Walter F. Mondale and the leaders of the Georgia Democratic Party made a first tentative stab at reconciliation and party unity here Friday morning after staring into the cannon's muzzle and seeing the grim prospect of a Reagan landslide this fall followed by a major Republican assault on state Democratic officeholders in 1986.

Mondale's troubles here are indicative of his problem nationwide: southern politicians agree that Mondale has to win some southern states to have a hope of beating President Reagan on Nov. 6. They consider Georgia to be one of his best prospects in the region, but it is impossible to find a white politician who thinks Mondale has a chance of carrying the state.

The Friday unity meeting was a big step toward improving Mondale's prospects here, however, particularly the active show of support and leadership by Sen. Sam Nunn (D-Ga.), the state's most respected politician.

Although Mondale's victories in the Georgia and Alabama primaries in March saved him from elimination from the presidential race and, despite his ties to former president Jimmy Carter, Georgia Democrats recently have viewed Mondale as just another carpetbagger.

His relationship with the party structure was so bad that one Democrat quipped that Mondale's visit to the state, his first since winning the presidential nomination in July, was intended to "dispel the notion that he can't come to Atlanta any more."

This is primarily because of lingering resentment over the humiliation of Georgia state party chairman Bert Lance during the Democratic National Convention, the belief by many that Mondale is too much a big-spending liberal for the state, and the Mondale campaign's amateurish operations in Georgia.

Grim necessity, however, brought about two dozen Democrats together for breakfast on Friday with Mondale. Among them were Nunn; Gov. Joe Frank Harris; Lt. Gov. Zel Miller; Lance; Atlanta Mayor Andrew Young; Hamilton Jordan, Carter's chief political strategist and White House chief of staff; the leaders of the other Democratic presidential candidates' Georgia primary campaigns, and most of the statewide officeholders.

Mondale expressed satisfaction with the meeting afterward.

"I think we've finally got ourselves together and are now unified," he said. "I know I'm behind, but I don't think it's anything like the 25-point gap the polls are showing. This campaign is just beginning."

Most of the participants also were enthusiastic publicly about the rapport they established at the meeting. One described it as a "virtual love-in," but probably more accurate was Jordan's assessment that Mondale "at least assured them that he hadn't written off the state."

Their major common ground was the agreement that Mondale's attack on Reagan's budget deficits was easily his strongest issue in the South. They urged him to argue that any tax increase will really be Reagan's and that it would be used to pay off Reagan's budget deficits rather than increase government spending.

Mondale did some tough talking about fiscal responsibility and indicated at the closed-door meeting that his plan to reduce the federal budget deficit, which he said he will unveil Monday, may include putting the receipts from the proposed tax increase into a trust fund to ensure that the money is used to reduce the deficit.

This reportedly elicited a warm reponse from Harris, who has been the most hostile to Mondale.

This is partly because of his initial opposition to Mondale's proposed tax increase -- Harris campaigned on a promise not to raise taxes -- but primarily because of his outrage over Mondale's ill-advised effort to dump Democratic National Committee Chairman Charles T. Manatt on the eve of the national convention and replace him with Lance.

When the subsequent uproar by Democrats who wanted no reminders of the Carter years (Lance was Carter's first budget director) forced Mondale to back down, he tried to salvage something by naming Lance general chairman of his campaign but was forced to back off this as well.

All this infuriated many Georgia party leaders and they retaliated by boycotting a rally in Macon that Mondale supporters tried to organize Aug. 15.

"They were openly contemptuous of Mondale's operation then and thought it was being run by idiots," one observer said last week. "Because of the way it was handled they all got their noses pretty well out of joint and this is what triggered Young's remark that they were 'just a bunch of smart-assed white boys who think they know everything.' "

There is no question, however, that Lance has remained a good trouper although his demeanor reveals how much his Mondale-inflicted wounds have hurt. He traveled from Calhoun to Atlanta Thursday night to persuade Harris to attend the breakfast and described himself as "very enthusiasic" about supporting the campaign.

The next morning, Lance introduced Mondale as "the man best qualified to be president of the United States."

Overriding these quarrels is the Democrats' realization of where their self-interests lie.

"Some, like Harris, were upset that Bert was mishandled," Young said. "Harris let his friendship with Bert override his judgment about the party and his self-interest. His self-interest is in Mondale's election because if Reagan carries Georgia, Macon Mayor George Israel, who is a very smart Republican, has $5 million to run for governor against Joe Frank in 1986."

Young sees defeatism as the Democrats' worst enemy, particularly since their hopes are based on turning out large numbers of black voters. Reagan carried the southern states over Carter in 1980 by margins as small as 5,000 votes and it is an article of Democratic faith that a turnout of 60 percent of the eligible black voters in the South could deliver the region for Mondale.

"But voting is a hardship and a sacrifice for working people," Young said. "They have to believe that it's going to make a difference or they won't bother."

If blacks vote in the same numbers as 1980, then Mondale will lose, Young predicted, "but if working people vote, the Democrats will win."

If there is a Reagan landslide, Young has a dire prediction about the future of the Democratic Party in Georgia: the party would splinter because disillusioned black candidates, feeling betrayed, might run in the future as third party or independent candidates.

"Blacks account for about a third of the vote and if, instead of a unified Democratic Party, most blacks were running as independents, in a governor's or senator's race this could be a powerful thing," he said. "Politics in Georgia would never be the same again, and to keep Georgia Democratic I think Mondale has to carry it."