The campaign aide's excitement fairly bubbled over the telephone last week. Brian Clarke, press secretary for Prince George's Council member Parris N. Glendening, was calling to proclaim: "Every endorsement that has been made so far has been for Parris."

As it happened, the zealous press aide was merely restating the obvious. When Glendening emerged as the man most likely to win the Sept. 14 primary and become the Democratic candidate for county executive in November, he inherited the political endorsement of almost every union and interest group in Prince George's.

"The momentum on our endorsements is like a runaway train," Clarke announced. But the fact is that most endorsements say more about candidates' chances of winning than their powers of persuasion.

Unless, of course, the endorser doesn't like someone.

For example, the Washington Metropolitan Council of the AFL-CIO, which has 65,000 members living in Prince George's, does not like 22nd District state Sen. Thomas O'Reilly because of what it claims is his anti-labor voting record. The result: so many area labor leaders showed up to back his challenger, Robert Redding, at a breakfast at the Capitol Centre last week, they had to scramble an extra tray of eggs.

"We are going to put Tom O'Reilly in the unemployment line," said labor council president Joslyn Williams with bravado. One by one, representatives of various locals crossed a grateful Redding's palm with $500 and $1,000 checks. Redding went home with $9,000 for his campaign coffers and promises of more cash and campaign volunteers to come.

"This will be a restoration for the Maryland General Assembly and a kind of renewal for the labor movement in the Washington area," said Williams. He was responding to questions about labor's political clout in Prince George's.

How much difference does labor support make?

"Just ask Steny Hoyer from his last congressional run," said Michael W. Mooney, president of Local 2250, Association of Classified Employees (AFSCME), whose 2,000 members pulled oars for Hoyer's successful 1981 campaign.

But Hoyer, in fact, is still backing O'Reilly, as are the incumbent senators from Prince George's and the smart money.

The unions are backing Redding, who, they say, as a state delegate voted "correct" on labor matters 92 percent of time. O'Reilly, the unions say, pulled the right lever only 36 percent of the time.

Perhaps the fact that Redding was a dues-paying member of Electricians Local 26 for 20 years before he became a lawyer had something to do with it. After all, professional affiliation helped at least two candidates for office win endorsements from the 6,000-member teachers union.

In general, Prince George's teachers endorsed incumbents and sure winners, except where there was a teacher running. Or, in one case, a relative by marriage of a teacher.

Although the joint leadership committee of the teachers and other school employes had recommended Tom Hendershot for the wide-open 3rd councilmanic seat in the New Carrollton area, long-shot candidate Charles Donnelly got the nod instead from a sparsely attended meeting of rank-and-file teacher representatives. His teacher wife, Tina, with a contingent of colleagues from Parkdale High School, made the difference in a close vote at the Aug. 2 meeting.

John d'Eustachio, a teacher-candidate for a 21st District delegate seat, made an emotional appeal to his colleagues' loyalty at the same meeting. As a result the teachers endorsed four candidates for the three delegate seats in the district.

"If you were a teacher you were endorsed. It was absolutely ridiculous," said one disgruntled candidate for state office. "They weren't looking for qualifications," the candidate charged.

Union board member Fran Doyle admitted that emotions skewed the endorsement process but said, "There was a very strong feeling that we go with our own."

During the primary campaign, teachers will take part in phone banks and polls and "do a lot of leg work," according to union president John Sisson. But the union will not be making any direct cash contributions. The union does not control the teachers' 6,000 votes because teachers pride themselves on being independent, Sisson says.

Indeed, most observers agree that in the long run, the candidates with the sorest knuckles -- not necessarily the most endorsements -- are the most likely victors. In the absence of the once-formidable countywide slate for county offices, old-fashioned door-knocking has become a highly touted campaign tool.

The most likely winners also have had financial support, and often the incumbency, long before the endorsements.

This may not be enough to comfort 3rd District council candidate Hendershot, a victim of endorsement cross fire between the teachers and the county police union.

Last June, Mahlon Curran, president of the Prince George's Fraternal Order of Police (FOP), talked to Hendershot about an endorsement, but only if Hendershot publicly would place public safety over education in county priorities. When Hendershot balked, Curran went with Jim Herl. Then the teachers dumped Hendershot for one of their own.

Said Curran: "The fact that Herl's brother is a Prince George's policeman and a member of the FOP board didn't hurt him."