When Tom McMillen was a senior at the University of Maryland his teammates on the basketball team hung a nickname on him: The Senator.

Tom McMillen would like to be a senator. He would like to be another Bill Bradley, athlete turned U.S. senator at a young age. Today, McMillen is a 29-year-old professional basketball player. But his work with the Atlanta Hawks is an October to April job. The rest of the time, McMillen works very hard at his job-to-be: politics.

He is enough of a realist to understand it will be difficult for him to go straight from the locker room to the Senate Chamber. Once, the House of Representatives from Maryland's 4th District seemed to be the best intermediate stop. Two years ago, apparently with that in mind, McMillen bought a town house in Crofton.

Then Rep. Marjorie Holt, the popular Anne Arundel County Republican, decided not to run for the Senate. Instead she will seek a sixth term from the 4th District this year. If McMillen thought he could beat Holt, he would probably retire after this season. But Holt's presence has probably delayed McMillen's entry into the political arena for two years.

It has not, however, quelled his ambitions.

In the last three years, by his own count, McMillen has made close to 400 public appearances, many of them on behalf of his alma mater. Last week, while most NBA players were home relaxing during the All-Star break, McMillen was in Maryland, meeting Thursday with Charles Manatt of the Democratic National Committee. A Friday meeting with Gov. Harry Hughes to discuss playing a role in Hughes' reelection campaign had to be postponed by the governor. Sunday, McMillen was in New York for a ceremony commemorating the birth of Franklin D. Roosevelt, there because he is a member of the Democratic Party's National Finance Council.

"I have always been aware that professional basketball is a terminal business," McMillen said dryly. "It comes to a conclusion very quickly."

McMillen has been actively preparing for that conclusion for a long time. At 17, he was the most highly recruited schoolboy basketball player in the country, making the cover of Sports Illustrated with a caption that read: "The Best High School Basketball Player in the Country." The magazine was wrong, Bill Walton was better.

But that did not deter the recruiters, among them Maryland's Lefty Driesell. Recognizing that McMillen was not your ordinary jock, Driesell turned to Joseph D. Tydings, then the junior senator from Maryland, for help.

"The first time I met Tom I thought he was the kind of person who would do very well in public service," said Tydings, now a Washington lawyer. "One of the ways I recruited him was by telling him how important a connection with Washington, D.C., could be to him in later life."

McMillen was involved in politics almost from day one at Maryland. With Tydings' help he got a summer job working in the Senate Cloakroom for then majority leader Mike Mansfield. He was on the Governor's Commission on Higher Education and the President's Council on Physical Fitness. He considered running for president of the Student Government Association his senior year.

"He's a natural born politician," Driesell said. "He never forgets a face or a birthday or people's kids' names, things like that. When we used to go on the road, all the other guys would be readin' the sports section and Tom would be readin' Newsweek and The Wall Street Journal."

At Oxford, he studied politics, philosophy and economics. He has maintained residency in Maryland since returning to play in the NBA, settling in Anne Arundel two years ago.

At the time, that seemed a clever move. Holt was expected to run for the U.S. Senate this year, leaving her House seat open for a newcomer. But Holt decided to run for reelection in the fall and McMillen has now backed off.

"Running against a popular incumbent, especially when I still have another year to fulfil on my contract, doesn't really appeal to me that much," McMillen said. "It would take extraordinary circumstances for me to run this year."

McMillen isn't running, but he is on the move. Early this month, at the behest of the university's development office he signed a letter that went out to 25,000 Maryland alumni requesting money. The letter is part of a new campaign, in which a prominent Maryland graduate writes to alumni who have not previously contributed to the university. The letter is followed a few days later by a phone call from undergraduate volunteers.

The McMillen letter has generated 1,000 contributions worth about $30,000 in its first weeks, according to Robert Smith, head of the university development office.

It has also helped make McMillen a visible figure again around campus.

