Even after it was over Sen. Patrick J. Leahy (D-Vt.) insisted it had not been a big deal to him.

His announcement last week that he was dropping his opposition to construction of a convention center in downtown Washington was big news - more than two-dozen reporters and photographers crowded around him during a press conference at the Capitol.

It was obvious that Leahy gesturing and raising his voice all the while, enjoyed the attention.

But was it "a load off your mind?" he was asked.

"No," he replied. "Frustrating," he conceded, "but it was just one of many items" a senator deals with.

Now if you want to talk about big deals, he said, how about his work "rewriting the complete pesticide legislation for the whole country" earlie this year. "That was significant," Leahy said.

It was that aloofness - arrogance, some critics called it - that had prompted supporters of the convention center to look for hidden motives behind Leahy's 20-month-long opposition to the proposal.

What did he care about the District of Columbia? they asked. He comes from a rural state whose population is smaller than that of the city over which he has such a strong control; he did not choose to live here, picking instead one of those new, expensive developments in McLean.

But there he was, giving the go-ahead of the convention center, and saying how much he believed in home rule for the city.

If some did not believe him, others did. Sen. Edward M. Kennedy (D-Mass.) recalled recently that Leahy was one of his major allies in getting the necessary two-thirds vote in the Senate for the proposed constitutional amendment that would give the city full voting representation in Congress.

Leahy says, he lives in Fairfax County because moving here from Vermont after his election in 1974 was "a very difficult adjustment" for his wife, Marcelle, and their three young children. McLean was "the place as similar to home as we could find" and Fairfax County had "fine public schools."

"Besides," grinned Leahy, "I didn't come down here with the burning ambition of becoming chairman of the District Committee."

Leahy, a liberal in the Kennedy-Humphrey tradition, nonetheless found himself cast in the bad guy role by boosters of the convention center, especially in contrast to his conservative House counterpart, Rep. William Natcher, who long ago agreed to approve the project.

He may be a small town boy from Burlington, but Washington is "the only large city I would feel comfortable living in," he said one day as he squeezed his 6-foot-4 frame into his Dodge Aspen for the 45-minute commute to Capitol Hill.

He has heard all the nasty things people said about why he fought the center for so long, and he scoffs at their reasoning.

One person who observed Leahy presiding over a hearing on the city's budget had suggested that as a senator from a state that, until recently, had more cows than people, Leahy was "overwhelmed at the sight of all those zeros" in the proposed $1 billion budget. The D.C. Department of Human Resources, it was observed, alone has 12,000 employes, which is more people than live in the third largest town in Vermont.

Two Congressional colleagues insisted that Leahy's opposition to the center could be traced to the day City Councilman Marion Barry denigrated Leahy as a "rinky-dink" senator from a small state.

Still another popular theory was that the real mastermind behind the opposition was Michael Hall, the subcommittee's professional staff member, who influenced Leahy in opposing the project.

Leahy has easy answers for all of those suggestions. The idea that he was dazzled by the size of the budget is "ludicrious," he said, nothing that he previously served two years on the Armed Services Committee, "where you get used to big numbers in a hurry." As for Barry's remark, Leahy points out it was delivered in reaction to his already announced opposition to the center. Finally, Leahy says that staffer Hall initially was neutral on the subject until the senator "spotted it as a ripoff of the taxpayers."

Leahy pointed to his willingness to compromise on the revised financing plan as proof that he supports home rule for the city. Otherwise, he said, he would have killed the convention center in committee because he personally views the project as "a lousy idea." But because "the duly elected officials favor it," he limited his concern to "making sure that the tax dollars that go into it" are committed with the assurance that the project will be self-supporting. He contends that most District of Columbia residents "will never use or benefit" from the center and that they would reject the idea if given a chance to vote on it, as now planned by its opponents.

Leahy freely acknowledges that his principal interest in being chairman of the subcommittee is, as it was for his predecessors, to use it as a stepping stone to a more prestigious assignment. But he is likely to retain the District post through the 96th Congress (1979-80) in return for a promise from the Senate leadership that, assuming he is reelected in 1980, he will get the job he covets, as chairman of the defense appropriations subcommittee.

