George McGovern's entry into the 1984 presidential race appears to be one more obstacle for Walter F. Mondale, the front-runner for the Democratic nomination, and a possible building block for Sen. John Glenn (D-Ohio), his major challenger, according to a Washington Post-ABC News public opinion poll.

The poll shows Mondale and Glenn still running far ahead of the other Democratic candidates. Mondale holds a substantial lead over Glenn--but it might be considerably larger without McGovern's entry.

In addition, both Mondale and Glenn continue to lead President Reagan in projected trial heats. Mondale leads by 2 percentage points, a narrow edge that falls within the poll's margin of error. Glenn leads by a considerably more substantial 10 points.

Mondale is now the choice of 41 percent of registered Democrats as their presidential candidate, according to the survey. Without McGovern in the race, however, the poll suggests that Mondale would be the choice of 48 percent.

Glenn is the choice of 28 percent of registered Democrats, but without McGovern that figure would rise to 30 percent, according to the poll.

In a Post-ABC News poll at the end of July and beginning of August, before McGovern had announced his candidacy, Mondale was the choice of 42 percent of the registered Democrats, Glenn of 27 percent.

McGovern entered the contest Sept. 13. He was favored as the nominee by 10 percent of those who said they were registered Democrats in the new poll. More than half his backers say their second choice is Mondale, but only 1 in 5 gave Glenn as their second choice.

The other announced Democratic candidates also appear hurt by McGovern, proportionally more than Mondale since each began at so low a level. A comparison of the new poll and the Post-ABC News July-August survey shows that:

Sen. Alan Cranston (D-Calif.) is now at 4 percent, down from 6; former Florida governor Reubin Askew is at 1 percent, down from 4; Sen. Gary Hart (D-Colo.) is at 1 percent, down from 3; Sen. Ernest F. Hollings (D-S.C.) is at less than half a percent, down from 1 percent.

The figures for these candidates remain the same whether or not the name of the Rev. Jesse L. Jackson is included. Jackson is the choice of 7 percent, most of whom otherwise would turn to Mondale.

In the presidential trial heats, Mondale leads Reagan by 48 percent to 46 percent and Glenn leads the president by 52 percent to 42 percent. That pattern of Mondale in a virtual tie with Reagan while Glenn substantially leads the president has turned up repeatedly in past Post-ABC polls, and in other polls.

Among independent voters, Reagan leads Mondale by 51 percent to 40 percent, but Reagan and Glenn are virtually tied, with Reagan at 47 percent and Glenn at 46 percent.

The poll also shows the so-called "gender gap" working for both Democrats. Among men, Reagan gets 49 percent and Glenn 48 percent. But among women, Reagan trails Glenn by 35 percent to 56 percent.

The president leads Mondale by 51 to 45 among men, but trails the former vice president among women, 41 percent to 52 percent.

For the survey, 1,506 people nationwide were interviewed between Sept. 22 and Sept. 26. Most of the figures cited are based on 1,069 respondents who said they were registered voters or 451 who said they were registered Democrats. The theoretical margin of sampling error is about 3 percent or slightly higher.