Mayor Marion Barry and his chief challenger for the Democratic nomination, Patricia Roberts Harris, are in almost perfect agreement on many of the key issues in the campaign.

They agree that handguns should be banned; they support statehood and an elected school board. Both are in favor of no-fault insurance, rent control, decriminalization of marijuana and government-funded abortions. Both are opposed to public employes having the right to bargain collectively and to strike; both are against mandatory sentencing.

There are some differences.

Harris gives qualifed support to increasing the District's authority to tax some real estate owned by nonprofit and church groups in the city. She favors returnable bottles as a way to keep streets clean, and legislation that would permit tenants to make repairs on apartments and deduct the cost from their rent. Barry opposes all three of those ideas.

Barry would sell bonds to finance repayment of the city's long-term debt; Harris would not. Barry gives a qualified yes to the notion that the District government should be the employer of last resort for city residents. Harris says no, leave that to the private sector.

"The difference between us," Barry said recently, "is leadership. Who has been involved locally. . . .What determines the real difference is not what I say or she says in a speech, but what we have done as private citizens, not in any official capacity, to show we care."

Harris agrees that the chief issue is leadership, but more specifically what she sees as Barry's lack of it. "It's a matter of leadership and planning," Harris says. "I'm willing to take charge. It's a matter of how I would deal with matters of importance. I'm organized. I'm not going to wait until the last minute to deal with important problems. . . . I want this city to be a model of efficient government, not a constant target for criticism from across the nation. It's a leadership question."

The similarity between the two front-running candidates for mayor is a revealing sign of the state of politics in the District of Columbia.

In a city dominated by liberal black Democrats, the politicans are of one mind on most issues. Even considering their differences, no one could call any two local politicans here ideological adversaries. There are no analogies to Jesse Helms versus Ted Kennedy, conservative southern Republican versus liberal northern Democrat in local Washington poltiics.

Harris and Barry have followed similar paths to political prominence. She was a civil rights activist of the 1940s and '50s. He, like most home rule politicians, was a black power advocate of the '60s--the same movement, only a more recent part.

Harris fought to desegregate schools and cafeterias and then to affirm that black people--particularly black women--could think, manage, and govern in the highest of offices. Barry fought for jobs and for economic development--for the next phase of power.

Since first coming into public office in 1971, Barry has moved from the role of local black radical hero to that of a relatively moderate mainline Democrat on the local scene.

Harris, once a standard-bearer for the best of the black race and a mainline moderate Democrat of national prominence, must demonstrate that she can mix with the little people who were Barry's original base of political support.

Barry says on the campaign trail that he knows he could have done better, he knows he has made mistakes, but that he has been a good mayor. Harris cites his administration's problems to say that the city needs competent leadership.

So at this stage, the election can be seen as a contest between look-alikes. Imagery often appears more important than substance because substance is already agreed upon.

The facts that Harris is a woman, has a hard edge to her personality or wears her hair a certain way become a basis for voters' decisions. Similarly, Barry's tendency to mumble or sweat, the suits he wears or the fact that he's a man become factors in his success or failure.

When there is an absence of real choice on the issues--or even an apparent absence of genuine, old-fashioned issues--the candiates and the voters look to differences of another sort.

Harris and Barry move to each other like two people trying to fit in the same suit, and the argument is over who would look better in the suit.