It started 10 years ago when then Rep. Larry Hogan was running for re-election in Maryland's 4th congressional district and a young Republican couple from Fort Washington, Ella and Robert Ennis, decided to help.

The political career of Ella E. Ennis snowballed from there. While Robert went on to become chairman of the Republican Central Committee in Prince George's County, Ella worked first as a legislative aide for former County Executive William Gullet, then as an assistant to the House of Delegates' minority leader.

Now, Ella Ennis will find out what a decade of dedication to Prince George's forgotten political party adds up to in votes.

While her old mentor, Hogan, takes on the leader of the county's dominating Democratic organization, Executive Winfield M. Kelly Jr., Ennis is tangling with three of the organization's most loyal, entrenched and powerful delegates in the 28th legislative district.

Democratic incumbents John W. Wolfgang, Joseph F. Vallario and William R. McCaffrey have singled out Ennis as the most threatening of their three challengers, and are waging an expensive and aggressive campaign to stop her.

More than a seat in the Maryland House of Delegates is at stake. The 28th district election will test the strength of one of the last strongholds of the disintegrating county Democratic organization, or "machine." It may determine whether the 28-year-old old tradition of all-Democratic county legislative delegations ages four more years.

And it will indicate how close the county's G.O.P. organization can come to putting its own regulars into office.

The 28th district, embracing most of the southern half of Prince George's County and a silver of northern Charles County, is not an unlikely place for a show of Republican strength. Not only did its largely-rural constituency help to elect Hogan twice to a congressional seat, but it is the home of some of the county's most conservative politicians, like Sue Mills, the former anti-busing crusader on the county school board who is now a candidate for the County Council.

The 28th this year had the most hotly-contested Republican legislative primary in the county. Ennis, James G. Panor and William P. Crandell emerged from a five-way race highlighted by some infighting among party factions committed to Hogan and his primary challenger, Martin Aragonal.

Democrats still outnumber Republicans by three to one in the district, however, and the Democratic incumbents, who faced no opposition in the primary, are probably among the most solidly-based local politicians in Prince George's.

They are also intensely loyal to each other, to Kelly and to the party leadership.

"We have the best legislative team in this district," says Sen. Thomas V. (Mike) Miller, the leader of the group. "Other people say it, but nowhere is it more true than in our districts. We work the hardest for the party."

"Some of the Democratic candidates in other districts regard Kelly's candidacy as an albatross, and they try to get away from it," Miller said. "I'm very confortable with it. I tout him for executive wherever I go."

Although all four Democrats discount the possibility of an upset win by Ennis, their joint campaign tells a different story. Each of the four has chipped in a little over $3,000 - in addition to a $2,500 contribution for county Democratic literature - for 140 four-by-eight foot signs, brochures, and mailings.

Ennis, for her part, plans to spend close to $10,000 before the campaign is over, helped by contributions from Republicans all over the county: Panor and Crandall, in contrast, will spend less than one-third of that amount.

Only one central issue has emerged in the campaign: the records of the incumbents. The Republicans are attacking their opponents for voting for a state sales tax increase of one percent in 1977 and for helping to approve large state budgets.

"The most significant thing I have to offer happens to be a negative approach," said Panor.

The incumbents counter that the sales tax was necessary to avoid further property taxes and that their records are fine. "I'm not ashamed to stand on my record," says McCaffrey. "It's as good as any representative serving in Annapolis."

Wolfgang, in particular, can campaign on experience. An eight-year veteran of the legislative, he is chairman of the Economic Matters committee and is consequently one of the more powerful members of the Prince George's delegation.

Most Republicans, however, feel that Wolfgang is the most vulnerable member of 28th slate. "Wolfgang and Vallario are hanging on Miller's coat-tails," said Panor. "Miller and McCaffrey have been able to get through to people in a way that Wolfgang and Vallario haven't."

Democrats, on the other hand, are hoping that a potential Ennis upset in the district will be diffused by the strong candidacies of Panor and Crandell, who beat Ennis in five precincts out of 16 in the primary.

Miller is likely to receive far less of a challenge than his running mates in his own race against Republican Ray Velesquez, an attorney who has run and lost in the district before.