"When I was up there to have my number retired, a lot of people came up and asked me about the letter," McMillen said. "A lot of people think the university is totally supported by state funding. But it's not. I had to explain that to them."

McMillen is good at explaining things. In many ways he is the prototype political candidate. He is articulate and approachable. And, at 6-foot-11 he is certainly readily identifiable. A McMillen-Holt campaign, young giant against 5-2 hometowner, would certainly pique a good deal of interest.

But at the moment, McMillen is keeping his options open. He is aware of Bradley's jump two years ago from private life to the Senate although the earliest he could run for a Senate seat is 1986. What's more, it is possible that Rep. Steny Hoyer and Attorney General Stephen B. Sachs will both be in that race and McMillen is not likely to challenge either.

The presence of formidable opposition on both sides of the political spectrum seems already to be giving McMillen a closed-in feeling. "Bill Bradley has really led kind of a charmed life," McMillen said, almost wistfully. "I really admire him for his methodical and plodding way of attacking things."

McMillen sees himself in a different mold. Tydings, who has been his political mentor since college, likes to talk about his energy. McMillen certainly looks dashing with his prematurely graying hair, his impeccable clothes, his constantly on-the-move image. (When Bradley played for the Knicks, his teammates offered to take him shopping in the belief he owned only one suit).

McMillen is also quite adept at handling the media. Last summer during the baseball strike, Tommy Stinson, who covers the Hawks for The Atlanta Constitution, received a call from McMillen. Would Stinson, McMillen wanted to know, be interested in talking to him about his political ambitions?

"I was surprised to say the least," Stinson said. "But when I thought about the call, it made sense. We were desperate for stories and it would have been interesting. He was up front about it, didn't try to hide what he was doing at all."

Recently, Dick Williams, a political columnist for The Constitution, wrote a story about McMillen's future. In it, Williams suggested that the ideal starting point for McMillen would be as Hughes' running-mate this fall. That is a spot McMillen would covet. Hughes' people say it won't happen.

"If that kind of offer came up I would seriously consider it," McMillen said. "I'm going to support the governor for reelection anyway."

"It would be a no-risk way for Tommy to start out," Williams said. "Hughes is heavily favored to win and, even if he lost, the defeat would be seen as a rejection of Hughes' policies, not of Tommy."

Williams has known McMillen for three years and admits to being on his bandwagon. "Tom wants to be a senator," Williams said. "I think he recognizes that it might be difficult to just jump into the Senate though. If he didn't run with Hughes this year, the best thing for him would just be to continue helping with fund-raising. He's terrific at that. He combines jockdom, Rhodes scholardom and business. He's got connections all over the place. Watching him work a party is fascinating. He's smooth and he's slick. He's the classic politician."

Two weeks ago, when the Hawks played the Washington Bullets at the Capital Centre, McMillen was host of a party after the game for about 100 friends. Many of those friends were political movers, including state Democratic Chairman Rosalie Abrams, Peter O'Malley and Tydings. McMillen is comfortable with these people now, calls them friends and expects their support when the time comes.

He will probably get it.

"Tom is someone who I like and I admire," said O'Malley, who could end up playing the same guidance role for McMillen that he has played for Hoyer. "He's an achiever. He's also someone who has always been keenly aware of the fact that there is a life beyond sports."

For the moment, McMillen is still working at basketball. Quietly, he has spoken with Hawks management about the possibility of being traded to the Bullets next season so he can be in Maryland during the last year of his contract. He underwent knee surgery two years ago but has recovered and is playing quite well for the Hawks. Still, all those he plays with are completely aware of his plans.

As McMillen walked from the Capital Centre locker room heading for his party two weeks ago, he paused briefly to ask a local reporter if he would be interested in coming. McMillen's coach, Kevin Loughery was standing nearby.

"Am I invited?" Loughery asked, grinning, "or is it just for important people."

"Of course you're invited," McMillen answered.

"Thank you," Loughery said. "I mean, thank you, Senator."

Tom McMillen didn't blink. He just smiled.