Sen. John Glenn (Ohio), a 1984 presidential hopeful, today became the fifth major Democratic figure in three days to campaign here for the party's mayoral candidate, Rep. Harold Washington (Ill).

Glenn endorsed Washington, who thanked him for "giving a broader view of what this campaign means nationally." But Glenn's appearance illustrated the danger that the "nationalization" of the Chicago mayor's race is beginning to pose for Washington.

Glenn unwittingly upstaged an event that probably was more important to Chicago voters than his endorsement.

Washington had called a news conference to release the major issues document of his campaign, a 52-page booklet detailing his stands on everything from crime to culture.

His staff had prepared charts illustrating Washington's positions and had invited dozens of Chicagoans to the event. But Glenn's visit dominated the news conference to the extent that Washington scarcely had a chance to talk about his positions.

Washington, the first black ever to win the Democratic mayoral nomination here, continues to insist that the help he is getting from national party leaders is furthering his campaign.

But others have doubts.

"I think they're making a very serious mistake," said Milton L. Rakove, a University of Illinois professor and leading authority on Chicago politics. "I don't think it's doing Washington any good, and it's beginning to backfire. This is Chicago. Nobody in this city cares who is president. They care who's mayor and alderman."

A poll released Monday night by WLS-TV, the local ABC affiliate, lent some credence to these remarks With two weeks to go before the April 12 primary, it showed that Washington's lead over Republican Bernard Epton had dropped by 11 points in two weeks, all among white voters, though the Democrat still leads by 48 percent to 34 percent with 18 percent undecided.

There is little question that the Chicago mayor's race, traditionally a strictly local affair, has become a national one. Both major parties say they feel they have an unusually high stake here. And both have committed more resources here than in any mayor's race in recent history.

For Democrats, it is a question of maintaining credibility among black Americans, the party's most dependable voting block. Party leaders say they feel they have to demonstrate that the party, despite widespread local divisions here, can deliver votes to black candidates.

"It is really not an exaggeration to say that the national integrity of our party is at stake in this election," Democratic National Chairman Charles T. Manatt wrote in one fund-raising appeal.

Democrats have formed a special Chicago task force. They have sent out two emergency appeals for money for Washington; scheduled fund-raising events for him in Los Angeles last week and in Washington, D.C., and New York Thursday; taped commercials with leading Democrats endorsing the congressman, and leaned on Chicago party leaders to get behind their candidate.

They also have persuaded a host of party and union leaders to campaign here. This week former vice president Walter F. Mondale, Rep. Claude Pepper (Fla.), AFL-CIO President Lane Kirkland, United Auto Workers President Douglas A. Fraser and Glenn have visited.

There are some here, however, who suspect that the visitors, particularly the presidential candidates, are at least as interested in helping themselves among blacks as in helping Washington.

Republicans, sensing a chance to make inroads in this traditionally Democratic city, have mounted a quieter effort. They have paid for a major public opinion poll for Epton, financed telephone banks and sent political organizers into Chicago.

"This is the most we've done in years and years in any mayor's race," said Republican National Chairman Frank Fahrenkopf Jr. "The disarray in the Democratic machine gives the Republican Party not only a chance to elect a mayor, but to do some party building."

The campaign has been dominated almost completely by the race issue, and Fahrenkopf conceded that this concerns some Republicans. But he added, "It certainly isn't the Republican Party that raised the race issue. It was raised by Democrats fighting among themselves."