Three of the members of Congress from Chicago are black -- the largest number of black representatives the city has ever had. But they are not rejoicing; rather, there is among them the cool tension of polite but ever-alert suspicion.

The reason for the mistrust is simple. Sometime later this year, as Illinois' political powers use information from the latest U.S. census to draw new boundaries for the state's shrunken congressional delegation, one of the black members of Congress from Chicago is almost certain to be bumped off the map.

The frustrating problem, as yet unresolved: which one?

The poor West Side Chicago district represented by Cardiss Collins lost more population than any other in the state over the last decade and might be expected to be the most vulnerable in the redistricting. But Collins' allegiance to the machine of Chicago Mayor Jane Byrne makes her a far more likely survivor than the two other black Chicago lawmakers, former state senator Harold Washington land newspaper publisher Gus Savage, who are both outspokenly against the machine.

Washington and Saveage, said one colleague, are "worried that Cardiss Collins will cut a deal with the machine to blow one of them. away. It's all very polite, as polite as those things can be."

There are a half-dozen other black members of Congress who have to worry these days about surviving the coming scrambles for seats after 17 in the Northeast and Midwest are given up in reapportionment to states in the Sun Belt.

Former black strongholds in the big-city enclaves of Chicago, North St. Louis, East Cleveland, Detroit, Harlem and Bedford-Stuyvesant are high on the list of congressional districts in the nation that experienced the greatest loss of population over the last decade. In the downtown Detroit district that had been represented by convicted former congressman Charles Diggs, more than 180,000 people -- one of every three living there in 1970 -- moved out during the last 10 years.

The Congressional Black Casucus optimistically counts only three seats as "possible losses": those held by St. Louis' William L. Clay, Cleveland's Louis Stokes and one of the three in Chicago.

But other black incumbants, even if not defeated, are certain to face vastly different, often whiter constituencies. The change already has affected, to some degree, the style of the Black Caucus, which came into being a decade ago when the black members of Congress -- then numbering nine -- walked out on President Nixon's State of the Union address.

"I kind of welcome moving beyond a total black environment," said John Conyers, whose north-central Detroit district lost more than 77,000 people. He said he and other members of the caucus have talked about the need to broaden their appeal, not only because of reapportionment but also to exert more influence in the new conservative Congress.

"We're the single most progressive political unit, not just in Congress, but anywhere in the country," Conyers said. "To make your most effective contribution to blacks, you have to talk about these things from a wider podium."

The threat to the black members of Congress comes after a decade of steady growth in their numbers and, ironically, at a time when they have more clout than ever before.

The seniority system on Capitol Hill they once railed against for keeping entrenched white southern members in power has, over time, served the big-city black members well.

"I was opposed to it [the seniority system]," said caucus Chairman Walter Fauntroy, the District of Columbia's nonvoting delegate. "But the longer I stay, the better I like it."

Four members of the caucus are House committee chairmen -- Ohio's Stokes heads the Ethics Committee, California's Ron Dellums chairs the District Committee, and Gus Hawkins of California and Parren J. Mitchell of Maryland control House Administration and Small Business. Nine members of the caucus chair subcommittees, including Fauntroy, who heads the House subcommittee on domestic monetary policy.

"We're going to be breaking out of the black bag and projecting members of the caucus as leaders of thought and policy-making at all levels of the American budget and the American condition," Fauntroy said. "What we are projecting is leadership that is indeed black but which focuses on dealing with the problem of the vast majority of moderate-income Americans. Most of the people who are unemployed in this country are white. So long as employment needs, public-assistance needs, tax-relief needs were portrayed as black concerns, racists could rationalize not dealing with them."

Much about the dispersal and migration of blacks from the central city that is now causing the Black Caucus such headaches has been a story of success. The lawns and newer homes of the suburbs have proved just as alluring for upward-moving black families as they have for whites. In the 1970s, after bars to open housing were lifted, sometimes with the aid of the black members of Congress whose seats are not threatened, black families marched steadily out of the city.

Often the homes and apartments they left were abandoned and boarded up, no longer the refuge of black migrants up from the South. For more than three decades, "Freedom Trains" had brought legions up by day-coach form the dusty little towns of Arkansas, Mississippi and Alabama to the factories and mills of the industrial North. But the new census indicates that, in the '70s, more blacks headed in the other direction, returning to a more benign South where the cost of living is lower and opportunity for blacks somewhat greater than before.

Some see in that trend new opportunities developing for would-be black officeholders in the southern states. "I do not see the overall picture as being threatening," Fauntroy said. "While we stand to lose some seats in the Frost Belt, there is also the opportunity to gain some in the Sun Belt."

That's a silver lining also seen by Radamese Carbera, an enthusiastic former Howard University graduate student who has been hired by the caucus to aid its members in surviving reapportionment.

Carbrera thinks there are opportunities in Mississippi and Georgia, and in such Sun Belt cities as Pensacola, Miami, Mobile and New Oreleans. He says he also believes that blacks may one day be able to run in such suburban areas as Prince George's County and win.

But he sees tough sledding this year. "My concern is that the Republicans are so much more ahead of the game," he said. "They're just leaps and bounds ahead of the feminist and most black and progressive organizations."

Tough sledding is almost certainly in store for Missouri's Congressman Clay, who is often on the phone these days to his district in St. Louis and to the Missouri state house, where the talk is of chopping his thin district in two. His aggressive style has offended some, and in the halls of the legislature and the columns of the newspapers, he has been accused of failing to tend to the concerns of whites.

"I have substantial white support, contrary to what the media is saying about the way white voters perceive me," Clay said. He is a down-the-line supporter of labor, environmentalists and civil libertarians, and says he will be calling on those groups for help in building support among whites.

The courts are an ace in the hole for Clay and other black members of Congress, who look for them to intervene if maps are redrawn deliberately to dilute minority voting strength. But in Clay's case, even the most favorable redrawing of his North St. Louis base would create a district in which only one of every three voters is black.

Blacks rarely have been elected to Congress from districts less than 40 percent black, although former senator Edward Brooke (R-Mass.) and Dellums, from the San Francisco Bay area, have been notable exeptions.

Conyers said he believes he and former Detroit judge George Crockett who succeeded Diggs, are safe. "If you want to destroy the remains of the Democratic Party in Michigan, try to cut either George or me," Convers said. "That's how strong we are."

Harlem's Charles Rangel and Brooklyn's Shirley Chisholm are expected to be protected in Albany by the caucus of black and Puerto Rican legislators. In Maryland, the hope is for Mitchell to have his district's boundaries extended from Baltimore to suburban Baltimore County to recapture 120,000 people who left the city.

Meanwhile, the trio from Chicago stays on guard.

In separate interviews, Collins and Washington said they hoped there would be an effort to retain all three Chicago congressional seats held by blacks, but they acknowledge that they and Savage have never managed to meet since being sworn in a couple of months ago.

"I think there's a way [to save the seats] if we put our minds to it," Collins said vagely. Washington said he believes that black civic and social organizations in Chicago would bring the three together and be an army for support to keep them.

"We can't play the traditional kind of politics because we can't control it," Washington said. "You go into a smoke-filled room, you get your a-- kicked and your public doesn't even know it."

So, what will he do?

"Obviously, I'm not going to sit still and get wiped out."