To members of the Board of Trade and other city leaders who had anticipated his reassignment at the end of this year, Leahy offers the caustic advice, "take two aspirin."

Under existing legislation, the city cannot issue its own bonds, so it borrows from the U.S. treasury and repays the loans from revenues. Because the federal payment - the money Congress appropriates each year to make up for the nontaxable federal presence - approximates one-third of the District of Columbia's billion-dollar-plus operating budget, Leahy figures that the federal risk in any city undertaking works out to about one-third.

Eventually, Leahy believes, the city should be given full home rule, including the ability to sell bonds, with U.S. involvement limited to zoning "so that no one could put a series of fast-food franchises on the mall" and security of federal officials and installations.

When that occurs, Leahy said, elected officials will "run everything" and if they want to do something foolish, such as build a convention center, no one in Congress will stop them. "But there'll be no federal bailout either," Leahy said. Instead of the variable federal payment, Congress would pay a fixed sum, in lieu of taxes, on federal properties within the District.

Jeannette Westover, owner-operator of Westover's grocery store and Texaco station, the only commercial enterprise in Middlesex, Vt., offered directions to Leahy's farm, a couple miles over dirt roads from her cross-roads general store.

"Pat's been a customer ever since we've had the store," which is 19 years, she said. She' never been inside the Leahy house - "not that I haven't been invited," she was quick to add.

The circa 1840 farmhouse is a modest, two-story frame structure, about one-third of the size of the Leahy's new place in McLean. From the front yard, a visitor gets a panoramic view of a dozen mountain peaks, lush with pine trees, and enough rolling open space to permit the crosscountry skiing that the Leahys enjoy during their winter visits.

On a hallway wall inside the house are pictures of Leahy with Sen. Kennedy and their notables, and headlines from two issues of the Rutland Herald. The issue dated Nov. 1, 1974, a survey of Chittenden County (Burlington) voters in bold type: "Chittenden Poll Dooms Leahy." The same paper, five days later, screamed in equally large letters: "Leahy Upsets Mallory, Wins Aiken's Seat."

Leahy's predecessor, the venerable Republican George Aiken, was first elected tothe Senate in 1940, which was, as Leahy points out, "the year I wes born." At 38, Leahy is younger than all of his Senate colleagues except Sen. Joseph R. Biden Jr. (D-Del.), who is 35. Because he is balding, Leahy looks considerably older than his years, a happy circumstance that permits him to emphasize his youth while looking mature enough to satisfy voters who were accustomed to elder statesman Aiken, who was 82 when he called it quits four years ago.

"You gave away my canal," complained State Sen. Seeley Reynolds Jr., of Leahy's vote last spring in favor of the Panama Canal treaties. Reynolds, a crusty Republican legislator greeted Leahy as they waited to take their places in a holiday parade through the picturesque college town of Middlebury.

"Seeley would rather hold it at gunpoint for years, at a cost of a billion dollars, and he's the only one in Vermont who could afford that," Leahy responded. Graspring the old man by the shoulder, Leahy continued, "Seeley is a bit to the right of Attila the Hun." Both men laughed, exchanged handshakes and headed for their respective convertibles for the ride to the town bandstand.

Leahy plunged into a leaden text that called for reduced military spending while ensuring that the U.S. remains "a first-rate world power." It is critical to reach agreement with Russia on the strategic arms limitation talks (SALT), Leahy said, noting that he is the father of three school-age children "who will live most of their lives in the next century, if there is one."

Leahy was invariably greeted as "Pat" or "Patrick," prompting him to remark later that "if they call me Senator, it probably means they didn't vote for me."

John Z. Williams, a Republican lawyer from Poultney, who had marched in the parade with the Cairo Temple Shrine band, has known Leahy since they took the bar exam together 14 years ago.

