Rep. Clement J. Zablocki (D- Wis.) is a son of Milwaukee's Polish South Side and Sen. Robert T. Stafford (R-Vt.) is a quintessential Yankee. But circumstances have made both part of an important political experiment, testing whether senior legislators unaccustomed to the techniques of TV politics can survive against modern, media-oriented foes.

What makes the test all the more intriguing is that Zablocki and Stafford have been saddled, by virtue of their seniority and leadership positions, with the added burden of defending Congress' reputation at a time when it is held in minimum high regard. The contests they face in next Tuesday's primaries may be a clue to the question whether incumbency is going to be a liability or an advantage to their colleagues in November.

The odds should be with them. Stafford has not lost an election in Vermont in 36 years. Zablocki has held his House seat since 1948. Moreover, both occupy power positions that should be of benefit at home. Stafford is the chairman of the Environment and Public Works Committee, with a major voice on issues from clean air legislation to highway construction. Zablocki is chairman of the Foreign Affairs Committee.

Yet both of them are facing unusually stiff nomination challenges this year, and neither is assured of victory. A number of circumstances contribute to their peril. At their ages -- both are 69 -- there is some feeling they should be ready to retire. Both happen to have opponents, a generation younger, who have been frustrated in past efforts to get into Congress.

Stafford's chief rival, businessman Stewart Ledbetter, barely failed in his 1980 bid to defeat Sen. Patrick J. Leahy (D-Vt.). Wisconsin state Sen. Lynn Adelman, Zablocki's foe, tried previously for the House in another district. Moreover, both challengers are candidates of the technological age, at home with computerized mailings and TV spots. They have forced their older rivals, who are comfortable with low-budget, hand-shaking campaigns, to fight with unfamiliar weapons.

Despite all that, the incumbents would still be solid favorites, were these man-on-man contests. What makes the races doubtful, and what gives them their special significance, is that Zablocki and Stafford are carrying the weight of Congress on their shoulders. To many voters, Congress has become a symbol for isolated, arrogant, self-centered power. The image may be a distortion, but it is a powerful one. A poll taken this summer in the central Illinois district of House Minority Leader Robert H. Michel found 85 percent of the interviewees gave Congress a low rating. This, despite the fact that Michel, as much as any man, has forged legislative majorities for the program of a president who remains personally popular in the district.

What seems to grip the public mind is the belief that Congress has been fiddling while the republic burns. All the months of travail on budget and tax bills do not seem to overcome the effect of stories about supposed scandals on Capitol Hill, on gyms for overweight senators, lobbyist parties for randy representatives, of honorariums, and tax breaks and travels.

Ledbetter has been hammering Stafford with charges of junketing as head of the congressional delegation to the Inter-Parliamentary Union. His radio ads say, "You are more like to find our Senator in Mexico, Switzerland, Tahiti or Singapore" than in Washington. Adelman is running TV ads citing deficits, higher utility bills and the loss of jobs in Milwaukee. At the end of each ad, he asks: "Where's your congressman?"

People know--or think they know--where their president is every day, because television shows him, apparently struggling with the country's problems. Congress has no such visibility, even with House debates and some Senate hearings on TV. Its reputation in the country is rotten, and that is why the contests for veterans like Zablocki and Stafford are worth watching.