Like the man in the song, Gary Hart is "another year older and deeper in debt." The Colorado senator is also short of sleep, he complains, because he went straight from two months of stumping for the Mondale-Ferraro ticket and other Democratic candidates to a six-week blitz of events aimed at paying off the big bills left over from his runner-up race for the Democratic presidential nomination.

But on the morning last week that Hart stopped for coffee and conversation in the Senate restaurant, he looked about as chipper as someone can who is carrying a $4 million debt.

"We could resolve it right now if I had $2.5 million," he said, almost cheerfully. "I hope it will be paid off by the summer or fall of 1985; certainly it will be by the end of next year."

Meantime, Hart is full of plans for 1985 -- an important year that will end with his decision whether to run for reelection to a third term in Colorado in 1986 or drop out of office temporarily and devote full time to another presidential bid in 1988.

He ran for the nomination in 1984 believing that the Democratic Party was on the brink of generational change and fearing that if he did not seize the moment, one of his contemporaries would snatch the prize. In 1988, he knows, Iowa and New Hampshire will be overrun by aspiring young Democrats, so he does not intend to be backward about exploiting the recognition he gained by what historians may judge his "premature" effort this year.

Soon after President Reagan's Inauguration and State of the Union Address, Hart plans to deliver a "New Patriotism" speech that he says "will try to recapture that phrase for those who think it should suggest contributing to society and not just yourself.

"It will," he says, "be both visionary and specific on ways in which people can make that contribution."

The Senate will be his main forum, and he is fortunately situated, with membership on the Armed Services and Budget committees that are certain to be arenas of major policy debates. He is seeking to return to the Intelligence Committee, on which he served in the '70s, "because if there is going to be mischief in Reagan's second term, the CIA will almost certainly be involved."

The 1984 campaign was a bittersweet experience for Hart, but the impression I have is that he has absorbed its lessons and is putting them to use -- just as he did when he moved from managing George McGovern's campaign in 1972 to planning his own first race for the Senate in 1974.

Privately, it must rankle Hart to reflect that, but for his own errors, he might have captured the nomination. Publicly, what shows is a disciplined determination not to repeat them.

"Early on, in the living rooms of Iowa and New Hampshire, I had time to make a thoughtful presentation of my ideas on military reform and economic-growth strategies, and people saw that they made sense," he said. "But after I beat Mondale in New Hampshire, the front- loading of the election calendar he had arranged forced me into a roller-coaster media campaign. I had the challenge of shorthand- ing my ideas in unfriendly forums, and I didn't know how to do it in seven minutes on an airport tarmac, before flying off to another city.

"By the time we got to Ohio and California, I had begun to learn -- but by then, he [Mondale] had it [the nomination] sewed up."

The implication of this is clearly that Hart believes neither he nor his "new ideas" suffered any kind of long-term rejection in 1984. "What has happened since Nov. 6," he said, "is that the Hart vocabulary of 1983-84 -- especially the emphasis on qualitatively improving our defenses and expanding our economy -- has become the language everyone in the Democratic Party is using. Four months ago, there were two different vocabularies in the Democratic Party. Now, all of the discussion is in the language and territory I staked out, and that is half the battle."

Hart clearly sees himself now as an insider in the party leadership -- not a young pretender seeking recognition from the throne. In that respect, he is certainly correct.

With Walter Mondale's retirement from the pursuit of elective office, Hart is one of only three Democrats (Ted Kennedy and Jesse Jackson are the other two) with national recognition and a network of active supporters reaching into almost every state. All of the other 1988 prospects, including New York Gov. Mario Cuomo, start further back.

Hart is positioning himself on the procedural questions now before the party -- the choice of a new chairman and the debate on the rules -- very much as a man who understands the distance he has come from his outsider status and the distance he still must go to reach his goal.

His history suggests that it would be a mistake to underestimate either his intelligence or his persistence.