They have already taken one enormous political blow this campaign, at the hands of cartographers, census-takers, judges and statehouse pols.

Now they are pressed to avoid the ultimate beating, at the polls on Nov. 2. Exactly half will succeed.

They are the 12 House members who, because of the vagaries -- and vengeances -- of the redrawing of their states' congressional districts to conform to the population changes of the 1980 census, were forced to decide to run against other incumbents.

In their six contests, stretching from California to Massachusetts, the Democratic incumbents have taken the offensive, trying to wed their opponents to the Reagan economic program and emphasize its failures.

Rep. Barney Frank, a liberal Democrat from Massachusetts, strolled down the aisles of Congress during the last session whistling, "Me and My Shadow." This was a gibe at his opponent, Republican Rep. Margaret M. Heckler, who in 1981 was gung-ho for the Reagan program.

When California's irrepressible John H. Rousselot (R), one of the most staunchly conservative politicians in the country, found himself the victim of a Democratic redistricting plan that forced him to decide to run in a heavily Hispanic district, he did what any self-respecting House member in his position would do. He promptly joined the Hispanic Congressional Caucus and set out to become more Hispanic than his opponent, a bona fide Mexican American, Rep. Matthew G. (Marty) Martinez.

When Frank, who is a member of the House Select Committee on Aging, scheduled a committee hearing in mid-August in the new part of his and Heckler's redrawn district to discuss inequities in the Social Security system, hopefully at Heckler's expense, she was not to be outdone.

The night before the hearing, she persuaded a fellow Republican to resign his seat on the committee and let her take it. When Frank gaveled the hearing to order the next morning, who was seated at his side?

There is no end to the insider's jockeying. Incumbents best know the foibles and weak spots of other incumbents, they know the procedural votes, the embarrassing inconsistencies, the glories of pulling things out of context.

Frank, for example, has made a big issue of Heckler's support of something called the Notch Act, a part of the Social Security Act of 1977 that Frank claims is inequitable to people born after 1917. He neglects to point out that the entire Democratic leadership of the House, from Speaker Thomas P. (Tip) O'Neill Jr. and Aging Committee Chairman Claude Pepper on down, supported the same provision.

But Heckler is taking no chances. Now she, too, is for the repeal of the Notch Act.

Heckler, in turn, has ads contending that Frank has voted to reduce the penalties against rapists. South Dakota Rep. Clint Roberts (R) makes similar charges against the incumbent he is opposing, Rep. Thomas A. Daschle (D).

Indeed, they did vote that way, but the Republicans neglect to note that the bill they voted on had to do with changes in the District of Columbia criminal code that were fully supported by women's groups and law enforcement officials as a way to increase convictions.

Since 1954, 92 percent of all House members seeking reelection have won. The figure for Senate races, where big-picture issues come more readily in focus, is 80 percent over the same period; in the three elections since 1976, it has fallen to 62 percent.

One reason for this is the incumbents' natural edge in fund-raising; in these races both candidates are magnets for money.

The showdown between Heckler and Frank is expected to be one of the most heavily funded in the country, with Heckler raising close to $750,000 and Frank more than $1 million.

Daschle has tried to force Roberts to give back big-business contributions. Roberts has refused. Both candidates are receiving heavy out-of-state contributions.

Some Republicans, where voter sentiment is hospitable, have joined the battle on the Democrats' terms and defend Reaganomics. More, however, they are bobbing and weaving, trying to play instead the more familiar games of personality, constituent service and seniority.

Some of the races are plain old-fashioned turf wars.

In New York, which lost two seats to reapportionment, Rep. Guy V. Molinari (R) brings to the race his Staten Island base while Rep. Leo C. Zeferetti (D) hails from the Bay Ridge section of Brooklyn. The numbers of the joined district favor Molinari, who had represented 70 percent of it before. Still, to buttress himself against an attack on Reaganomics, the Republican took the precaution of voting to override the president's veto of the 1982 supplemental appropriations bill.

Just north of New York City, in what the state legislature dubbed the "fair fight" district, Democratic Rep. Peter A. Peyser wants to talk about the nuclear freeze and the economy, while Republican Rep. Benjamin A. Gilman, who has represented more of the district, prefers to stress constituent service.

And in Missouri, Democratic Rep. Ike Skelton likes to lambast Reaganomics, while the Republican, Rep. Wendell Bailey, talks about the now repealed tax deduction Congress voted itself. Bailey didn't take advantage of it; Skelton did.