In 1981, US Sen. Paul E. Tsongas presented his vision of the Democratic Party's future - a vision that in many ways diverged from traditional liberalism.
That streak of political independence was apparent again Wednesday when he endorsed Sen. John Glenn (D) of Ohio's bid for the 1984 Democratic presidential nomination.
The endorsement's impact on Senator Glenn's prospects or on Senator Tsongas's political future is uncertain. But it would appear to put considerable distance between the Bay State senator and fellow liberal Democrats.
Backers of former Vice-President Walter F. Mondale are particularly displeased, not so much because Tsongas passed over their candidate but because he said he doubted that any liberal Democrat - a category that would include Mr. Mondale - could beat President Reagan should he decide to run for a second term. His endorsement, he explained, was based substantially on the Ohio senator's ability to be elected and capacity to govern effectively.
Some political observers say Tsongas stood little likelihood of having a major impact on the Mondale campaign. They say that being the first US senator to back his colleague from Ohio could allow Tsongas to gain a position of potential influence in the Glenn campaign.
Others note that Tsongas's brand of ''Atari Democrat'' liberalism is different from that of traditional liberals such as Mondale, Sen. Alan Cranston (D) of California, and others. This would help explain why he might not have much influence on the Mondale camp.
To what extent the endorsement might alienate Tsongas from other Democratic liberals, such as fellow Massachusetts Sen. Edward M. Kennedy, is uncertain. If nothing else, it could make it a bit harder for Senator Kennedy to remain neutral as candidates scramble for the Democratic presidential nomination.
Here in the Bay State, Mondale's bandwagon has attracted a number of politically prominent riders who cut across a broad spectrum within the party. Early Mondale supporters include liberal US Rep. James N. Shannon, Reps. Edward Boland and Joseph Early, both of whom are somewhat to the right of Representative Shannon, and Thomas W. McGee, the conservative Speaker of the Massachusetts House of Representatives.
Whether the Tsongas endorsement induces other highly visible members of his party here or in Congress to endorse Glenn remains to be seen.
Some veteran observers say that Tsongas could lose some support for his own reelection campaign next year because of his endorsement. But other Bay State Democratic activists generally doubt the move will jeopardize Tsongas's second-term prospects.
Some, however, are quietly shopping around for a strong challenger to him for the party's 1984 senatorial nomination.
Those state party members who would topple Tsongas had counted on coming up with a contender from the party's conservative ranks.
In fact in some quarters former Gov. Edward J. King, who certainly would fit that mold, had been discussed as a potential candidate.
Such pursuits and their prospects for success may have been made tougher by the fact that Tsongas has endorsed a moderate rather than a liberal.
Tsongas critics among Massachusetts Democrats, especially those who might have designs on his seat for themselves or somebody more to their liking, note that in winning the party's senatorial nomination in 1978 he polled less than one-third of the Democratic primary vote. In topping the then five-candidate field he bested his closest rival by but 37,955 votes among over 900,000 cast.