About the beginning of October they began to trickle in to the Prince George's county clerk's office: a letter here, a casual note there. "Dear Mr. Chairman," they usually begin, "As a lifelong/resident/businessman/citizen/ of the county, I am writing to inform you of my support for" one cable company or another. Sometimes they give reasons why.

Now the letters are filed away in a folder grown three inches thick over the last few weeks, stuffed with everything from handwritten notes on humble lined paper to the flowery script of law firm letterhead. As the county council nears its Nov. 17 deadline for awarding the two franchises that virtually will monopolize cable delivery in Prince George's for the next 15 years, the trickle has become almost a flood of pleas from citizens on behalf of the various firms fighting for the county's lucrative cable business.

"We've been getting all this stuff," said councilmember Sue Mills,"from people, well, like you wouldn't believe."

"We're getting them all," said a weary Ann Lombardi, "But you want to know the truth? I don't even read them anymore."

Out of a desire to avoid litigation and to follow the rules of fair play, Prince George's passed a resolution in September 1980 requiring that contacts between cable companies and officials be avoided or made public. In July, Montgomery County passed a similar rule, but there, violation carries a fine of $1,500 or a sentence of six months in jail or both. As a result, firms anxious to make a good impression without putting Montgomery officials in an awkward spot have begun assiduously to court county community groups instead.

The Prince George's rule carries no such penalty, and the fat file of what are officially known as "ex-parte contacts" reflects both a vigorous campaign for community support and a willingness by the cable operators to step forward on their own behalf.

While pondering its choice among five competing cable companies, the Prince George's council has heard from local mayors, a state delegate and community activists like senior citizen advocate Cora Rice. There are letters from Jack Winegardner, president of an Oxon Hill Chevrolet dealership; Lawrence Abell, vice president of Fedco Systems, Inc., and Arthur Fox, president of a local chemical company, as well as from groups of lawyers and a handful of State Farm Insurance agents.

Most of the letters are written in support of Cross Country Cable, a firm whose proposal was passed over by the commission charged with recommending cable operators to the council.

One message came from Camp Springs insurance man Donald Meehan, who wrote that, while he is not familiar with all the proposals, "I am aware that Cross Country is what I consider to be a local candidate . . . I feel that they have the sincere desire to see that Prince George's County is serviced properly by a company that is ultimately responsible to the citizens of Prince George's County."

Ten percent of Cross Country's stock is locally owned; State Del. Frank Komenda(D-Prince George's) owns 25 percent of the locally-held stock and serves as general manager.

"I know Frank Komenda," said Meehan when asked why he wrote the letter. "I just thought I'd try and help him. I just did it because I like him.

"In fact," he added later, "I don't even live in Prince George's County . . . I figure someday, well -- he'll be in politics for a long time."

Josie Bass, president of the Prince George's NAACP, telephoned councilmember Deborah Marshall Oct. 21 to ask "various questions on the cable process," Marshall wrote, "and supporting Storer and Cross Country". Storer's local cable subsidiary is headed by former county executive Winfield Kelly Jr., a powerful figure in the county Democratic party. Storer's proposal was another passed over by the cable commission.

"With wholehearted support," former state delegate Decatur Trotter recommended Cross Country for the southern franchise area, even though the Glenarden activist does not live there. Trotter's Oct. 24 letter to councilmember Roy Dabney -- who has announced that he will abstain from voting on any cable franchise because he formerly owned some cable stock -- neglects to mention that Trotter has worked as a paid consultant to Cross Country. "It says I have worked with them," said Trotter, "I meant to cover that."

Komenda said he never asked his friends to write the letters. "Rather, by virtue of our 2 1/2-year effort, a number of people contacted us and asked us if they could be of assistance," he said.

Moreover, Komenda said, a number of local Cross Country board members decided on their own to approach their friends and business acquaintances for help. As for the five mayors who have written, all of whom have awarded franchises in their towns to Cross Country, Komenda acknowledged that unless his company receives a county franchise or picks up other municipalities, Cross Country will not be able to build the system originally promised them.

Some cable contenders have preferred to make their pitches directly, rather than working through local friends, and of that group Winfield Kelly has made the most liberal use of the telephone.

Kelly called Ann Lombardi on Oct.12 and again on the 28th, lunched with Dabney on Oct. 21 and spoke to Glendening -- who called him -- on Oct. 26. But Kelly's most constant contact has been Gerard McDonough, a longtime political ally whom Kelly called on Oct. 8, 9, 14, 15, 23, and 27, according to the record. Most recently, wrote McDonough, Kelly called "regarding the scheduling of the cable TV work sessions and the Council procedures."

McDonough said he regards the ex-parte rule as "strictly a political ploy on County Executive Lawrence J. Hogan's part to try to make the council look bad." McDonough compared the file to a court docket: "The whole essence of this thing is that all individual parties know that one side has communicated with the decision makers. There's nothing sinister about it; it's a fair play kind of thing."

Besides, said McDonough, "How do you sell something if nobody'll let you sell? Let 'em get in there and . . . may the best man win."

Others say they have refused to discuss the issue with friends who call. "What I've gotten is a series of calls from political friends asking a more sensitive question: 'Are you seeing anyone? Would you like to talk about cable?' And my answer is the same: put it in writing," said Ann Lombardi.

"Just outright dealing isn't my style and this isn't the place to start."