DES MOINES, JAN. 31 -- You could call it the greening of Michael Dukakis.

In the final week of the Iowa campaign, the governor of Massachusetts is trying to close the passion gap, which is about all that prevents him from becoming the instant favorite for the Democratic presidential nomination.

He has fought his way into contention in the Feb. 8 caucuses against his main rivals, Rep. Richard A. Gephardt of neighboring Missouri and Sen. Paul Simon of next-door Illinois. Although still as out-of-place in a discussion of farm policy as a bond salesman in a barnyard, the urbane Dukakis has assembled an organization as good as any in the race and made his name familiar through 78 days of campaigning.

His greatest remaining problem in winning the caucuses and thereby solidifying his favorite's role in the Feb. 16 New Hampshire primary and subsequent contests was capsulized in the lead sentence of the profile in today's widely read Des Moines Sunday Register: "Michael Dukakis appeals to the head, not the heart."

On the stump the past two days, and in a constant barrage of television ads, Dukakis is doing his best to convince the activist caucus-goers that he is not simply the "brainy, cool technocrat" the Register profile described, but a man who shares their dreams -- and their anger.

His principal vehicle is the issue of aid to the contras, a hot-button question for the kind of Democrats who pick the delegates here. Dukakis has consistently condemned the "illegal, improper" Reagan administration effort to apply military pressure to the Sandinista government in Nicaragua by financing the contra forces.

But his rhetoric is newly emotional, fueled in part by his reaction to a front-page story in last Thursday's New York Times, written by his one-time protege and aide, Stephen Kinzer.

With a catch in his voice, Dukakis on Saturday gave audiences in Cedar Falls and Cedar Rapids a paraphrase of what Kinzer had seen in a Managua hospital.

"You almost always see a child or two or three, without an arm, without a leg, without vision," Dukakis said, "a child who has been hurt in a contra raid, a child who has been playing around with a Sandinista mine, a victim of this war. And when spokesmen for the administration talk about keeping up the pressure on the Sandinistas, we have to know what pressure really means: kids with their arms blown off, pregnant women assassinated and raped, villages and farms destroyed. That's the human face of 'pressure' . . . . And that's why I say: Not one dollar of contra aid. Not one. Not one!"

As he spoke those words, audiences of several hundred who crowded the liberal churches selected for his appearances first grew silent and then exploded in applause. "It was a side of him I'd never seen before," said a woman in Cedar Rapids. "I could feel his rage."

Aides talk about the phenomenon as "connecting on the human level," something they concede has never been easy for the methodical, task-oriented Dukakis, even with his own staff members and advisers.

It has become a critical factor in caucus politics, because Gephardt and Simon already have managed to make that connection: Simon by identifying himself with the compassion and tradition of Hubert H. Humphrey, and Gephardt by borrowing some of Jesse L. Jackson's populist anger at the large corporations and "the establishment."

Ethel Klein, a political scientist and Gephardt adviser, commented today that Dukakis is wise to show more emotion. "People have to make a leap of faith to vote for someone for president," she said, "and there are a lot of voters still hesitating. They recognize Michael has leadership ability but they don't feel they have access to him because he has been so emotionally closed. He can't just say, 'I care'; but by talking about something he really does care about, he conveys that message subliminally and makes himself more accessible."

The same message was reportedly given to Dukakis by a Harvard law school classmate and old friend, former Iowa senator John Culver, who endorsed him and campaigned with him across the state on Saturday. Culver, a bear of a man with a renowned temper, almost totally overshadowed Dukakis at a joint appearance, but later toned down his own rhetoric while encouraging Dukakis to put more passion into his speeches.

Dukakis' speaking efforts complement a pair of new ads by Ken Swope, a media consultant who joined the campaign at the beginning of January. One is on the contras, the other on homelessness, with Dukakis reacting to almost unbearably poignant scenes of street people in the shadows of the White House.

When the governor returns to the state Wednesday for his final push, plans are for him to focus the same kind of emotional rhetoric on his basic theme of jobs and economic growth. "I've told him," said his new press secretary, Francis O'Brien, "I don't want to hear any statistics. I want to hear you talk about people you know at home and people you've met in Iowa, and what you've learned about how you and your leadership can help make their lives better."