One of their chief complaints is that "proportional representation" is really "perpetual representation": Sitting councillors almost always win. Critics of PR point to the most recent council election in 1987, when nine incumbents sought re-election, and nine incumbents won.

Others argue that PR does not really provide for minority representation. Although the system was designed to loosen the hold political parties have on government, many city residents argue that here, a two-party system has simply adapted itself to PR.

The City Council has been split for decades between two distinct groups. On the current council, four members are endorsed by the liberal Cambridge Civic Association (CCA), the good-government group that originally backed Plan E. The other five bill themselves as Independents.

"It doesn't seem to make a hell of a lot of difference whether we have [PR] or not," says tenant activist William B. Cunningham. "If it's supposed to give minority representation, it doesn't, because we basically have a two-party system."

In recent years, rent control has been the dividing line between the two. CCA candidates back the existing rent control system, while Independents--with the crucial exception of Mayor Alfred E. Vellucci--want to weaken it.

But today's race may mark the beginning of the end for the CCA Independents split. Fringe groups on both sides have formed new organizations this year with slates of candidates to counter the CCA's established one.

On the left, the grassroots Working Committee for a Cambridge Rainbow has endorsed a platform calling for a radical overhaul of city government.

On the right, a coalition of Independent council hopefuls have joined with Councillor William H. Walsh to form the "Today's Independents" slate. The slate's members are unified by their support of Proposition 1-2-3, a ballot referendum that would drastically alter rent control.

The slate's formation also splits the Independents into at least two other groups: old-line Independents like Councillors Walter J. Sullivan and Thomas W. Danehy, who base their support on neighborhoods, and protenant candidates like Timothy J. Toomey, Jr., who support rent control and oppose 1-2-3.

ACCORDING to the most recent census, Blacks constituted 10.9 percent of the city's population in 1980--slightly less than one-ninth.