The Republican Party openly conceding that it has long ignored the nation's cities, set up an urban beachhead in a glistening downtown complex here yesterday in an attempt to teach party leaders how to woo black, blue-collar white and ethnic voters.

It did so with a great deal of uncertainty and self-criticism.

"The Republican Party ignored the urban question in 1976," Michigan Gov. William G, Milliken said in a speech that set the tone for the day. And I think, as a direct result, we lost the election."

Speaker after speaker at a joint meeting of the Republican National Committee and GOP urban county leaders urged the party to seek out labor and minority group voters.

The key message was these voters need no longer be the sole property of the Democratic Party and their votes may mean the difference between victory and defeat for Republicans this fall.

"My word of advice is you have to reach out to the blue-collar worker," said Joseph M. Margiotta, the GOP chairman in Nassau COunty, N.Y. "Lets show them that we Republican are humans beings, that we enjoy beer, playing golf or going to a ball game. It's harder for them to hate us if they know us."

But a question a few minutes late vividly illustrated the uncertainty that plagues Republican efforts in the cities.

"I don't drink beer. Idon't play golf. I don't like to go to ball games and I don't know any labor leaders who are women," a female delegate from St. Paul. Minn., said. "What do I do?"

The GOP Urban Conference at Detroits impressive new Renaissance Center is the latest in a series of moves that the struggling Republican Party has made since the 1976 election to broaden its base of support.

GOP national chairman Bill Brock has previously made overtures to organized labor organization and blacks and emphasized party organization at the local level.

But this week's three-day meeting takes place at a time when Republican can spirits are higher than at any time since Jimmy Carter took office, largely as a result if the president's sagging popularity.

There is also general feeling that lingering cloud of the Nixon years has lifted.

The party is still not looked on favorably by the majority of Americans "but we are not so badly off as we were seven months ago," pollster and political consultant Arthur J. Finkelstein told the group. "At that time, one in three Americans said the Republican Party is the party of Nixon and Watergate. Now only one in five say it is."

"We have an opportunity to develop symbols in urban politics," he said. "The abortion issue is an obvious example. I don't think it's a good idea to carry a fetus around. But a meeting with a bishop, a local priest, a nun or even a drink in an Irish pub might not be a bad idea. It's also not wrong to identify with a cop in trouble if his name is O'Riley."

Robert Teeter, pollster for the Ford presidential campaign in 1976, recommended that the party target its major efforts on blue-collar workers between the ages of 18 and 40, 72 percent of whom have never attended college.

"If that group of voters, who don't vote much now, move in behind a candidate, a party or an ideal, they will form a majority coalition that will last for two or three generations," he said.

Both Teeter and Michigan Gov. Milliken said that national taxpayers' unrest represents a golden opportunity for Republicans.

But Milliken warned, "we cannot use tax limitation as the current euphemism for the anti-black, anti-Spanish-speaking and anti-poor sentiments of some segments of the population.

"If Republicans succumb to short-term expediencies and forsake long-term commitments, the inevitable reckoning will only leave a smaller minority party than ever before and we will have forfeited any right to be a national political party," he said.