Democratic National Chairman Charles T. Manatt made an unusual peace-keeping mission here today and found that deep divisions in the Chicago party pose a genuine threat to the election of Democratic mayoral candidate Harold Washington.

The visit was intended to be a symbolic gesture to identify the national party with Washington, the first black ever to win the Chicago Democratic mayoral nomination, and to rally support behind him.

Washington, a two-term congressman who upset Mayor Jane M. Byrne and State's Attorney Richard M. Daley in a heated Feb. 22 primary, remains favored to win the April 12 general election. But two white Democratic aldermen have endorsed Republican Bernard E. Epton, and other party ward leaders are openly considering supporting him.

Cook County Democratic Chairman Edward R. Vrdolyak, Byrne's most influential supporter, underscored the split today by refusing to say whether he would campaign for Washington or authorize party funds to be used in his behalf.

Manatt's visit, made at the request of state party chairmen, was designed to smooth over party divisions.

He lunched with state Sen. Phil Rock, the state chairman, Washington, state party officeholders and influential labor leaders who have yet to endorse the Democratic nominee. Vrdolyak was invited to the luncheon, but didn't attend.

Manatt then dropped into a meeting of the State Democratic Central Committee, which passed a resolution endorsing Washington. But before the resolution was approved, Roman Pucinski, an influential alderman and ward leader, stalked out of the meeting, refusing to endorse Washington.

"I think the state and county committees ought to take action before the state party does," complained Pucinski, who represents a largely white area. "We really haven't talked to the candidate. We don't know where he stands on issues that are important to my constituents."

Pucinski said that 95 percent of his constituents are homeowners and concerned about what might happen "to their community." He denied that race was a factor.

At a news conference moments later, Washington refused to hold out an olive branch to Pucinski or any other reluctant party leaders. Support from Democratic leaders, he said, "is presumed," and he pledged to seek it "cordially, civilly and firmly."

Manatt, declaring that "no election in America" is more important than Washington's this year, brought with him a $10,000 campaign donation, and pledged that the party would raise another $40,000 at fund-raisers in Washington and Los Angeles.

He also attempted to smooth relations between Washington and the 1984 Democratic presidential frontrunner, Walter F. Mondale, who angered Washington by endorsing Daley.

It was the first time in history, Manatt said, that the traditionally independent Chicago Democrats have sought or received a direct donation from the national party.

Manatt also issued a formal call for party unity saying: "The Republican Party would like nothing better than to splinter the strength of the strongest of Democratic strongholds."

Manatt also strongly hinted that the chances of bringing the 1984 Democratic National Convention to Chicago would all but disappear if Epton wins the election. Chicago and San Francisco are the leading contenders for the convention.

Manatt later visited privately with Mayor Byrne, who has endorsed Washington, and Chicago business leaders.

Washington appeared pleased with the visit.

"He Manatt brought the kind of tidings we wanted to hear," he said. "There is a direct connection between the national scene and what is happening here . . . . We relish the connection."