By most rational assessments two months ago, the suggestion that a militant black civil rights leader would be elected mayor of heavily Italian and Slavic blue-collar Buffalo was akin to predicting a warm day in January.

It wasn't that state Assemblyman Arthur O. Eve, the first black politician ever to run in a mayoral election here, had an identity problem. He is well-remembered for his unsuccessful attempt to seek a peaceful solution to the bloody 1971 riots at Attica State Prison, and for later introducing a bill to impeach then Gov. Nelson A. Rockefeller for his role in ending that rebellion.

It is just that the only successful political crusade based on neighborhoods ever waged here happened to have been by a Polish-American, Joseph Mruk, and that was in 1949. And the connection between his ethnic identity and his success was not accidental.

But these are hard times in Buffalo, a gray, depressed city on the eastern tip of Lake Erie, where the unemployment percentage and the degrees above zero Fahrenheit seem to always stay the same - about 10.

And Eve, the only black New York state legislator elected outside of New York City, has a campaign that appears to be catching on across color lines. If he wins, he will be the state's first black mayor.

Democratic Mayor Stanley M. Makowski, dumped by the regular party after four years of controversy over his management of the city - including critism of snow-removal efforts in last winter's disastrous storms - is not seeking re-elections.

Most polls put Eve even with state Sen. James D. Griffin, a conservative Democrat who lost the primariy to Eve last month and is running on the Conservative Party line. A distant third is Republican John J. Phelan.

The Eve-Griffin match-up is a classic liberal-blacks, conservative-white contest, and for the first time in 20 years the entrenched Democratic machine is watching from the sidelines.

A loser in the primary was Joseph F. Crangle, the powerful Eric County Democratic chairman, who has controlled Buffalo politics for years. His hand-picked candidate, city corporation counsel Leslie G. Foschio, ran third.

In the primary, Eve campaigned against Crangle's "bossism," and corruption, while carefully organizing the black community, which comprises 25 per cent of Buffalo.

Almost 10,000 new black voters registered at the prodding of Eve's organization.

"This is the first time the black community has been organized politically," said William Gaiter, Eve's field coordinator and director of a social action organization founded here in 1967 by Chicago community organizer Saul Alinsky.

"On electiond day, the old pols couldn't believe what they were seeing - poll watchers everywhere and black voters pouring out of their houses. We had 150 organizations and they [regular Democrats] didn't think we could do it," Gaiter said.

Eve's strategy in the Nov. 8 general election is duplicate his strong primary showing in black districts - which comprised about a fourth of the city's white votes, including the liberal university district, predominantly Jewish neighborhoods in north Buffalo and among moderates in other white middle-income areas. IN the primary he won about 10 per cent of the white vote.

Eve said he is attempting to build the coalition of labor, business and ethnic groups that Coleman Young has had some success in creating in Detroit. With that coalition, Eve said, he could turn Buffalo from a dying city to an urban showcase.

Eve's conversation and political speeches - in contrast to his earlier years of militancy - frequently are laced with references to "bringing people together." At the urging of his media consultants, he shaved off the beard that was viewed as a trademark of his causes.

But Eve says now that anything hsort of an ethnic and ideological coalition in conservative Buffalo would spell disaster for the city.

The Buffalo area is dotted with steel mills, chemical plants and automobile factories whose payrolls rise and fall with the national economy. Factory closings and layoffs are common, and the central business district is a maze of boarded-up stores.

"I've sat up there in Albany just watching the city continually die. I feel I can do better than the machine has," said Eve, who is deputy majority leader of the Assembly.

Eve's strategist and media planner is Frank O'Brien, of the Boston political consulting firm of Payne, Marttia and O'Brien, which is also handling Young's re-election campaign in Detroit.

O'Brien says the theme in the final weeks of the campaign will be that of a moderate who is trusted by the predominantly white business and labor leaders. The steel> auto and communications workers unions have endorsed Eve, as have some major business and religiou leaders.

However, Griffin's campaign manager, Joseph Martin, said, "Just because the steelworker's union endorsed Eve doesn't mean the rank and file will vote for him." The union leadership, he said, simply could not bring itself to endorse either a Republican or Conservative Party candidate.

Besides soliciting union backing, Eve, 44, has spent much time meeting with business leaders to convince them that the city's outward migration - population has dropped from 580,000 to 400,000 since 1950 - can be reversed by imaginative economic development programs.

While Eve frequently points to the business leaders who have endorsed him, there are many more who are remaining silent, or are backing Griffin, Eve's supporters concede.

Since the issues of the campaign are roughly the same for both candidates - jobs, industrial development, better schools, better city management and better police protection - Griffin has been forced also into running a massive voter organizing and registration drive to match that of Eve.

"We don't have a grand strategy. We've just trying to get out the vote and get people to look at both men's records," said Martin.

The record that Griffin cited in a series of appearances this week is one contrasting conservativism and liberalism: Griffin for the death penalty, Eve against; Griffin against liberal abortion laws, Eve for; Griffin for more severe prison sentence, Eve for increasing prisoners' rights; Griffin against increasing business taxes, Eve for it.

Griffin and his supporters emphatically deny they with appeal to race consciousness, but they note pointedly that "Buffalo is a blue-collar town" and essentially conservative.

"Eve and Griffin have well-defined personalities and followings. People will be crossing party lines on election day," Martin predicted.

Griffin lost the Democratic primary only because Eve let Faschio and Griffin "beat each other's brains out, and slid in with his 37 per cent," Martin said.

"I think he's peaked in terms of strength. We're going to get out the vote on election day in the way he did in the primary," Martin said.