Nonprofits across the country, including key institutions in Boston, are pressing lawmakers in Washington to preserve tax breaks for charitable giving, as they wrestle with negotiations over the fiscal cliff.

In a concentrated lobbying effort, hundreds of nonprofits sent representatives to talk to members of Congress this week, cautioning against doing anything that would take away or limit charitable deductions. Republican and Democratic leaders have indicated they would consider raising revenue by limiting tax deductions, but nonprofits argue the charitable deductions are a powerful incentive for people to donate.