Slowly, painfully and sometimes hesitantly, the major pieces of the old Democratic coalition -- the big unions, the big-city mayors, the Jews and the blacks -- are coming together in Pennsylvania for Walter F. Mondale as they haven't come together for a Democratic presidential candidate since 1968.

Mondale, like his mentor Hubert H. Humphrey, the party's 1968 nominee, is the coalition's kind of candidate.

And if labor and the mayors could deliver votes as they once could, Mondale would carry the Keystone State, which has voted Democratic in four of the last six presidential elections.

But they can't, and so with the election 10 days away, Mondale is locked in a close race with President Reagan in this state.

The campaign was slow to moblize black support, for example.

"In order to carry Pennsylvania, Mondale needs a 75 percent or better black turnout in Philadelphia," said one key black supporter. "I'd say you're looking at a 60 to 65 percent turnout at this moment."

Ethnic blue-collar workers, who voted in large numbers for Reagan in 1980, are another question mark, despite the most unified effort by organized labor in decades.

Nevertheless, Mondale has a good chance of carrying Pennsylvania, a key part of any scenario that envisions him winning the presidency. At recent rallies in Pittsburgh and Philadelphia, he drew two of the biggest crowds in either city's history, largely as a result of his performance in the first presidential debate.

"People who were avoiding Walter Mondale like he had leprosy are now fighting to stand next to him when the camera is around," Philadelphia Mayor W. Wilson Goode, a longtime Mondale supporter, said this week.

But there is great uneasiness even among some of Mondale's most loyal supporters. They are worried about the impact of national polls showing Mondale trailing Reagan badly and reports that Mondale has been told by campaign chairman James A. Johnson that the race is almost hopeless.

"If the media keeps saying it's over, it may just cut the heart out of us," said Philadelphia District Attorney Ed Rendell, one of Mondale's national co-chairmen.

"We can carry Pennsylvania, and we can't let polls affect what we do," he added. "I'm afraid if the press keeps pounding, we'll collapse."

According to political experts, Mondale's Pennsylvania hopes hinge on carrying Philadelphia with a margin of 200,000 to 250,000, winning Pittsburgh and the rest of Allegeny County in the west by more than 100,000 and holding down Reagan's margin among younger, white-collar voters in suburban areas.

Mondale has assembled an impressive bunch of supporters, uniting often warring factions of the Democratic Party. Goode is one of his state co-chairman; Pittsburgh Mayor Richard S. Caliguiri is the other. Erie Mayor Lewis Tullio, who supported Reagan four years ago, and Scranton Mayor James McNulty, who backed Sen. Gary Hart (D-Colo.) in the primary, are also working for the Mondale ticket.

Mondale's strong showing in the first presidential debate brought other leary Democratic officeholders into the fold.

"We were looking for a Reagan landslide and asking how do we survive it," said state Sen. Vince Fumo, who represents an ethnic, white district in south Philadelphia that Reagan carried by 10,000 votes four years ago. "All of a sudden people were proud to be Democrats again. People starting talking about voting a straight Democratic ticket again."

"Prior to the debate every day you got up you were a little more frightened," said Rep. Robert A. Borski Jr. (D-Pa.), who represents a predominately white district in northeast Philadelphia where he reports that Jewish and Catholic voters are returning to the Democratic column.

Jewish voters are a significant group in Pennsylvania, and Republicans had hoped to make a historic breakthrough this fall by capturing more than 50 percent of this vote. But Richard Fox, a Philadelphia businessman who heads the National Jewish Coalition, a GOP campaign arm, said there has been "some slippage since the summer. Largely it's because the church-state issue has become an issue."

Organized labor, brutalized by high unemployment in the steel industry, is going all out for Mondale. State organizations of rival teachers' unions, the American Federation of Teachers and National Education Association, are working together, for example. Teamsters locals in Pittsburgh and Philadelphia are bucking their international union and working for Mondale.

"It's the most unified effort since the '40s. I've never seen anything like it," said state AFL-CIO President Jules Uehlein. "We're all just scared to death of Reagan."

Uehlein said labor has 100 phone banks operating in the state. He added that he is hoping Mondale will win the votes of 75 percent of the union vote. But he, too, is worried about the impact of national polls. "I'm very concerned that people may stay at home," he said. "Yesterday I was campaigning at a textile plant and a woman came up to me and said 'What's the use. It won't do any good anyway.' "

The key to Mondale's hopes in black areas is turnout. But key black leaders complain that until recently they were ignored by Mondale's national campaign and that Goode has been ineffective in rallying the remnants of the city's Democratic machine.

"The lines of communication weren't there. We wanted to be part of the planning and strategy," said City Councilman John White Jr., one of the city's most effective ward leaders. "It took a long time for them to realize that in order to carry Pennsylvania we have to be happy. We haven't been happy until the last 10 days or two weeks."

"The situation isn't where it ought to be, but the pieces are now on the table, and with a concerted effort in the next 10 days, we can produce," said Rep. William H. Gray III (D-Pa.). "If they lose and it's close, it may be because of a low turnout in the black community."