After four months in office, Jimmy Carter is winning over the factory and office workers in middle-class Midwestern suburbs who nearly elected Gerald Ford last November.

That unmistakable conclusion is based on our day-long interviewing this week, assisted by pollster Patrick Caddell's Cambridge Survey Research, in Ward 6 of Franklin Village, southeast of Milwaukee. Although Ford easily carried the ward, there is no doubt he would lose it - and lose it badly - if the Ford-Carter election were rerun today.

The reason is exemplified by the young wife of a standards engineer who voted for Ford last November but would back Carter now. "At first I thought Carter was too liberal, but now I think he's safe enough," she told us. "He's kept a lot of his promises. He's active doing things."

This image of a straightforward, energetic conservative shows the Carter political strategy succeeding brilliantly. Although some Carter voters complained that he has moved to the right, they are not disturbed enough to express support for Ford. Moreover, substantial opposition to the President's gasoline and gas-guzzler tax proposals clearly have not diminished his popularity.

Ward 6 was selected for us by Cadell as a middle-income (median: $16,175) suburb with heavy Ford support last November (58.3 per cent). The first evidence of change was that 69 registered voters interviewed reported to us their 1976 votes as follows: Carter, 37; Ford, 31 (with one voter underage last year). Caddell's experts say this is a case of Ford voters' telling a pollster, consciously or not, thatthey voted for the winner rather than the loser.

But even among 31 admitted Ford voters, four back Carter today and four more are undecided. Thus, our interviews show 41 for Carter, 23 for Ford and 5 undecided - exactly reversing the outcome here seven months ago.

Furthermore, 25 of the 69 voters interviewed, including 16 Ford voters, said their opinion of Carter has improved since the election. Eighty-five per cent of the voters interviewed approved of the President personally (compared to 83 per cent for Ford) and 76 per cent approved of his general performance as President. Yet, they were hard put to find anything specific to say about his actual performance.

Rather, favorable statements concerning the President are within Carter's own chosen frame of reference: style, not substance. A construction worker's wife, switching from Ford to Carter, told us: "He hasn't brought his religion into office like I was afraid of. I've learned more about him and come to trust him. He's a truly honest person." Other voters praised him for "openness" or "being a good family man" or "being close to the people."

A case in point: These voters disapproved of the President's proposed gasoline tax by 2 to 1 and disapproved of his gas-guzzler tax by 3 to 2. But their whole here exceeds the sum of the parts. All but 6 voters of the 69 are convinced by the President's plea that the energy situation is "serious," and they approve of his overall energy plan by a substantial margin.

With self-styled "conservatives" outnumbering "liberals" by nearly 2 to 1 among the voters interviewed, Carter gains here by being perceived as moving rightward. "He doesn't take the liberal Democratic approach," said a middle-aged engineer, a Republican switching to Carter. "He approaches things logically, rather than politically."

But a retired foreman complained, "Carter was going to do so much for the old people. But what has he done? Nothing," Similarly, 55-year-old machine operator told us. "He's for all the rich, as it turns out - just like Ford." But both, avowed Democrats, say they would still back Carter over Ford.

Others complaints about the President tended toward style - his grin, members of his family, his informality - and what was perceived as indecisiveness. Some voters accused the President of being impetuous, citing his treatment of Maj. Gen. John K. Singlaub.

"I don't like what he did to that general," said a 39-year-old part-time nurse who is married to a teacher. A Ford voter last year, she told us, "Carter has come across better than I thought," but she does not know who she would vote for today because she fears the President is too impulsive.

Her position may have been influenced by nation wide publicity given the Singlaub affair immediately before our interviews. Nevertheless, it may also suggest that President Carter's support is as shallow as it is wide, that approval of style may be superseded by disapproval of specific action. Having switched loyalty, the voters of Ward 6 could change again.