THE aura of ''inevitability'' that had hovered over the Mondale candidacy for the Democratic nomination before the campaign began - and which Gary Hart's New Hampshire upset had dispelled for a while - has been quietly gathering again.
Such political concepts as inevitability - or momentum, or ticket balance - are basically insubstantial. They are descriptive terms, hardly concrete measures. But like Banquo's ghost, they can cause powerful reactions among those who think they see them.
Now it is Gary Hart who seems unsettled by inevitability's return. The delegate numbers partly show the story: Walter Mondale, 1,237 delegates; Gary Hart, 670; the Rev. Jesse Jackson, 207, with 1,967 needed to win. Just as important, Hart's sharpening attacks on Mondale - the latest reminding voters of the Tehran hostage seizure under Jimmy Carter and Mondale - appear to have a declining effect. Hart's initial appeal as a candidate of new ideas has been slipping into the tired old mode of ad hominem attack.
It is hard to see how Hart can continue such a campaign style beyond this next week, with the Texas caucuses Saturday and the Ohio primary next Tuesday. On style alone, he risks tilting voter sympathies increasingly to Mondale. It is Hart's option to compete all the way to the convention. But at some point soon he should consider how to influence the direction of his party in order to preserve what remains of his candidacy's capital.
Jesse Jackson, by contrast, appears not at all troubled by Mondale's steady recovery. The euphoria of Jackson's victory in the Washington, D.C., primary this week, and his respectable finish in Tennessee, reflect a candidacy at the moment on the rise.
Jackson's delegate total is negligible. His impact on the campaign is not. He is bringing out new Democratic voters in droves. He has campaigned with intelligence and verve, offering ideas outside the mainstream.
There is a debit side to the Jackson candidacy. He has a potential to hurt his party among Jews, Southern whites, and white blue-collar workers. Critics brush aside his domestic proposals - a surtax on incomes, a 20 percent slash in defense outlays, and a $50 billion jobs ''infrastructure'' program. His foreign policy proposals - a heavy third-world emphasis and reliance on negotiation rather than military might - miss the mark of the Democratic Party consensus.
Yet a candidate who appears on the covers of two major newsmagazines the day before he had won his first victory must have something going for him. This could be Jesse Jackson's big week, his 1984 hurrah.
The Democratic campaign will likely quiet down after next Tuesday. The California-New Jersey finale in early June could come down to a test of whether Mondale's relative position has held.
In the time between, the Democratic candidates will have to adjust to the July convention perspective. Mr. Jackson has already begun to negotiate his differences with the party leadership. Hart has yet to deal with the demands of a secondary role. Mondale will have to start setting himself for the general election.
A period of anticlimax holds risks for candidates. It was in the Texas debate four years ago, his candidacy already reduced to a run for the vice-presidency, that George Bush tagged Reagan's campaign promises ''voodoo economics.''
Mr. Mondale, as rival Hart keeps reminding him, faces an apparition of ''inevitability'' himself: the conventional wisdom that Mr. Reagan will likely be re-elected. Mondale may be moving into hailing position for the nomination in July. But he still has to shift his campaign strategy from Democratic rivalry to representing his party as Reagan's opponent for November.