The calendar says it's only April, but two local State House incumbents are already locked in a deadly game of musical chairs.

This year's state redistricting plan reflects drops in the Cambridge and Somerville populations by merging the districts of Reps. Peter A. Vellucci (D-Cambridge) and Marie E. Howe (D-Somerville).

The 29th District of Middlesex County, which they are competing to represent, is now divided almost evenly between the cities, with 9000 voters in Cambridge and 8000 in Somerville.

As a result, these two veteran politicians will square off in what could be the most expensive state representative campaign in Massachusetts history. While the two Democrats ran almost unopposed in 1986, they are certain to wage heated campaigns throughout the spring and summer months, culminating in the September 15 Democratic primary.

Although the candidates promised positive campaigns using differences on issues, both Howe and Vellucci have already attacked each other over endorsements, finances, volunteer strength and voting and attendance records. Both campaigns have also hinted at charges of personal indiscretions.

Peter Vellucci, son of Cambridge Mayor Alfred E. Vellucci, is in his third term in the State House. To defeat token Republican opposition two years ago, the Cambridge representative spent $4000. But this time Vellucci says he will be in serious trouble unless he receives contributions totaling $75,000 to $100,000.

"If we get anything less than $60,000, the situation will really be grim," said Clifford Truesdell '66, Vellucci's campaign manager. He said Vellucci has already received more than $24,000 in contributions.

Howe has been in the State House for more than 23 years. The Somerville representative declined to say how much she had raised, but a campaign finance report shows she had received more than $21,000 by the end of 1987. Her contributors include many trade unions and Senator President William M. Bulger (D-Boston).

She called Vellucci's claim to have raised nearly $30,000 "nothing but a lie" and also said the Cambridge representative exaggerated his claim to have sent 180 volunteer canvassers to the polls on Super Tuesday.

Vellucci describes himself as a neighborhood politician who cares about the working people of his district. "This is going to be a neighborhood-oriented grass-roots campaign," Vellucci said.

Along with the support of his working-class constituents, Vellucci has actively sought backing from political colleagues in Boston and Washington. He said he has received endorsements from U.S. Reps. Edward Markey and Barney Frank, both liberal Democrats from Eastern Massachusetts.

The name of Sen. Edward M. Kennedy '54 also appears on invitations to a Thursday night fundraising dinner for Vellucci.

Howe charged this week that the name appeared there without Kennedy's permission, but Mark O'Connor, a spokesman for the senator, said that although Kennedy planned to remain neutral during the primary, his staff had given Vellucci permission to put his name on the invitation.

Howe derided Vellucci's efforts to receive endorsements. "The only way he can get a vote is through hype. He is not in touch with the people. That's why he has to have the heavyweights," she said.

Meanwhile, Vellucci has focused on differences over issues and on publicizing "flip-flops" among Howe's votes on Beacon Hill.