When local politicians seek votes in Washington there often is a woman on their minds.

She is black, middle-aged and middle-class. She either works for the government or her husband is a government employe. Her home, which she owns, is in Petworth" Northwest Washington, Brookland in Northeast, or perhaps near Fort Davis Park in Southeast.

On Sunday she usually goes to church. On Election Day she votes Democratic, and during the past half decade of local D.C. politics she has supported Walter Fauntory for congressional delegate and Walter Washington for mayor.

How she will vote in next month's primary for mayor is one of the chief questions facing the city's politicians, because the woman sketched above is the most typical of Washington's voters. Her portrait is based on U.S. census data and figures from the D.C. Board of Elections - a composite of the most frequent characteristics of D.C. voters and residents.

"Can you win an election with just that group of voters. Of course you can't." said Matt Reese, a political consultant working for Mayor Washington. "Do you have to appeal across the board? Of course, you do. Can people win elections without them?

Of course, they can. But the voters like that are very, very important. There are more like them than anybody else."

With 690,000 residents and 246,965 registered voters, there is a lot more to Washington politics, of course, than appealing to any one group of voters. The electorate is diverse, ranging from welfare mothers in Anacostia's public housing to "uptown" lawyers earning $100,000-plus a year; mid-level bureaucrats living a Northeast; street-wise "dudes" in Shaw and middle-class "urban pioneers" moving in near them.

But the wards that produce the most Democratic votes - 4, 5, and 7 - are the ones with the most black, middle-aged, middle-class voters.They form the crucial center of city politics. The characteristics that describe these Middle Voters - such as the woman sketched above - are the ones most widespread in Washington's electorate:

Race: Black. According to the U.S. Census Bureau, the District's population is 72 percent black - about the same proportion as in 1970 although there are 66,000 fewer residents overall. Of residents over age 18 and therefore eligible to vote, 67 percent are black.

In recent elections blacks accounted for roughly two-thirds or slightly less of those actually voting, political analysts believe. A Washington Post poll last spring indicated that among Democratic voters 70 percent are black.

Sex: female. Among city residents over age 18, women outnumber men by 65,000. According to census data, black women vote more regularly than black men.

Occupation: government employe. Of 303,000 D.C. residents who are employed, 41 percent hold government jobs - 106,000 with the federal governmmnt and 20,000 with the city. Although the Hatch Act prohibits government employes from taking part in partisan politicsc, they vote in substantial numbers.

Social Status: middle-class. Median family income in Washington was $14,000 in 1975, the latest year for which Census Bureau estimates are available. The figure is substantially less than that in the Maryland and Virginia suburbs but about the same as the United States as a whole. The proportion of D.C. residents living in poverty (as defined by federal guidelines) was 12.5 percent. About a third of all Washington families earned between $12,000 and $25,000 a year.

Homee Ownership. Although about 55 percent of city residents are renters, those who own homes are far more likely to vote. In the Post poll, 70 percent of the Democratic voters owned their houses.

During the past half decade, the number of owner-occupied units in the city has risen by about 9,000, while there are about 4,000 fewer rental units, many having been converted to condominiums.

Age: 45-64. Half of D.C. residents eligible, to vote are over age 41, according to census data. The proportion who actually do vote is highest between ages 45 and 64. Since 1970 the only age groups to grow in the city's generally falling population have been the 25- to 35-year-olds and the senior citizens, those over age 95. Senior citizens vote just slightly less frequently than those who are middle-aged, according to census data; the under-35s vote far less frequently.

Church Attendance: heavy. No firm data exists on church attendance here, but both politicians and political analysts believe the churches are crucial in reaching black voters. The Post poll found that about three-quarters of the city's Democrats said they attend church often.

Political affiliation: Democratic, Washington is an overwhelmingly one-party city - now 79 percent Democratic in registration with just 11 percent of its voters registered as independents and 9 percent as Republicans. In the 1976 presidential election, 81 percent of D.C. voters supported Jimmy Carter, a higher percentage than any state in the union. In the Sept. 12 primary, Democrats expect about 90,000 voters, Republicans about 10,000.

Only one of the city's 137 precincts [WORD ILLEGIBLE] more Republicans than Democrats precinct 9 in Spring Valley.

