Two formidable members of the House of Representatives who were forced by redistricting to run against each other in the newly redrawn congressional district here have been beating each other up this fall in a thumping, high stakes, big money, heavy media political battle over -- what?

Define the key issue of this race and you've probably declared the winner.

To freshman Rep. Barney Frank (D), witty, wisecracking and remorselessly liberal, the fight is all about the devastating impact of Reaganomics, pure and simple.

In the district's biggest city and foremost battleground, the depressed garment town of Fall River, Frank is counting on a heavy anti-Reagan vote from an ethnic, blue collar and 13.4 percent unemployed constituency.

To his Republican opponent, Rep. Margaret M. Heckler, a feisty eight-term veteran and the most senior woman in Congress, the real issue in this campaign is her exemplary constituent service contrasted to Frank's "harebrained" record as a state legislator. He took positions then, she informs the voters, that were soft on prostitution, pornography and crime, disrespectful of the flag and tax happy.

The Heckler media assault began 10 days ago, and Frank, who considers it an outrageous smear, thinks it will fail.

"When unemployment is over 10 percent, you're insulting the voters by harping on the way I wanted to zone Commonwealth Avenue," he says, a reference to his controversial proposal in the mid-1970s to create a "Combat Zone" in Boston's tenderloin district. It was intended to halt the spread of prostitution and adult book shops onto Commonwealth Avenue and into the other residential neighborhoods he represented.

But Heckler, who watched her once substantial lead in the polls turn into a small deficit by early fall under Frank's blistering attack on Reaganomics, believes Frank has a glass political jaw.

"His state house record is a political consultant's dream," says Elliot Curson, the media specialist who put together Heckler's TV ads. "It might get Frank elected in Back Bay or San Francisco, but nowhere else I can think of."

"The whole strategy is to take the heat off Margaret," says Heckler press secretary Joshua Reznek. "It's designed to keep Frank from marrying her to Reagan."

But marry he does. At a debate here earlier this week, Frank, the beefy, disheveled son of a New Jersey truck stop operator, took every opportunity to remind voters that Heckler went down the line for President Reagan in 1981, and only started putting some distance between herself and the president in 1982 when she knew she would be facing Frank.

"I haven't had such a good influence on someone since my kid brother got too old to listen," quips Frank, who takes credit for influencing Heckler's votes this year on the balanced-budget amendment, the MX missile and the 1982 budget.

Heckler fires back that she is "not a Reagan clone. . . . I've served under five presidents, unbossed and unbought."

She ticks off her service to the district and reminds the audience that Frank, in one of those gibes that have sometimes been costly to him, once referred to constituent service as "slopping the hogs."

"Just what is the slop?" Heckler asked, her Irish magnificently up, "and who are the hogs?"

The arithmetic of the newly created district cuts both ways. Heckler represented 70 percent of the people in the district before redistricting, Frank only 26 percent. But there are nearly three times as many Democrats as Republicans.

When the new lines were drawn, Frank complained bitterly that he was victimized by fellow Democrats who were settling old scores, but since the first of the year he has been a tireless campaigner.

He is a colorful candidate and a fund-raiser with few equals. His war chest will exceed $1 million, making this one of the rare races this year where the Democrat will be better financed than the Republican.

Still, the issue here isn't money -- both have plenty. The issue will be Reaganomics -- or "slopping the hogs"?