The race for Massachusetts State Treasurer has developed into a contest of reputations and personal ethics, with both sides slinging and dodging veritable landslides of mud.

The past week's developments include fresh allegations, especially from the campaign of L. Joyce Hampers. Observers predicted even after her nomination at April's Republican Convention that she would use her trademark hard-hitting campaign style. Her attacks on 22-year incumbent Robert Q. Crane's private finances have been consistent enough to inspire a television spot from Crane accusing her of "mudslinging."

Crane's greatest challenge is to combat his image as an old-time Bay State pol building his personal finances on his ability to to dispense patronage.

The Massachusetts Lottery, which Crane administers, pays out a larger percentage of the profits to winners, and has almost twice as many employees as any other state lottery. It also spends an unusually large amount on advertising, which has included television spots featuring Crane, and full-page newspaper ads filled with names, which are headed "Treasurer Crane's Unclaimed Money List."

These expenses deprived Massachusetts of about $265.5 million in state aid which the lottery could have funded if it had turned a 40 percent profit, the national average.

However, Hampers' own record must be cleared to ensure victory against an incumbent with the support of a far stronger party than her own.

The office of Democratic Attorney General Francis X. Bellotti has released a letter contradicting Hampers' statements on a case she handled as State Revenue Commissioner in 1980. The letter concerns a suit by Benjamin Rosales, who was fired from his position as the department's legal counsel in 1978 by Hampers' predecessor, Laurence Fitzmaurice.

Hampers denies signing the letter. However, handwriting experts retained by WBZ-TV (Channel 4) determined last night that she had in fact signed it.

The department settled out of court with Rosales for a payment to him of $29,000, and Hampers has said she agreed to do so because the Attorney General's office had told her the department had little chance of success.