Even though all eight wards into which the city is divided had virtually the same population in 1970 - about 94,500 residents, Washington's political geography, as messured by where the voters are, is vastly uneven.

The section with the largest number of registered Democrats and consistently the largest Democratic vote and upper income area in upper Northwest (east of Rock Creek Park) and Northeast. It now has about 32,000 registered Democrats and is followed by awards 5 and 7 - two other areas with large black middle-class neighborhoods in Northeast and Southeast. They have the about 27,000 registered Democrats apiece.

On the other hand, Ward 8 in Anacostia, which has the city's greatest concentration of poverty, has by far the lowest number of voters. Only about half as many are registered as in Ward 4 and in elections only a third as many voted. One reason for the low vote in Ward 8 is the age of ii ts population. The ward has more children than any other part of the city. But even among those over 18, voter turnout has been exceptionally low. Only 21 percent of those eligible to vote there did so in 1976, compared to a 33 percent turnout city-wide. (The national average was 55 percent.)

In 1974 when Washington defeated Clifford Alexander for mayor, Ward 8 was the only one that Alexander carried, but its turnout was so light that it has little impact.

"Voter participation is usually a matter of socioeconomic status," said Robert E. Martin, a professor of political science at Howard University and former chairman of the D.C. Board of Elections. "Where there's low income, a low education level, and low job status (as in Ward 8), they vote less across the board. Where there's higher income and more education, people are more likely to be active politically."

Ward 3 (west of Rock Creek Park), which is 95 percent white and mostly upper-income, has more registered voters than any other part of Washington. Among them, however, are far more Republicans (they made up about a quarter of the ward's electorate), a factor which cuts its turnout in Democratic primaries to below that in wards 4, 5, and 7.

The remaining three wards - 1 (inner city-Northwest), 2 (Foggy Bottom, downtown, Shaw, and Southwest), and 6 (Capitol Hill and part of Anacostia) - all have relatively modest voter turnouts though substantially higher than Ward 8.

Since 1970, parts of all three of these wards have undergone substantial social change. Deteriorated row-houses have been bought and restored, mostly by young white professionals without children. Many of the low-income blacks who used to live in them moved away, although according to D.C. government estimates, all three wards continue to have substantial black majorities.

How great an impact the neighborhood change will have on this year's participation is low among both old election is uncertain because voter low-income residents and the new better-off ones.

Richard G. Smolka, a professor of political science at American University, said the whites with professional jobs might be expected to vote more because of their incomes and education, but in many cases, he said, they stay registered in the states where they used to live and continue to vote by absentee ballot.

Smolka said that even though physical change is "very evident" in many neighborhoods, "I don't think they 've added many white voters yet."

Overall, Smolka said, the city's white voters - about a one-third minority - occupy a position that is "the reversal of the black minority vote in many other parts of the country."

In some cases they [the white voters] can make a difference," Smolka said. "In a close election they might be critical, but this time I think all three of the main Democratic candidates [Washington, Marion Barry and council chairman Sterling Tucker] will get some proportion of the white vote, so it probably won't have that much impact."

Unlike nearly all other big American cities, Washington has no significant white working clans and no white ethnic vote either. The city'ss only substantial minority ethnic group are the Hispanics, numbering about 15,000, according to the Census Bureau. Latino leaders claim more, but many are recent immigrants - legal and illegal - who can't vote.

Since 1970 there has been a major movement of D.C. blacks to the suburbs, particularly to Prince George's County.Many of the movers, demographers said, are young middle-income families with school-age children. Several political analysts suggested that despite their move, many may still vote in Washington because they are interested in politics here and the D.C. voter rolls aren't kept completely up-to-date.

The outcome of the Sept. 12 primary will be decided by the District's election laws in addition to how people vote. The laws are unusual for a predominantly one-part area because there is no provision for a run-off if the high vote getter receives less than a certain percentage of the vote, usually 40 or 50 percent. Thus, one candidate can win even if both his opponents combined receive almost twice as many votes as he does.

The absence of a run-off probably favors the incumbent, Mayor Washington, because opposition to him is split between two major candidates. In a similar situation last year in New Jersey, Gov. Bredan Byrne won the Democratic primary with just 32 percent of the vote.