, but there have been instances of wretched excess such as Beard's ads against Sasser in Tennessee.

One depicted a wooden crate labeled "Foreign Aid" with hands grabbing stacks of money from it as a voice says: "When it comes to spending taxpayers' money, Sen. James Sasser is a master. Take foreign aid. While important programs are being cut here at home, Sasser has voted to allow foreign aid to be sent to committed enemies of our country. Vietnam, Laos, Cambodia, Marxist Angola and even Communist Cuba. You can bet James Sasser is making a lot more friends abroad than here in Tennessee."

An actor portraying Fidel Castro lights a cigar with a $100 bill and sneeringly laughs: "Muchas gracias Senor Sasser."

The ad was pulled, but not before The Nashville Banner withdrew its endorsement of Beard.

"The Beard stuff will live in history," said Bob Squier, who does media work for Sasser and other Democratic candidates. "That commercial cost that young man 10 points. We had him beaten, but that ad did him in."

A benchmark of sorts has occurred in the Virginia 8th Congressional District race, where former Democratic representative Herbert Harris is trying for a comeback against the man who beat him two years ago, Rep. Stanford Parris.

Each has filled the local television screens with unflattering accounts of his opponent's record. The result: Harris and Parris both lost support. Parris' pollster, Lance Tarrance, attributed the decline of both candidates to their negative ads.

A wry way of dealing with such tactics was crafted by the Buckley-Rothstein political consulting firm for a quiet Democratic incumbent, Sen. Quentin N. Burdick of North Dakota, who was charged with being a big spender.

Burdick's responding television ad opened with a voice: "Some people say this man is a big spender. Quentin Burdick a big spender?"

This was followed by a videotape of Republican Gov. Allen I. Olsen, ridiculing Burdick at a roast: "The fact is he's cheap. When [President] Reagan went after the school lunch program, you thought our Democratic senator would be concerned. He was worried about where he was going to have lunch next week. Quentin has viewed Jack Benny as a role model throughout his life and he's the only guy who thought Scrooge was really on the right track."

Not all the negative ammunition has been fired at members of the other party.

In New York's hotly contested Democratic gubernatorial primary, Mayor Edward Koch put some heavy fire on Lt. Gov. Mario Cuomo, who won the primary. One, designed by Garth, began by telling viewers they had 20 seconds to think of a single thing Cuomo had done as lieutenant governor. What followed was a clock that ticked away the 20 seconds for what seemed like an eternity.

Garth has had second thoughts.

"The ticking clock was based on fact," he contended. "But it certainly was a heavy spot -- one that might have gone across the border as being too heavy."

That may also have been true of the latest ads for Republican Sen. Harrison H. Schmitt of New Mexico by GOP media expert Roger Ailes. Schmitt watched his once-commanding lead over state Attorney General Jeff Bingaman dwindle to nothing.

Schmitt aired an Ailes ad charging that Bingaman played a major role in "freeing a convicted felon" by recommending a "pardon" for a man who had been on the FBI's most-wanted list.

What it did not say was that the U.S. Justice Department had requested that the man be released in its custody as the key witness in another trial, for the murder of a judge in Texas. When the governor requested a legal opinion on whether this could be done, Bingaman reported that a conditional parole -- not a pardon -- could be arranged.

Ailes had based the Schmitt ad on an incorrect article in The Albuquerque Tribune, a story that the newspaper later retracted.

"There is nothing false in those spots," said Ailes. "We relied on a press report."

In another ad, Schmitt charged that Bingaman refused to prosecute inmates responsible for a prison riot in which 33 prisoners died. Actually, Bingaman had spoken out repeatedly at the time promising that the inmates would be prosecuted, but it was the task of local district attorneys.

"He Bingaman tried to take advantage of the New Mexico prison riot for his own political advantage," said Ailes. "So it's fair game."

What would Ailes be saying if he were on the other side of the ads?

"Oh, I'd scream foul," Ailes quickly replied. "But my responsibility ends with the act. Maybe folks can say I'm an unethical guy. But it's not my job to make his Democrat Bingaman's case."

Charles Guggenheim, who authored some mild media for Sen. John C. Danforth (R-Mo.), said: "I don't like negative ads and neither does the candidate." "But . . . we are beginning to get blips in our direction, since they've been running. It's awful that you have to do this."

He hesitated, and then added: "It's not a very good profession."