For as long as anyone can remember, "Clem" has been as much a Milwaukee institution as brats and beer, as fixed in the order of things as Sunday Mass or the labor rally in Serb Hall.

In the 34 years since his first election, Rep. Clement J. Zablocki (D-Wis.) has climbed the seniority ladder to the chairmanship of the House Foreign Affairs Committee. And in the 34 years since he replaced the one-term Republican usurper in his heavily Democratic south side district, he has been home regularly, at the wakes and weddings, but rarely has been troubled with a real campaign.

Neither in Washington nor at home has Zablocki rocked many boats. He has been a reliable supporter of the defense budgets and foreign policies of presidents of both parties, whether those policies meant war in Vietnam, negotiations with Russia or peacemaking in the Middle East. At home, he has minded his own political business and resisted efforts to enlist his prestige in other campaigns.

But this year, as it must to all men, redistricting came to Clem Zablocki. Milwaukee's population loss forced him to double his territory, bringing in new western suburbs and more than 100,000 new constituents. The shrinking population of many American cities documented by the 1980 census has resulted in similar redistricting of many urban congressional constituencies.

Zablocki's neighboring north side Milwaukee congressman for the last 28 years, Rep. Henry S. Reuss (D-Wis.), saw his district similarly altered and decided to retire. Reuss is 70 and married.

Zablocki, 69 and a widower, was not ready to retire. But with the new territory came a new challenger, a suburban liberal state senator named Lynn Adelman, and the kind of fight in the Sept. 14 Democratic primary that Zablocki has never faced before.

The financial reports the two men filed in July were startling. Adelman had raised $50,967--a healthy if not overpowering sum. Zablocki had raised $3,188. "I never had a fund-raiser in 34 years," Zablocki said the other day. "I just took whatever people wanted to contribute and sent out little reminders to vote."

But Adelman was a different kind of challenger. The 42-year-old attorney, an honors graduate of Princeton and Columbia University Law School, ran for the House in 1974 in the suburban Republican district where he grew up, and lost to Republican Robert W. Kasten Jr., now a U.S. senator.

Two years later, seeing that the incumbent Republican state senator in a neighboring district was in trouble, Adelman moved to the suburb of New Berlin and won the legislative race in his new home district. Now in the middle of his second term, he has become chairman of the Judiciary and Consumer Affairs Committee and earned a reputation as an effective legislator.

Adelman mobilized the technology of modern campaigning early--before Zablocki really knew what he faced--putting together a computerized list of some 85,000 likely Democratic primary voters and a series of targeted phone and mail appeals.

Judy Schiera, Zablocki's district assistant for the last 17 years and now on leave to run his campaign, said she learned of the threat when friends of the congressman said they had been called three or four times by Adelman workers. But it was not until July that Schiera opened a headquarters for Zablocki. "In the past," she said, "we never had a full-time campaign office; I'd just go over after work and on Saturdays and a few people would help me get out the mail."

Polls this summer showed Adelman within easy range of an upset. But in the last few weeks, Zablocki has begun to fight back. He held his first fund-raisers in Washington and Milwaukee in late July and August, and raised enough, he said, to match or exceed the $125,000 Adelman says he has budgeted for the primary.

There is no Republican candidate in November, so neither man has to hold back funds for the general election.

This week, Zablocki's first TV spots ever went on the air--two 30-second ads in which he is seen but not heard, presiding over his committee, mingling with constituents and being praised by old friends in this community. The tagline is: Zablocki Delivers. A Proven Man for Difficult Times.

Adelman has more varied spots, some with himself talking, some with actors. They play to his themes that Zablocki has done nothing to halt the loss of jobs, the mergers, the rise in utility prices that Adelman says have hurt the 4th District. An actor and actress in rocking chairs ask each other a series of questions about deficits, taxes, economic woes, then look at the camera and ask, "Where's our congressman?"

On the stump, Adelman hits away from a variety of angles--questioning why Zablocki was the only Wisconsin Democrat to support the 1981 Reagan tax bill, why he has "the third-worst spending record" from the National Taxpayers Union, and why he supports tobacco and sugar subsidies, nuclear power and public works projects in other states, and such defense programs as the MX missile and the B1 bomber.

In a cable television debate the other night, Zablocki offered explanations for all these votes, but seemed most pleased when he said that if Adelman was so concerned about jobs in Milwaukee, he should not have contracted with a Denver computer firm and a Virginia pollster to do the consulting work on his campaign. "I'm proud to say that my media consultant is at 1717 S. 12th Street in Milwaukee , right in the heart of my district," Zablocki said.

Just below the surface is a religious issue which both candidates seem uncomfortable discussing. Adelman is Jewish and last month, the Milwaukee Journal publicized a fund-raising letter Lawrence R. Appel had sent to 5,000 Milwaukee Jews, saying Zablocki was "no friend of Israel" and the man who would succeed him as chairman of Foreign Affairs, Rep. Dante Fascell (D-Fla.), is "a friend of Israel."

Adelman acknowledged clearing the letter, but when asked in an interview if he agreed with the characterization of Zablocki, he hesitated a long time before saying, "I think his vote on AWACs was a mistake." Zablocki supported President Reagan's decision to sell the sophisticated reconnaissance aircraft to Saudi Arabia, over Israeli protests.

Zablocki said in a separate interview that the fund-raising letter had "boomeranged. It brought me contributions from people who wanted to be sure I was not out-spent." Zablocki said he was "concerned about the issue of Israel getting injected, because there is latent anti-Semitism here and we want to keep it under control."

There are few Jews in the heavily Catholic district. Zablocki came to the TV debate with a right-to-life symbol in his lapel, and is doing a special mailing this week to a list of 12,000 anti-abortion voters. Zablocki has the support of the Milwaukee labor federation and of both local papers, the Sentinel and the Journal.

But as Adelman headed out one day last week to knock on more doors, he said, "Thirty-four years is a long time for anybody. I think people realize it's time to change."