After eight months of campaign hoopla and more than a touch of impassioned rhetoric from the candidates, District of Columbia voters will go to the polls to vote today for their party candidates for mayor and for seven other offices.

Nearly half of the city's 246,965 registered voters are expected to cast their ballots at the city's 137 polling places, which are open from 7 a.m. to 8 p.m. The turnout could be diminished sharply if it rains today. The weather service forecasts a 50 percent chance of afternoon and evening thundershowers.

Just as in the first D.C. political campaign in 1974 - after the city won a limited home rule form of government from Congress - most of the attention has been focused on the race for the Democratic nomination for mayor.

Voters throughout the city will find that the intense campaigning that has marked the contest will continue right up until the polls close. The staffs of the three candidates - Mayor Walter E. Washington, City Council Chairman Sterling Tucker and Council member Marion Barry - all have planned elaborate get-out-the-vote operations, as well as a few gimmicks to remind Democratic voters that their man's the best.

Voters who look up in the sky to see if it is raining might find an airplane flying overhead with a "Vote Washington Sept. 12" banner trailing behind. Voters who open their front doors might hear sound trucks blaring the political gospel according to Sterling Tucker and Marion Barry.

Democratic and Republican voters will have a chance to pick their mayoral candidates, while Democratic voters also will pick candidates for City Council chairman, an at-large seat on the Council, the council seats in Wards 1, 3, 5 and 6 and four delegates to the national Democratic party's mini-convention next December in Memphis.

Democrat Walter E. Fauntroy, the District of Columbia's monvoting delegate to the House of Representatives, and Hilda Mason, the Statehood Party's at-large representative on the City Council, are unopposed for renomination, while six other council contenders running as U.S. Labor. Republican, Statehood and Socialist Worker candidates are unopposed for nomination.

When voters get to the polls - possibly in a car driven by an election day worker for one of the candidates - they will come face-to-face with poll workers for all three mayoral candidates handing them sample ballots. Not surprisingly, workers for Washington, Tucker and Barry have all prepared ballots that mark their candidate as the preferred choice.

Tucker workers planned to have new campaign signs posted at polling places. Throughout the race, the City Council chairman's red, white and black signs have said, "Tucker. Mayor. Now." On election day they will say, "Tucker. Mayor. Today." Barry workers at some polling places plan to wear sandwich boards promoting their favorite candidate.

D.C. residents who haven't voted by mid-afternoon, but who have been identified as supporters of one of the contenders, are almost certain to get a call from one of the hundreds of campaign workers assigned to the candidates' telephone banks today.

One of Tucker's strategists, Harley J. Daniels, said the Tucker campaign has identified 15,000 Barry voters. The Washington campaign refused to divulge such information.

Daniels predicted that 115,000 of the 195,019 registered Democrats would vote, while Donaldson predicted 105,000 to 110,000. Warren Graves, the director of field operations for the mayor's campaign, estimated that 95,000 to 98,000 Democrats would vote.

Representatives of all three campaigns said they will have legal staffs standing by to help solve any problems that arise during the day, and to monitor the vote count conducted by the D.C. Board of Elections and Ethics at the Martin Luther King Memorial Library. All three plan to have poll watchers at all precincts as well.

Shari B. Kharasch, the chairperson of the elections board, said that ballots cast from 7 a.m. to about 12:30 p.m. will be picked up and brought under police escort to the library to be counted this afternoon. The results from the morning voting will be announced when the polls close at 8 p.m., she said, and then the rest of the votes will be counted throughout the evening.

Washington, Tucker and Barry spent part of yesterday closeted with their campaign aides discussing last-minute plans for election day operations. Tucker and Barry also did some campaigning in various parts of the city, while Washington confined his public schedule to a brief television interview.

About 9,000 of the citys 21,498 registered Republicans are expected to vote in the GOP mayoral primary, according to city party chairman Paul Hays. The choice is between Arthur A. Fletcher, a former Nixon and Ford administration official, and Jackson R. Champion, the party's mayoral candidate four years ago.

Susan Pennington is unopposed for the U.S. Labor Party mayoral nomination.

Three Democrats - City Council members Arrington Dixon and Douglas E. Moore and a Baptist minister, the Rev. John G. Martin are seeking the party's nomination for City Council chairman.

Nine more Democrats are vying for the party's nomination for an at-large seat on the council, while four are seeking the Democratic nod for the Ward 1 seat, two in Ward 3, eight in Ward 5 and four in Ward 6.

Residents of Precinct 123, who formerly voted at the old Congress Heights Elementary School, now vote at the new one, at 6th Street and Alabama Avenue SE. Residents of Precinct 24, who in the past have voted at the Morgan Elementary School, now will vote at the Marie Reed Learning Center, at 2200 Champlain St. NW.