Paul Tsongas: flirting with 'Republicanism'?

''Before I announce my conversion to the Republican Party . . .''

It's a line that could have come from several congressmen recently. Rep. Eugene V. Atkinson of Pennsylvania has already switched to the GOP, and there are rumors afoot of similar changes by three other members of the House.

But Massachusetts Senator Paul Tsongas, one of the most formidable of the young rethinkers of the Democratic Party?

The line is his. It came in a carefully conceived speech in Boston Jan. 14 at a luncheon sponsored by the Greater Boston Chamber of Commerce. It was intended as a joke, and it drew laughter from his free-enterprise-minded audience.

But for all the comedy, it was not altogether in jest. For it appears that Paul Tsongas, the pragmatist's pragmatist, is doing something that might seem dangerous in this overwhelmingly Democratic state: moving ever closer to the Republican camp.

When he made the remark, he had just spent several minutes berating his own party for being in ''disarray.'' And he had praised the Republicans for doing three things which he said no Democratic presidency could have accomplished - cutting federal regulations, trimming government spending, and emphasizing that there is ''nothing wrong with being in business.''

Nor did the issue of his ''conversion'' end with his prepared text. Fielding questions after his speech, he was asked to amplify his earlier remark. ''I grew up in a conservative ethnic Republican household; I know what its about,'' he said in an equally enigmatic answer.

Nobody expects the Bay State's junior senator to shift horses - however challenging it may be to canter in the shadow of his senior colleague, liberal standard-bearer Senator Kennedy. In fact, he downplays any discussion of change. He says he has not changed his positions on what he calls ''the social agenda'' of ''tripwire issues'' like capital punishment or abortion. ''I can't be accused of trying to win over the Moral Majority,'' he said last fall.

But the junior senator, who describes himself as a ''revisionist,'' has been riding a different path from the ''traditionalists'' for some time. His ''liberalism at the crossroads'' speech to the Americans for Democratic Action in June 1980 attacked many tenets of the 1960s idealism associated with Senator Kennedy. His recent book (''The Road from Here'') amplified his criticisms of the Democratic Party.

But what did he mean by saying ''I know what it's about''?

The son of a Greek immigrant in Lowell, Massachusetts, he grew up driving a truck for his father's independent dry-cleaning establishment. So he had, presumably, a foundation of free-enterprise values which even his Dartmouth-Yale-Peace Corps education could not expunge.

Did he mean to suggest, then, that these early values were reasserting themselves?

They seem to be. His interests in foreign affairs have shifted - away from such typically liberal issues as Africa and the third world and toward a fascination with the Soviet Union that is customarily the domain of conservatives.

In the Boston speech, too, he praised President Reagan's European arms control initiative as ''marvellous.'' He then told his audience that next month he will be going to Europe - as a Reagan supporter, drumming up interest in a flagging NATO.

His attitudes on domestic policy also seem surprisingly conservative for a man whose record recently gave him one of the highest ratings in the Senate from the staunchly liberal ADA. He fights excessive government regulation. He pegs high deficits - which usually worry Republicans more than Democrats - as the major threat to the US economy.

He told his Boston audience that President Reagan is ''very appealing'' and ''a genuine human being.'' He even admitted a preference for college loan programs instead of the vocational-education programs sponsored by the now-withering Comprehensive Employment and Training Act - a confession which, here in the heart of CETA country, could well alienate that part of his constituency for whom Reaganomics is anathema.

So what was he doing?

His speech may simply have been a response to his Jan. 14 audience. They live in a state so left-leaning that even the president of the Chamber of Commerce is active in Democratic Party affairs. But they are movers and shakers in a business community that typically holds conservative views.

Or it may have been an attempt, not to move closer to the Republicans, but to move them closer to him in the run-up to the president's State of the Union message Jan. 26. He told his audience that ''there is a remarkable amount of agreement'' between Republicans and Democrats - once you eliminate the extremes on the left and the right. And he noted that the President himself is moving toward the center, as evidenced by his moderation on Poland, his arms control initiatives, and his decision not to sell the FX fighter to Taiwan. Emphasizing his own moderation might make it easier for Republican policymakers to share his views.

It may even have been pure pragmatism - a recognition that, as the nation drifts to the right, he better drift with it. It may be symptomatic of the kind of attitudes that have handed victories to President Reagan even in the nominally Democratic House.

But it may be something more - a private realignment by a still-young man, who may have been swept into prominence without having quite enough time for the profound thinking of which he is capable.

He is at a time of life when some men shift their careers and others their philosophies. It is a shift well worth watching.Rushworth M. Kidder is a Monitor staff writer.