Massachusetts Democrats went on the attack against Republican US Senate nominee Gabriel E. Gomez for a $281,500 tax deduction he took in 2005 for an historic preservation deal on his Cohasset home, calling on the candidate to release the details of the agreement.

“It seems outrageous,’’ said John Walsh, the state Democratic Party chairman, at a Boston press conference. He called on Gomez to make public his tax returns for that year and other details about how the deduction was calculated on the home that the candidate and his wife bought for $2.1 million in November 2004.

“It may show a guy very much different than the picture he’s trying to draw,’’ Walsh said.

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The Gomez campaign on Thursday again rejected the Democrats’ request to release the 2005 tax return, arguing that the candidate has been transparent with his taxes, having already released returns for six years beginning in 2006. The Globe first requested Gomez’s 2005 return on Wednesday.

The Republican’s campaign also struck back, saying US Representative Edward J. Markey, the Democratic nominee, is criticizing him for taking the deduction for a program the congressman himself voted for.

The campaign spat was sparked by a Globe story Thursday that revealed the Gomezes had taken the deduction as part of an agreement in which they gave an easement on the facade of their home to a controversial nonprofit historic preservation group that has been targeted by the US Department of Justice for its marketing tactics. As part of the agreement, the couple gave control of the exterior of their home to the Trust—a concession that was counted as a monetary donation.

Federal prosecutors and the Internal Revenue Service said the National Architectural Trust had arranged for “unwarranted’’ claims by homeowners for the deductions. The IRS had at the time listed the preservation agreements such the one the Gomezes signed with the Trust as one of its “Dirty Dozen tax scams.”

The easement requirements for which the Gomezes got their large tax deduction was already legally binding on them by local by-laws, raising the question as to whether their donation—the price of which is based on the loss of value in their real estate—had any monetary worth.