The talk of the town in these closing days of the British election campaign is not defense, disarmament or unemployment. It is the way Prime Minister Margaret Thatcher treated Sir Robin Day, the BBC's renowned television interviewer.

Day takes an aggressive, Mike Wallace-like stance in questioning everyone. He minced no words in putting to the prime minister condensed and pointed versions of opposition criticisms of her record, in a prime-time interview nine days before Thursday's election.

The sparks flew for 40 minutes as they sat across the table from each other. As is her habit with the less renowned journalists she faces at her campaign press conferences, Thatcher not only responded volubly, but freely criticized the quality of the information and the precision of the vocabulary of her questioner.

What struck everyone--and was discussed for three days on the front pages --was that she repeatedly referred to the man her own government had knighted, not as Sir Robin, but as "Mr. Day."

After a good many "you did; I did not" exchanges, Day closed off the interview by apologizing for his own interruptions. "That's all right, Mr. Day," the lady said. "I can cope with you."

That her manner with Sir Robin has become a matter of such moment shows the extent to which Thatcher's personality has become a dominant question in this election. And quite a personality it is.

When I first saw her, campaigning as the challenger to a Labor government in 1979, the best phrase I could find for her was "Jack Kemp dressed up to look like Mrs. Miniver." She appeared then to be a spunky politician, so full of nervous energy that she had trouble keeping her voice and hands controlled. Having learned her version of the Great Free Market Truth from her mentor, Sir Keith Joseph, as Kemp learned his from Arthur Laffer, she was Kemp-like in her eagerness to spread the gospel and thereby save a nation's soul.

Today, she is something quite different, transformed by four years in office and particularly by the Falklands victory, into a dominant and domineering figure.

To the extent that there has been any suspense in the closing days of this campaign, it has not arisen from the challenge to Thatcher from the hapless Labor leader, Michael Foot, or the able but lisping third-party leader, Roy Jenkins.

With all the polls forecasting a Conservative victory, the question is how far Britons will trust the self-styled "Iron Lady" with their futures, and how large a mandate they wish to give her for the next five years.

What set her off, in the interview with Day, was his opening question: "Do you understand, prime minister, why some (people) worry about your getting too big a majority?"

"No, I don't," she said.

"Well," Day pressed, "even some who support you say that an elective dictatorship of a huge Commons majority should not be in the hands of someone so dogmatic and strong-willed as yourself."

"That is ridiculous," the prime minister replied. "I believe in certain things. I place those things openly before the people. Those do not change with the size of my majority. . . . I will carry on in exactly the same way as in the last four years."

That way of carrying on is both her strength and her weakness. On the positive side, The Economist's Simon Jenkins noted: "She has reestablished the preeminence of the prime ministership, not just over politics but over the nation as a whole. . . . By her idiosyncratic egotism and emphatic views . . . she has contributed a new definiteness to British politics. Everyone knows what she thinks."

For all that, she enjoyed only a shaky 49-46 percent margin of approval over disapproval as prime minister in a Gallup Poll taken at the time she called the election last month. Her negatives, according to Robert Worcester, another leading pollster, "are personal. She is seen as hectoring people, talking down, lecturing, as someone who won't listen to advice." That reputation--reinforced by the way she has dismissed critics from her Cabinet and converted the Conservative Party into her personal campaign organization--is what her own advisers feared the public saw in her interview with Day.

In this respect, she is almost the opposite of her good friend, President Reagan. She is much sharper in debate than he is, much more in command of the facts. But she lacks his ability to disarm critics, as he did so memorably when he turned to Jimmy Carter in debate and said, "There you go again, Mr. President."

You can bet Ronald Reagan would never have called Sir Robin "Mr. Day." Because Thatcher is "no more Mr. Nice Gal," this election finds her, in effect, running against herself. And that is the toughest competition she's got.