"Vermont has always been a three-party state, two Republican and one Irish," said Williams, analyzing Leahy's success. With the retirement of Aiken, many progressive Republicans, which Williams said included himself, swung to Irish Catholic Pat Leahy because the conservative wing had captured the GOP.

Another parade participant, State Sen. James Douglas, 26, a Republican who represents Middlebury and Ripton (hometown of poet Robert Frost) in the state legislature, credited Leahy with "good public relations. He keeps in touch."

Douglas is among some Leahy watchers who critize him for dropping the names of "Jimmy, Fritz and Hubert" into his conversations, but conceded, "Leahy's popularity is extremely high, though not as high as that of Jeffords." (Rep. James M. Jeffords, a Republican who is the state's lone member of the U.S. House, is a contemporary rival of Leahy's but he has announced he will not challenge Leahy for the Senate in 1980.)

Sen. Robert T. Stafford (R-Vt.) asked to assess Leahy's performance, cautions, "I should tell you, we've become close personal friends. Pat's a good senator. We vote differently on purely political issues, such as organizing the Senate, but otherwise, there's no more division than when Aiken and I were here. Pat's a moderate Democrat; I'm a moderate Republican."

Stafford supports Leahy's contention that what happens within the D.C. subcommittee "doesn't make much difference back home," although he concedes that it might help the liberal-leaning Leahy if his frugal Yankee constituents are aware that "he stood for economy" in opposing the convention center.

Although Leahy insists "I have never issued on press release or once raised the subject in Vermont," his long-standing opposition to the project was widely and favorably reported in home state newspapers.

As one of the more junior members, Leahy is required to preside over the Senate for one hour each week. One day before the end of the session he drew the doubly dubious distinction of presiding over an especially early opening hour, beginning at 8:30 a.m.

Leahy raced from his office at 8:22 a.m., and punched the "senators only" button on the elevator. Whisked to the basement level of the Russell building, he summoned an electric subway car by sounding a "senators only" bell.

The buttons were just a few of the perquisites that a Senator enjoys daily, and as Leahy said later that day, when a Capitol policeman stopped traffic so he could cross Constitution Avenue against the light, "It's all a bit much."

At 4:30 p.m., after an uneventful and routine day, Leahy returned to his office, kicked off his shoes, proped his feet on a coffee table and was briefed by his top aides on plans for the next day.

"Expect a Proposition 13 speech" from Sen. William Proxmire (D-Wis.) at a HUD appropriations session, cautioned legislative aide Marty Frank, who predicted Proxmire will "scream" at one item. "And he's have his remarks reproduced at $300 a page in the (Congressional) Record, and 25,000 reprints distributed," added Leahy.

The HUD budget markup, however, would give Leahy an opportunity to object to what he views as an "urban bias" within the agency. In a column he writes for weekly newspapers in Vermont, Leahy has complained that HUD typically awards grants for subsidized housing in units of 20, which makes ineligible small towns that may only need and can only support two units.

Shortly after 6:45 p.m., just 12 hours after he began his day, Leahy was en route home. He passed up an invitation to a working dinner at the White House, saying "if you accepted all the invitations, you'd never see the family."

Marcelle Leahy, a petite woman of French-Canadian parents, was fixing rigatoni and watching Walter Cronkite on the portable color TV set when Leahy entered the kitchen at 7:20 p.m.

He changed quickly from his stripped suit to jeans and sweatshirt, popped open a beer for a dinner guest, fixed a mixed drink for himself and poured a glass of wine for his wife, who feigned embarrassment and said, "I just read that white wine is out."

The children romped in from various parts of the sprawling house to greet their dad. Mark, 8, punched the senator in the stomach; Alicia, 12, offered a ladylike peck on the cheek and Kevin, 14, asked how soon he was going to get some help erecting a basketball backboard over the garage door.

About 90 people sipped gin-and-tonics on an unseasonably hot Vermont evening in the lobby of the Middlebury Inn, as Leahy, featured speaker at a dinner meeting of the Addison County Democratic Party, held forth on one of his favorite gripes, year-round sessions of Congress.