D.C. Mayor Marion Barry, the Democratic incumbent who apparently is on his way to an easy reelection victory in Tuesday's election, declined an invitation for a televised appearance with his three opponents that was scheduled for Monday.

The station has canceled the event, denying Republican E. Brooke Lee, Socialist Workers Party candidate Glenn White and independent Dennis Sobin their only opportunity for televison exposure.

The incident illustrates the extraordinary degree to which Democrats dominate District of Columbia politics. Indeed, Barry not only has been able to control the tone of the general election but, in effect, decide that there would not be much of a campaign at all. Democratic control in local Washington is so complete as rarely to be questioned.

Democrats in the District outnumber Republicans 9-to-1. Going into Tuesday's election, there is only one Republican on the 13-member City Council, the Rev. Jerry A. Moore Jr., and he has benefited from city law that operates to save two at-large council seats for non-Democrats. Hilda Mason of the D.C. Statehood Party holds an at-large seat and is expected to win reelection on Tuesday.

All of the other council members are Democrats.

Local political insiders point out that Republicans have not made an effort to become a force in the city. Arthur Fletcher, a highly regarded Republican candidate who ran for mayor in 1978, received little financial help or advice from the party. Only two of the city's eight wards have Republican organizations, and blacks are noticeably absent as a factor in the citywide party's leadership.

The District's Republicans even have deferred from the idea of endorsing Democrats when those Democrats appear unbeatable. Councilman Moore said that at one time he suggested that the party endorse Barry, as the Republican Party had endorsed Democratic mayor Edward Koch of New York City in his last election when all indices showed him a near-certain winner. By endorsing Koch, New York's Republicans at least assured themselves of access to the mayor and continued existence as a factor in the city's politics.

"The breakdown of Democrats versus Republicans is as lopsided in some areas of Maryland and Northern Virginia as it is in the District," said council member John Wilson (D-Ward 2). "But the Republican Party there manages to field candidates and finance campaigns. I don't think they think they can win, and they have never tried."

"There has never been a concerted effort by Republicans in the District to convert people to the Republican Party," said William B. Simons, head of the Washington Teachers Union.

"You could look at it in racial terms," Simons added. "When whites left all the city but Ward 3, the Republicans left all the city but in Ward 3, and they haven't come back or even made the effort to come back."

D.C. Republican Party chairman Robert B. Carter could not be reached for comment. GOP mayoral candidate Lee, who has only lukewarm support from his party, said he believes Democrats are dissatisfied with Barry and will turn to him, despite his Republican affiliation.

The city is 70 percent black, and blacks traditionally have aligned themselves with the Democratic Party since the "New Deal" programs of Democratic president Franklin D. Roosevelt. Union leader Simons, for example, recalls that during the Depression he received $15 a month from the National Youth Administration for working in the cafeteria of Miner Teachers College.

In addition, Democrats generally have been more supportive of the District's fight for home rule than Republicans. One of the reasons Republicans give for their reluctance to see the city get voting rights in Congress is that any D.C. representative is almost certain to be a Democrat.

"At this day and time, many blacks find it embarrassing to identify with the Republican party because of the incumbent president," said the Rev. A. Knighton Stanley of the People's Congregational Church. "It's not a bad idea for blacks to identify with the more progressive elements of the Republican Party, in order to let Democrats know blacks aren't in their hip pockets . . . but for right now, blacks are leaning farther and farther away from the Republicans in the nation and in the city."

In other races, D.C. Del. Walter E. Fauntroy, a Democrat, is running against Republican John West and independent Viola E. James for reelection as the District's nonvoting delegate to Congress.

Democrat David A. Clarke is running unopposed for chairman of the city council. The D.C. Court of Appeals on Friday ruled invalid the candidacy of Gregory Rowe, who is listed on the ballot as the Statehood Party candidate.

The court ruled that Clarke's Democratic Party membership does not bar him from being the Statehood Party's candidate. Clarke won the 1,500-member Statehood Party's primary with 72 write-in votes, against one write-in vote for Rowe, a Statehood Party member.

Democrat Betty Ann Kane and Mason of the Statehood party face no opponents for reelection to their at-large council seats.

In the Ward 1 council race, Democrat Frank Smith is opposed by Republican Charles B. Fisher and independents Maurice Jackson and Ester L. McCain Jr. In Ward 3, incumbent Democrat Polly Shackleton faces Republican Lois DeVecchio.

In Ward 5, Democratic imcumbent William Spaulding is being challenged by Republican W. Ronald Evans, Statehood Party member Martin Chivis and independent Virgil Thompson. And in Ward 6, Democratic incumbent Nadine Winter is running against Republican Julie M. Servaites, Statehood Party candidate Walter M. Lee and independent Charlotte Holmes.