HARRISBURG, Pa. (AP) _ Pennsylvania, the next state in the Democratic presidential marathon, may well offer a smooth path for front-runner Michael Dukakis but for rival Jesse Jackson the April 26 primary is strewn with obstacles.

''We could win the state and lose 80 percent of the delegates,'' lamented Jackson delegate coodinator Steve Cobble.

Pennsylvania's popular vote is merely a beauty contest and the real winner in the primary must succeed in direct delegate elections in the state's 23 congressional districts. A prize of 178 delegates, fourth largest of any state, is at stake.

Jackson's core support is concentrated in three districts in Philadelphia and Pittsburgh, the state's two largest cities and home to most of the state's blacks. Blacks make up about 11 percent of the voting age population.

Though turnout there may be the highest in the state and Jackson could pull close to Dukakis in the overall popular vote, the delegate races likely will swing overwhemlingly to Dukakis, observers say.

''These direct delegate elections are unfair to blacks. It's one of the last vestiges of Republican-type politics in the Democratic Party and it ought to be done away with,'' Cobble said.

Personal popularity of the individual delegate candidates doesn't count for much since the name of the presidential hopeful appears beside each committed delegate candidate.

Dukakis was the only Democrat to field a complete slate of 116. Jackson's campaign, in contrast, has only 79 district delegate candidates and has turned to delegate-candidates pledged to former contender Rep. Richard Gephardt of Missouri and other former candidates to fill out the gaps. The remaining 62 delegates will be apportioned in June according to the district outcomes.

Some political veterans say Jackson could win no more than 30 delegates.

''Even in a best scenario how he (Jackson) gets more than 30 delegates I don't know,'' G. Terry Madonna, director of the Center for Politics and Public Affairs at Millersville University.

State Democratic Chairman Larry Yatch, who along with Gov. Robert P. Casey is neutral in the race, doesn't think Jackson's slate-filling effort will be fruitful.

But Jackson's campaign said late Tuesday afternoon it has commitments from eight to 10 delegates of departed candidates to support Jackson and is working on more.

Jackson will win Philadelphia, where he is endorsed by Mayor W. Wilson Goode and Rep. Bill Gray, but Dukakis is favored everywhere else, Yatch said. Pittsburgh Mayor Richard Caliguiri and Erie Mayor Louis Tullio both have endorsed Dukakis.

The latest pre-primary poll, conducted Thursday through Sunday by the Pittsburgh Post-Gazette and Philadelphia Daily News, showed Dukakis with a 59 percent to 20 percent edge over Jackson. Gore fell far back at 9 percent. The survey of 647 likely Democratic voters had a margin of error of 5 percent.

''Mike is taking nothing for granted,'' said Dukakis' Pennsylvania spokesman Pat McCarthy. He said a win could ''put Mike over the psychological top'' on the road to the nomination.

A strong showing in New York would help Jackson in Pennsylvania, but Dukakis is favored right now, delegate coordinator Cobble said.

Jackson, who won 16 percent of the vote and 18 delegates in the three-way 1984 presidential primary, hopes to woo disaffected blue collar workers, particularly in southwestern Pennsylvania, where unemployment still runs into double digits in some areas. Statewide, he is hoping at least to duplicate his record of attracting 15 percent to 20 percent of the white vote.

''We'll do better than in 1984, exceed expectations and give Dukakis a hell of a run for his money,'' said Janice Fine, a national campaign aide. ''We're very competitive in Pennsylvania.''

Democratic Lt. Gov. Mark Singel, also neutral, disagreed.

''The more likely scenario is that Dukakis uses Pennsylvania to solidify his front-runnership, his hold on the nomination,'' Singel said.

Gore's moderate stands should have meshed with Pennsylvania's voters, who gave the state to Ronald Reagan both in 1980 and 1984 and who haven't elected a Democrat to the U.S. Senate since 1962. In fact, Gore was endorsed as the most electable of the candidates by the state Senate Democratic leadership. But Gore hasn't won anywhere since Super Tuesday in his native South and the New York primary results could signal the end of his campaign.

There is also a Republican race, at least on paper, but Vice President George Bush is expected to easily brush aside his only remaining rival, Pat Robertson, in the contest for 78 delegates.