Here in the climactic week of Political 1980, if much of the congressional campaign rhetoric sounds as though it comes from the same mold, there's a reason.

It does.

There may have been a time in American politics when originality was a hallmark. But today's television, videotapes, offset presses and marketing techniques that peddle candidates as easily as shampoo have fairly well doomed that side of congressional campaigning.

"This is a major television campaign, and there are only so many things you can do on TV," said Steve Sandler of the National Republican Congressional Committee. "So yes, you're getting a lot of the same messages."

Consider these tidbits:

The RNCC is doing close to 300 separate television spots for candidates, but many are identical, with space left for Candidate X to appear and give viewers the illusion of omniscience on a complex issue.

Many of the candidates have gone through campaign workshops, run by their parties or by political consultants, to teach them technique, scheduling, press relations, personal appearance.

Democratic House incumbents in Ohio and Iowa are opposed by challengers from remarkably similar backgrounds, whose positions on issues also sound remarkably alike. They were produced by the same Washington consulting firm.

Tucson radio listeners hear two incredulous voices discussing the record of Rep. Morris K. Udall (D-Ariz.). "He better explain this one," says one of the voices, which then reads the telephone number of Udall's office and urges calls to the man himself. Several thousand miles away, Rep. Al Ullman (D-Ore.) is getting the same treatment -- same issue, same statistics, same incredulous voices, same telephone call-in approach.

This little lever for getting under an incumbent's skin is the brainchild of the Murphy-Powers consulting firm in Los Angeles, handling publicity for the opponents of Udall and Ullman.

"We found the effect of the ads better in Arizona than in Oregon -- a helluva lot more effective -- because the incumbent took the bait and responded," said consultant Mark Barnes. "In Oregon the candidate had the presence of mind not to respond."

Udall campaign aides, taking the bait or not, thought the mock-sincere actors' voices in the ads of Republican Richard Huff were neither effective nor clever. "A few calls came in, but what's more offensive is the cynical packaging of the candidates," one said.

Sal Guzzetta of Campaign Management Associates, a Washington firm, pleads guilty to packaging, but he sees nothing at all cynical in giving candidates the same scripts. Guzzetta did it for four candidates, two of whom are still running. One withdrew and the other lost in a primary.

"There is not a member of Congress [actually, there are a number] who doesn't have someone putting out his press releases and statements," Guzzeta said. "If it is not wrong for one, why is it wrong for two, three or more candidates to use the same material? I don't see a problem with it."

Democratic Reps. Thomas L. Ashley (D-Ohio) and Berkley W. Bedell (D-Iowa), opposed by look-alike, sound-alike candidates from the Guzzetta stable, saw plenty of problems with it, earlier this month publicly blasting the canned material as the "cynical and arrogant" misleading of voters.

William J. Skow, Ashley's campaign coordinator, said that Republican "targeting" of Ashley "has meant outside-the-district money, outside candidate training, outside campaign consultants, outside polling, outside computer capabilities and outside campaign managers and staff."

Pish-posh, said Guzzetta. "That material had been around for a year and never was controversial," he said. "Three hundred political action committees in this area got my candidate campaign kits . . . . And Ashley's attack backfired. Our candidate's [Republican Ed Weber] poll went up the week Ashley attacked him. If we win, we intend to send Ashley a thank-you note."

In the area of "targeting" of incumbents, Rep. Bob Eckhardt (D-Tex.) is under siege from every angle of the right: school prayer, antibusing, anti-abortion, gun control, antiliberal -- you name it.

His opponent, Jack Fields, has strong support from the RNCC and the National Conservative Political Action Committee (NCPAC), a premier fund-raiser and targeter for New Right causes.

Last week one of the targeters became controversial in Houston. Mark Florio, a researcher here with NCPAC's sister Conservatives Against Liberal Legislation (CALL), sent a letter and a $5 campaign contribution to Eckhardt.

Florio's letter praised Eckhardt for his firm liberal stands on controversial issues, saying all the right things to inflame conservative passions.

Then he sent the letter and money to Fields' campaign office, where it was opened "by mistake." Fields' people forwarded Eckhardt's mail, but not before issuing a news release excoriating the congressman for his liberal campaign connections.

Bryan Wirwicz, Fields' press secretary, ruefully took credit for reading Eckhardt's mail. "I've learned a very nice lesson. I won't use other people's mail to put out press releases . . . we have apologized to Mr. Eckhardt," he said. Wirwicz refused to go further. "I am tired of answering questions," he said, and hung up the telephone.

Florio has declined to be interviewed about the matter, and his boss at CALL, Rhonda Stahlman, said Florio would not be disciplined for aiding liberals. "He has lost his cover with us," she said, "but I like him and I don't want to fire him. I was hurt more than anything else when I heard about this."

Fields, a candidate with no previous record, is getting considerable help from the RNCC. It analyzed Eckhardt's votes in Congress, counseled Fields to accuse the congressman of being "out of touch," and instructed him to talk about how he would have voted on similar issues, Sandler said.

"We've stressed that out-of-touch theme, with the economy issue under that," Sandler said. "But that has been out theme in many races -- economics, high unemployment, out-of-touch incumbents."

That message is heard across the country from GOP challengers, with such repetition that congressional district lines seem to blur and candidates' voices blend into one homogenized monotone.

"More than Republican platform ideas," Sandler said, "it is the polling that moves us in this direction. Economic issues overwhelm everything else. They are overriding this year."

Sandler, who oversees radio-TV projects for RNCC, agrees that the sameness of the challengers' messages is not just an illusion. "If you've got something that works, you plug parts of it into a lot of races," he said. "We have applied the man-on-the-street approach in many of these ads, just as Madison Avenue has done so successfully with beer and detergent."

"You see this over and over on TV, and it seems the same. The content may differ -- and we have done a number of ads on local issues -- but the technique is about the same. Ten years ago it was handshakes and yard signs. That all seemed the same, too."