He was just one of hundreds of Capitol Hill employes who have greeted Michael D. Barnes, Montgomery County's congressman-elect, with the words, "you know, I live in your district." But the large gray-haired man went on to reminisce about the time he gave one of Barnes' Republican predecessors a hard time about a local issue.

"I wouldn't do that to you," added House Speaker Thomas P. (Tip) O'Neill Jr., who lives just across the District line in Sumner. "You're a Democrat."

After spending a week being taught about the nuts and bolts of Congress, Barnes said yesterday, his overriding impression is that "you need to be a Democrat, part of the majority, to get anything done."

That whole argument, was not something Barnes had mentioned during his successful campaign against one-term Republican Rep. Newton I. Steers Jr. "It's not a very sophisticated argument," he said.

But after the past two weeks, he believes more than ever it is true.

Although there will be nearly as many new Republicans (37) as there are Democrats (41) in the 96th Congress, Barnes said yesterday, "I haveyet to meet one."

Last week, freshmen Democrats got their indoctrination on the House floor while the Republicans at a meeting room in the Dulles Marriott motel were learning about procedures. This week, the Democratic Caucus met on the House floor, while the Republicans used the nearby Capitol Hill Club, and got on the floor only when the Democrats did not need it.

"It's mostly a confirmation of what I knew," Barnes said yesterday, "but I didn't know how true it was."

Another revelation that falls in the category of "something I knew would happen, but I didn't know," is "the mail. The phone calls. It's astounding how many people just want a few minutes of your time."

Many of those are among the hundreds of job-seekers who hope to fill one of the 18 slots Barnes has available within a staff budget of $273,132 a year.

So far, Barnes has hired only two members of his staff. Keith haller, who was his campaign manager and will be his administrative assistant, and Judy DeSarno, who will be his executive assistant.

Barnes has been kept busy learning such essential details as how Congress' computer system works, how many bells means a roll call vote, how to insert the plastic card coded with his name into the voting machines on the House floor ("that was easy enough") and that there are perquisites for the asking, including a bank inside the Capitol that disburses only freshly printed money.

There has been little time for "the heavy stuff," but that will come next week when Barnes and some of the other new members will spend six days at Harvard being briefed at the Institute of Politics on domestic and international issues.

And virtually every night he has been wined and dined. Last night it was a dinner with the president at the White House. Before that, there were dinners with O'Neill, Majority Leader Jim Wright, the Maryland legislature in Annapolis and a private fund-raiser at the Monocle Restaurant, sponsored by O'Neill and George Meany to help reduce Barnes's $20,000-plus campaign debt.

On Monday, Barnes will learn the location of his permanent office, but he cannot move in until January. Until then, he and a staff of volunteers will continue to work out of cramped transition space in a former FBI file building several blocks away.

Barnes also said he has been offered space in the Cannon Building in the suite of Rep. Nick Joe Rahall II (D-W. Va.). As a freshman member of the 95th Congress, Rahall has been assigned to Barnes as "my buddy." Rahall's suite, incidentally, is next door to the office that Steers will vacate when his term expires Jan. 3.

Barnes said he called Steers the other days and thanked him for "the great cooperation" he has received from Steers' staff during the transition period.

Because he will commute from his home in Kensington-by Metro rail from Silver Spring "as often as practical"-Barnes will be spared one of the biggest problems of a new member, finding a second home.

But the proximity to the home district brings its own problems. Yesterday it meant that Barnes had to eat two breakfasts, first with constituents at a Bethesda restaurant, then with the League of Women Voters at the Capitol. Next came two lunches, one by himself in the House cloakroom and another with constituents who dropped by shortly afterward. Finally, there was dinner with his wife Claudia at the White House. CAPTION: Picture, MICHAEL D. BARNES . . . inundated by mail, calls