The top executives at Independent Sector and the Council on Foundations are the latest voices in the chorus of advocates who say President Obama’s tax plan would hamper giving to nonprofit groups and foundations.

“We are opposed to proposals which will significantly depress incentives for charitable giving,” said Steve Gunderson, president and chief executive of the Council on Foundations, an organization that represents grant makers. “In these hard economic times, we need to make sure tax and regulatory policy encourage growth in philanthropy.”

Independent Sector, a coalition of major charities and foundations, said in a statement that the plan would encourage some donors to “cap their gifts.”

“This could be a problem for many struggling nonprofits vital to our communities that are already facing a very difficult fund raising environment,” the organization said.