Sen. Gary Hart (D-Colo.) was greeting supporters at a reception in Boston Monday when he found himself face to face with 9-month-old Genevieve DiNatale.

As photographers pressed in on him, Hart resisted the stock shot of politician-kissing-baby, shaking the baby's hand instead. Grinning broadly, he said to the photographers, "I don't pose," then ducked his head close to the baby's.

And shielding the baby's eyes from the flashing cameras, Hart remarked to Genevieve, "If you're going to run for president, you've got to get used to it."

Gary Hart got used to it this time a year ago in his unsuccessful bid for the 1984 Democratic presidential nomination. This year, he is back at full speed, ready to claim a place at the front of the pack for 1988.

Hart says he has not decided about running for president, or even about running for the Senate in 1986. But he has set out for himself the kind of agenda that will keep him as visible as possible.

Hart's plans for 1985 contrast sharply with the course followed four years ago by Walter F. Mondale, the man who defeated Hart last year. In 1981, the former vice president went underground for a year of "reeducation" and rethinking. Hart says he has no intention of disappearing in 1985.

"There's an enormous vacuum in the [Democratic] party," he said in an interview. "I did my rethinking in 1981, '82, '83 and '84. What the party needs right now is . . . someone to fill the vaccuum thematically."

The "true patriotism" speech Hart delivered in Boston's historic Faneuil Hall Monday is the beginning of Hart's effort to do that.

He said he plans to take the themes of his Monday speech to party dinners and gatherings around the country this year. Some of those gatherings will be used to help retire Hart's $4.3 million debt left over from 1984, a job he hopes will be nearly finished by the end of this year.

Hart also plans to deliver roughly a speech a month on such topics as economic growth, defense policy, military reform, tax reform and arms control. One goal is to "challenge" others in the party to get into the debate.

In addition, he is planning to establish a think tank to bring together "creative thinkers" on public policy, is writing a book on defense policy, is planning to rejoin the military reform caucus in the Senate and says he will "be involved" in Senate debates about the budget, tax reform and other issues.

"The plate is full," he said.

He also has made a number of staff changes, beginning with hiring William Dixon as his administrative assistant. Dixon worked for the House Judiciary Committee during the Watergate days and was most recently Wisconsin banking commissioner. Hart said his former administrative assistant, Buie Seawell, will soon become Colorado Democratic Party chairman.

Hart also plucked Jeremy Rosner from Mondale's campaign speechwriting team, filling a void that existed last year, and hired as his legislative director David Dreyer from the staff of Rep. Stephen J. Solarz (D-N.Y.). He named Beth Smith of his staff to replace his well-respected press secretary Kathy Bushkin, who has gone to work for U.S. News & World Report.

Hart said he doesn't know where all this will lead. "I have not made a decision, and will not until the end of 1985, about 1986."

Many Colorado Democrats believe that Hart will not run for reelection to the Senate if he intends to run for president in 1988. Hart says the choice is not that simple, although he notes there is a "fundamental question of fairness" to the people of Colorado involved. But he dismisses talk that he might have trouble winning another race for the Senate. "I can win reelection in Colorado," he said. "There's a sense in Washington that I hurt myself in Colorado by running for president. That may be true to a degree. But many people are proud of the race I ran. It was a modest net-plus."

Hart also served notice to critics who say the party will turn away from the 1984 field of candidates in 1988. "I would disavow and disabuse people of the notion that anybody involved in 1984 is out of contention for 1988," he said, noting that he has developed a strong network around the country.

Meanwhile, he is staying ready. "The leader of our party in 1988 will be the person who best articulates a message for the future of our country in 1988," he added. "It will be the person with the vision and the message, period. And that will not be decided in 1985."

Hart acted on Monday like a man who had put the disappointment of 1984 behind him. On the flight to Boston, he was explaining why he had decided to give the "true patriotism" speech when a reporter asked: "Senator, why did you change your name?"

Hart paused, started to say his mother had done it, and then dissolved in laughter.