John F. Kerry, the Massachusetts lieutenant governor who will step into the shoes of Sen. Paul E. Tsongas, is a traditional liberal Democrat.

In a bitter race against conservative businessman Raymond Shamie, Kerry attacked President Reagan's economic program, called for slashing the military budget, backed the proposed Equal Rights Amendment and a nuclear freeze and opposed intervention in Central America.

Tsongas was a somewhat diffident intellectual, an iconoclast who called for a new Democratic agenda and didn't shy away from the details of legislating. The handsome, 6-foot-4 Kerry, with his war record, Yale degree and Irish name, offers a Kennedy-like glamour and a zest for the speech-making and flesh-pressing of politics.

Kerry, 40, achieved national fame in the early 1970s as the photogenic leader of dramatic protests by Vietnam Veterans Against the War. As a Navy lieutenant who commanded boats in the Mekong Delta, Kerry received the Silver Star, the Bronze Star and three Purple Hearts. He made a show of throwing away what the news media assumed to be his own medals at a demonstration on the Capitol steps in 1971, but it was later revealed that the medals belonged to another veteran -- a fact that Republicans gleefully raised this year.

Kerry said he never agreed with the policy to throw away medals and was complying with the request of a veteran. During the campaign, Kerry focused on his Vietnam experience. His slogan was, "Once you have seen war, you never stop fighting for peace."

Kerry's comfortable 56 percent share of the vote is partly due to his success in painting Shamie, who once flirted with the John Birch Society, as a right-winger.

Tsongas, who decided to retire after learning he had cancer, is expected to step down early to give Kerry a seniority boost that may help him get Tsongas' seat on the Foreign Relations Committee