Long-scorned lake-front liberals and Hispanic voters have emerged as the key to Tuesday's close and bitter Chicago mayoral election.

With massive and offsetting turnouts of white ethnic voters for Republican Bernard E. Epton and black voters for Rep. Harold Washington (D-Ill.) expected, both are concentrating on the less predictable and usually less courted small constituencies on the lake front and in the northwest and South sides.

Concern about his eroding position in lake-front wards brought Washington to two rallies in the last 24 hours at the old Belmont Hotel, overlooking Lincoln Park and the yacht harbor.

He drew enthusiastic crowds of several hundred at both a Saturday afternoon event, where author Studs Terkel introduced a parade of liberal white politicians, and this morning at a lox-and-bagel breakfast aimed at the important Jewish communty.

The Belmont is in the 44th Ward, halfway up the string of six wards stretching north from the Chicago River to Evanston. Washingon received about 32,000 votes, 22 percent of their total, from these wards in the February primary. But he must at least double that to have a chance against Epton.

The lake front is normally hospitable to liberal candidates. Anti-organization Democratic aldermen and state legislators such as William Singer, Martin Oberman and Dawn Clark Netsch have nurtured their organizations here.

The academic and integrated neighborhood of the far-north 49th Ward, around Loyola University and Mundelein College, boasts such institutions as the Heartland Cafe, a vegetarian restaurant and political clubhouse whose proprietors, Kathleen Hogan and Michael James, endorsed Washington when he ran for mayor in 1977.

The keynote of lake-front politics is reform, and reform is what Washington preached in rousing speeches at the Belmont and Mundelein. "The issue is not a black man for mayor," he said. "The issue is reform."

In his lake-front appearances, Washington said Epton would be a pawn in the hands of Washington's vow to curb patronage and force open bidding for city contracts.

Washington repeated these arguments today in interviews on "This Week With David Brinkley" (ABC, WJLA) and "Meet the Press" (NBC, WRC). Epton pulled out of a scheduled appearance on the latter program because he objected to the presence on the panel of Vernon Jarrett, a black columnist for the Chicago Tribune whom Epton accused of being openly pro-Washington.

Washington's pitch about Epton as a potential pawn is a good one for lake-front liberals, but everywhere Washington went last week, reporters were told that his support in these crucial wards was slipping.

Singer, who once ran for mayor as a reform Democrat, told a friend he has not been able to persuade his father to vote for Washington. Roberta Lynch, the Washington coordinator in the six lake-front wards, has the same problem with her mother.

"It's the tax thing," said Jim Terman, a part-time advance man for former vice president Walter F. Mondale, who is helping Washington. The same explanation comes from Christopher Cohen, a liberal pro-Washington attorney, and from Mike Brady, the 49th Ward committeeman who was a political adviser to defeated Democratic Mayor Jane M. Byrne and now is a nominal but inactive Washington supporter.

Although Washington has said he was punished "inordinately" in being jailed for 36 days for failure to file tax returns for four years, the lake front's sensibilities have been offended by that story and by more recent revelations of other unpaid bills.

"I'm amazed at the depth people are going to, to find reasons not to vote for him," Michael James said. "They just don't want to say it's his race."

Many lake-front voters deny it is Washington's race, and they were given support today when influential Chicago Sun-Times columnist Mike Royko, who earlier had assailed what he called "racist" support for Epton, turned against Washington.

"If Harold loses, he'll have only himself to blame," Royko wrote. Citing the string of financial revelations and infighting in Washington's campaign as deeply unsettling to "the liberal, independent white vote," Royko wrote: "There have been too many things to wonder about. And too many people who couldn't help but wonder."

The defections have provided hordes of workers for normally anemic Republican organizations.

Catherine Rondinelli, the 44th Ward Epton coordinator and a veteran of many losing GOP campaigns, marveled at having "at least two workers per precinct. We don't have to give them reasons to be for Bernie. They come to us."

While spending much of their time on the lake front, Epton and Washington have made strong runs at the city's 90,000 registered Hispanic voters, promising them a better deal on city hiring, contracts and policy-making for the city's estimated 465,000 citizens of Spanish extraction.

With unemployment rates in some neighborhoods running about 25 percent for Hispanic males, the promise of jobs has a special ring. The question is whether it will spark a higher than usual turnout by these voters.

Washington's Hispanic corodinator, Peter Earle, predicted today that about 68 percent of the Hispanic voters will vote and that Washington will receive about 65 to 70 percent of that, or about 36,000 votes. That would be a huge gain from the primary, when Washington ran a poor third behind Byrne and Cook County State's Attorney Richard M. Daley, receiving only 13 percent of the Hispanic vote.

Washington's strength in this segment of the electorate lies with the heavily Democratic Puerto Rican community, which is about half of the Hispanic population. Miguel Santiago, the lone Hispanic alderman, is Puerto Rican and adds strength to Washington's campaign.

But Washington's staff is nervous about the Mexican vote. In the 7th Ward, which is 20 percent Hispanic, 60 percent black and 20 percent Polish, Mexican-American Ray Castro is considered the front-runner in a City Council runoff Tuesday against a black opponent. Although Castro says Washington will receive a substantial plurality, Washington has not endorsed either man.

Usually heavily Democratic, the Mexican vote in this election may be more Republican, in line with anti-black sentiments in the white ethnic wards. Cubans, who comprise only 3 percent of the Hispanic block, generally vote Republican. Marlen Vilas-Roth, a Cuban who chairs Hispanics for Epton, said the campaign has attracted "people who never were involved in anything."