The politics of the Senate . . .

Massachusetts Sen. Paul Tsongas has chosen not to run for reelection this fall, cutting short an impressive Washington performance to reposition his career in his native Lowell, Mass., for family and health reasons. Whatever one thinks of Mr. Tsongas's brand of high-tech liberalism, the young Democrat's quiet, thoughtful approach characterized many of the new generation of Capitol Hill leaders in both parties - pragmatic and idealistic, looking toward a politics of the future built on a regard for the past.

The revival of Tsongas's Lowell, which has recycled its once-empty textile factories into a vigorous community, has made people think twice about the prospects of old New England - much as Tsongas helped lead a healthy rethinking of Democratic Party directions.

Senator Tsongas's exit, however, will do little to alter the basic lay of the nation's Senate races next fall. Unsurprisingly, Massachusetts still seems likely to follow Tsongas with another Democrat.

Overall, it appears the Republicans will retain control of the Senate, where they now lead 55 seats to 45. Republicans talk of losing two or three seats, Democrats say double that. ''Democrats are going to come close to taking control ,'' says one Democrat. ''But we will need some breaks to get over the top.'' Another Democrat active in Senate races says: ''There are five Republican seats in the northern tier where we can win - Maine, New Hampshire, Illinois, Minnesota, Iowa. We are favored in four in the South - Texas, Mississippi, Tennessee, and North Carolina. That's nine Republican seats we've a shot at.''

On a state-by-state basis, Republicans rate the 1984 Senate races virtually the same as do the Democratic professionals. In fact, the ratings have changed very little since the parties began work on 1984, the morning after the 1982 elections. The Democrats look secure in all 14 seats they now hold, while the GOP is vulnerable in half the 18 incumbent seats that will be up for new elections in the fall. The main difference between now and a year ago is that the parties know which are potential challengers and which are not.

In the past three elections - 1978, 1980, and 1982 - the Republicans won virtually all of the really close Senate contests. The Democrats hope this time the cut will go the other way.

But are there broad political currents that will help them? The Democrats look to stubborn pockets of unemployment, as in Minnesota's iron range and in Mississippi, to help them. If the recovery removes the economy from attention, secondary issues may come to the fore: the so-called fairness issue, arms control, the environment. Central America and Lebanon raise the question of how large a foreign arms commitment the people want, Democrats say.

Sad to say, for the Tsongases of Congress, how quickly the impersonal forces of politics like New England snow sweep in to cover one's tracks. But we are seeing more politicians today mature enough to recognize their individual careers at some point might take them away from the drama and allure of the Potomac. When they make the decision on the basis of affirming family priorities , one can only wish them well.