While the race for the governor's seat has dominated headlines in the state throughout the summer, three local incumbents have been fighting in particularly close contests that will be decided in tomorrow's Democratic primary election.

Middlesex County Dist Atty John Droney, State Sen. George Bachrach and 29th District State Rep. Michael J. Lombardi each face strong challenges from several contenders.

In undeniably the hottest race, 16-year-incumbent Lombardi squares off against Peter A. Vellucci, son of Cambridge Mayor Alfred E. Vellucci, in a clash between the two oldest political families in the East Cambridge neighborhood. The other candidates, Salvatore Gulla and Eugene Gobby, are both considered longshots.

The Lombardi-Vellucci feud, settled by only a few hundred votes in the 1980 election is described by many as a battle between the oldtime, neighborhood-centered politics of the incumbent and the more liberal progressive reform of the challenger.

Droney's toughest opponent is Cambridge lawyer L. Scott Harshbarger '64, who came within three percent of the vote of the incumbent in the 1978 election. Suffolk County Register of Probate Paul Cavanaugh is also considered a serious contender in the race. The third challenger, Edward Gargiulo, was an assistant district attorney under Droney from 1975 to 1981.

Droney's health is the central campaign issue. A neurological disorder struck the 23-year incumbent just before the 1978 election and he is unable to communicate without an interpreter.

Bachrach drew much of his support in the last election from his hometown of Watertown and is trying to fend off a challenge by the city's town clerk, James Fahey. A split of Watertown loyalties could give the election to Cambridge Police Lt William H. Maher, who has launched an expensive, last minute media blitz. Albert DiNicola is the other candidate in the race.

Fahey has challenged Bachrach's party loyalty throughout the campaign. The senator left the Democrats to run as an independent to avoid a tough, three-way primary battle. His opponent has attacked him for returning to the party after he won the general election.