THE race for State Representative in the Second Middlesex District (Wards Five, Six and Eight in Cambridge, containing most of Harvard) is a fairly quiet one this year. Its most notable aspect is that all three contenders seriously predict they will win. Cynics will say that all politicians are chronically optimistic, but this election could be surprisingly close.

The three candidates are:

* Thomas H.D. Mahoney, the Democratic incumbent. Mahoney is seeking a second term in the House after a long involvement in city politics--three terms on the School Committee and four terms on the City Council. He is a professor of History and Political Science at MIT.

* Joseph W. Downes, a Republican who failed to gain his party's nomination for this seat two years ago but now hopes to restore it to the Republicans, who held it for several terms before Mahoney's election in 1970. Downes is an attorney with an office in Boston.

* Nicholas Peck, running as an Independent this time after losing the Democratic nomination to Mahoney two years ago. Peck is a former reporter for the Boston Globe and a conscientious objector.

It is Peck's presence in the race that makes the outcome difficult to predict. He is young and has been waging a more spirited campaign than either of his opponents. He says that he expects to receive votes from many Republicans as well as Democrats and Independents.

But Mahoney is a formidable incumbent with a record that seems to suit his liberal constituency. He was one of only six House members to receive a 100 per cent score from the American Civil Liberties Union, and one of twelve to get a perfect score on nine education bills from Citizens for Participation Politics.

Peck says that the abortion issue was the major reason he decided to run again. He believes that abortion is a matter for a woman and her doctor to decide, and that the State should take itself out of the controversy. "I think it's going to make life better for all people, men and women, when that question is clear," Peck said last week.

Mahoney answers, with barely suppressed condescension, that he was not one of 16 representatives in the 240-member House to support abortion on demand, but notes that he did join a small number of colleagues who unsuccessfully tried to have the issue decided by a state-wide referendum.

Peck also quarrels with Mahoney over the "PR" question. Peck is strongly in favor of proportional representation, saying that it is an invaluable aid to minor political groups who would otherwise not be represented in the City's governing bodies.

The Legislature voted earlier this year to abolish PR, even though Cambridge is the only city in the Commonwealth that uses it. Mahoney attached an ammendment to the bill that would have allowed Cambridge to vote on the issue directly. But the matter became academic when the courts ruled the statute unconstitutional.

Peck charges that Mahoney should have known the bill would be declared invalid (since it applied to only one community) and that he should have prevented it from ever reaching the floor of the House.

The third major issue for Peck is the district's need for a full-time representative, which he promises he will be if elected. Mahoney says it's a bogus issue because he has had a 99 per cent attendance record at the House, and has found sufficient time to do the committee work required of all legislators. "It's like academics," Mahoney said recently. "Some people can work 20 hours a day and accomplish nothing. Other people can accomplish much more in a relatively short period of time.

Downes has been emphasizing what he calls "bread-and-butter" issues. He wants to see a system of portable pensions instituted in the state, which would guarantee that a person could hold several jobs in his lifetime but not end up with just "a watch and a hand-shake."

Downes also wants to have a new law enforcement department established in Massachusetts, comparable to the national Federal Bureau of Investigation, to combat both street crime and organized crime.

Harvard does not seem to be an issue in the campaign, unlike the September primary in the First Middlesex District when City Councillor Alfred Vellucci continually spoke out against Harvard's expansion in Cambridge and called for taxes on University property.

"The politicians in Cambridge have been using Harvard as a whipping boy for years," Downes said last week. "I don't go for that town-and-gown stuff. Harvard is there and it's tremendous asset to the community," he said.

Mahoney says he is most proud of legislation that he sponsored which give state authorization for the City to issue parking stickers to Cambridge residents to park on their own residential streets, which he claims are now pre-empted by non-residents. He is also pleased to have co-sponsored legislation that called upon the Governor to provide rebates to Massachusetts motorists for over payments to insurance companies.

However, Mahoney is disappointed that his bill to tax the gross income of Cable Antenna Television (CATV) "was brushed aside." He wants the state to follow the lead of Connecticut, Illinois and Rhode Island in this matter. His bill to reduce noise pollution caused by vehicular traffic never got out of committee.

Normally, an incumbent like Mahoney would be impregnable in this district. But it has a strong Republican tradition, and if Downes finds enough support them Peck could sneak through with a coalition of young people, radical liberals and independents. On the other hand, if Mahoney and Peck divide the left-of-center vote, then Downes might win. Mahoney has trouble hiding his irritation with Peck's persistent criticism, and accuses Peck of undermining the American two-party system by running as an Independent. But Mahoney seems confident that he has staked out the broad center of the political spectrum and will win going away.