THE PRESIDENCY ''I'VE MELLOWED A BIT''

By Hugh Sidey

On Wall Street, Don Regan was called disruptive, cold, irreverent--and he led Merrill Lynch to the top. At the Treasury Department, he was said to be insensitive, arrogant, unsophisticated--and he helped launch the nation on one of the longest peacetime booms in history. As chief of staff of the White House, Regan has been accused of being unfeeling, narrow-minded, egotistical --and the President he serves now stands at an all-time high in prestige and acclaim. ''When you are on the inside, quite obviously you always think you are doing a better job, perhaps, than some of the outsiders may consider,'' says Regan, explaining the seeming paradox between his reputation and his results. He knows he gets paid to take it on the chin for his boss. Just last week he was lambasted for reshuffling the White House speechwriting department, easing out two of the top wordsmiths, in order to make it more to his pragmatic leanings than to those of the resident ideologues like Patrick Buchanan. Many in Congress speak wistfully of the days when James Baker ran the White House and there was a lot more cajoling and stroking. But Regan appears on his way to becoming another of those people in this Administration whose political obituaries were premature. Regan, perhaps the second most powerful man in the Government, will probably go the distance with the President. But it will be an increasingly difficult task, as power inevitably wanes and every issue becomes tainted with politics in the struggle to choose a Reagan successor. ''It is absolutely uncanny,'' muses Regan. ''Something small that you think is just a nit and a gnat, and all of a sudden, if not handled correctly or somebody gets offended by the way it is handled, it blows up into a major issue. Take the Mike Deaver case. It never occurred to me that that is the kind of subject that could suddenly become a lead story. You open up the paper in the morning to see who the hell has leaked what now, or what we are being castigated for.''

It is the business of trying to anticipate trouble that consumes him and marks a major difference between his corporate and political lives. ''In the corporate experience, you had more time to think, more time to visit individuals, more time to look around at the organization,'' Regan declares. ''The problems are brought to you here. I miss my ability to, it's not quite snoop, but to look around rather than to have it fed to you through others all the time.'' If the day ever comes when he talks to students of Government about running the White House, Regan will hammer the point that too much attention is paid to theories of management and not enough to personalities. ''At least three-quarters of this job is personal,'' he says. ''One person replacing / another on a job can make an entire difference, good or bad. I would tell them, in deciding why things happen, to make more of a study of the people involved.'' For all his wariness, Regan obviously loves the battle and gives back brickbats of his own. ''Having a reasonably short fuse, I am usually very unhappy at a less than perfect performance. We are always under the spotlight. There are always many decisions that a corporate executive makes in a hurry. They are crisis decisions, but he doesn't have a thousand reporters watching him and 5,000 who are going to write or comment on how he handled it. ''It's the art of the possible,'' says Regan, who believes he is calming a few critics. ''I think people see what we are doing, and they've grown accustomed to it. Perhaps I've mellowed a little bit. I said a little bit.''