IT WAS PROBABLY mere coincidence that Rep. James M. Shannon (D-Lawrence) and Lt. Gov. John F. Kerry were wearing identical blue suits and red ties at the Sep. 6 Democratic Senate candidates debate. But the physical and ideological similarities between the two are not. The "liberal twins"--as more moderate contender Michael J. Connolly, the secretary of state, calls them--are both young. Catholic well-educated, and products of a post-Vietnam War backlash that has led them to impassioned opposition to U.S. military involvement abroad and the nuclear arms race on moral rather than political grounds.

You're not going to find two more liberal pols than John Kerry and Jim Shannon, even in the People's Republic of Massachusetts, and the temptation is to assume they are identical. Resist it. Scrape away the often stale liberal rhetoric and the image mongering, and you will find that Jim Shannon deserves and is more qualified to replace Paul E. Tsongas as this state's junior senator.

Not only does the 32-year-old Shannon have more Washington experience than Kerry, whose lieutenant governor post serves as little more than a political stepping-stone, but he has managed to reach an influential post on the House Ways and Means Committee without compromising his liberal voting record. Shannon, only 32 years old and finishing only his third term representing the Fifth District north of Boston, has already mastered the art of getting things done. He is a liberal with clout, and should continue to be effective in stopping and reversing the arms race and pushing for meaningful negotiations with the Soviet Union. A non-interventionist whose rhetoric occasionally borders on the isolationist, Shannon will nonetheless provide a strong opposition to President Reagan's tendency toward military adventurism.

Shannon has been among the strongest opponents to Reaganomics in the House, resisting the flow of Democrats votes to the Republican side in the passage of Reagan's 1981 budget and massive three-year tax cut. Voting in Congress, of course, is more than the pulling of a lever, and we think Shannon, an instant protegee of Speaker of the House Rep. Thomas P. (Tip) O'Neill (D-Cambridge), will prove that he is more than a litmus-test liberal.

To do this, Shannon will first have to catch up to front-runner Kerry, who holds a narrow lead in the polls, a lead due largely to the fact that just two years ago he ran a statewide campaign for lieutenant governor. Kerry, whose main political asset at this point is his high recognition, has persistently attempted, often unfairly, to undercut Shannon's record and has continued to reach for the moral high ground, a spot for which he is not well suited.

The biggest story of the summer's campaigning has been sniping over a $14 million tax break this spring put through the House Ways and Means Committee to a Springfield insurance company that had previously donated $6,000 to Shannon's senate campaign. According to local press reports--denied by Kerry--the Kerry campaign actively peddled the story to area media and then volunteered that he was shocked and astounded by the revelations. It was perhaps poetic justice that an executive of the company revealed that Kerry had approached the same insurance firm for campaign donations, only to be turned down.

It is symptomatic of Kerry's campaign that even his safest proclamations of the liberal would of morality and care should sound hollow. He has yet to adequately convince voters that he has evolved from his brash days as leader of Vietnam Veterans Against the War, when he gained a reputation among many as a hollow; unconvincing activist.

Neither of the other two contenders have shown they have much to offer. David M. Bartley, a university president after leaving the speakership of the Massachusetts House, has been of a disappointment, and his sudden pledge to oppose any tax increases reeks of the worst Reaganaut pandering. Connolly is distinctive only in that his appearance and gestures bear a remote resemblance to those of Bill Murray.