The lawyer from Arnold & Porter wasn't apologizing, exactly, but it was true, he told a caller, the law firm wasn't aiding any of the bidders in the competition for Conoco Inc.

"We do represent people who are interested," he added, lest the caller think that one of Washington's largest legal establishments was missing out on the billion-dollar Conoco sweepstakes.

The heavens have opened and the mamma is falling already on Washington's legal establishment as the bidding for Conoco spreads and becomes more complex. Or, as one lawyer put it indelicately, "The meter is running."

"There must be 10 or 15 firms in this," said Michael F. Butler of the Washington office of Andrews, Kurth, Campbell & Jones. The Houston-based firm (which White House Chief of Staff James A. Baker III left to join the 1980 presidental campaign) is part of the legal army retained by the Seagram Co. of Canada, whose offer for Conoco stock set off the bidding war. "It looks like Martindale-Hubbell," said Butler, naming the huge, multivolume encyclopedia of law firms and attorneys.

Mobil Corp. entered the fray yesterday, seeking to silence potential criticism by insisting that the acquisition of the ninth-largest U.S. oil company by the second-largest would not violate Justice Department antitrust guidelines.

To buttress that opinion, Mobil called on Sanford Litvack, the assistant attorney general in charge of the Justice Department's Antitrust Division in the Carter administration, as well as Howrey & Simon, one of Washington's leading specialists in antitrust matters. Litvack, who had worked on antitrust matters for Mobil as an attorney with Donovan Leisure Newton & Irvine in New York City before joining the Justice Department, has returned to his old post.

Conoco's legal representatives include the firm Skadden, Arps, Slate, Meagher & Flom and, in particular, merger expert Joseph Flom in New York City. The firm's Washington staff includes Lynn Coleman, former general counsel of the Department of Energy in the Carter administration.

Another Skadden, Arps member happens to be Irving S. Shapiro, who retired as chairman of E.I. duPont de Nemours & Co. in April, two months before Du Pont surprised most of the financial world by making a bid to acquire all of Conoco.

Shapiro, who played an important role in preparing Du Pont's bid, says he hasn't gone near Conoco. "They put up a wall between me and the law firm, and that agreement was never breached," he told The Wall Street Journal.

Stock offers and antitrust policy are only two of the issues that radiate from the Conoco bidding.

Some of Washington's best known legal names are on opposite sides of a lobbying fight over a possible governmental crackdown on Canadian investment in U.S. energy companies -- a response to Seagram's bid and other financial power plays by Canadian firms since the spring.

Thomas H. Boggs Jr., of Patton, Boggs & Blow, and former representative James O'hara have been urging restrictions on Canadian investment on behalf of Conoco, their client.

James T. Lynn, former budget director in the Ford administration, is working for Cities Service Co., whose potential merger with Conoco was nipped in the bud by the Seagram offer, and which has its own stock fight with another Canadian firm. Lynn is with Jones, Day, Reavis & Pogue.

On the other side of that lobbying fight is Seagram.Butler, chairman of an American Bar Association committee on foreign investment in the United States, isn't alone. The Canadian liquor company also is represented by Van Ness, Feldman, Sutcliffe, Curtis & Levenberg of Washington, whose lineup of energy experts includes Charles Curtis, chairman of the Federal Energy Regulatory Commission in the Carter administration, and William J. Van Ness, former chief counsel of the Senate Interior an Insular Affairs Committee.

Fried, Frank, Harris, Shriver & Kampelman is aiding Seagram, too -- its partners include Sargent Shriver, Sen. Edward M. Kennedy's brother-in-law, and Max Kampelman, one legislative counsel to the late Sen. Hubert H. Humphrey. Another former Humphrey associate, Patrick J. O'Connor, heads the firm O'connor & Hannan, another of Seagram's allies, with former Sen. Edward Brooke among its members.

And not to neglect the Reagan camp, Seagram also is represented by Anderson, Hibey, Nauheim & Blair -- Stanton D. Anderson was a prominent member of the Reagan transition team.

There is a lot of chuckling about all this, a prominent Washington antitrust lawyer said, asking for anonymity. President Reagan's arrival in Washington seemed to inaugurate a dry spell for Washington's fraternity of antitrust lawyers, who assume there won't be a flood of suits by the Justice Department to keep them busy. "But they've done the next best thing," he said.

In appearing to open the door to possible mergers that would have seemed unthinkable before, the administration has sown legal confusion far and wide, and that is fertile soil for law firms, he said.