PORTLAND (AP) — Democrat Cynthia Dill on Monday challenged Republican Charlie Summers and independent Angus King to join her in releasing 10 years’ worth of federal income tax returns to provide transparency in the U.S. Senate race.

Dill said her idea is for all three candidates to release their tax returns at the same time through a trusted third party. Dill, who sent letters to Summers and King on Monday, said releasing the tax returns would help the candidates “set the national standard for public accountability.”

King’s spokeswoman said he was at an out-of-state fundraiser and was unable to respond immediately to the request.

But Summers immediately agreed to the release his tax returns. “Charlie welcomes the opportunity, and we’re gathering all the necessary information and look forward to Gov. King being fully transparent with Mainers,” said Drew Brandewie, Summers’ campaign spokesman.

Dill’s proposal comes against a backdrop of a similar debate in the presidential race in which Republican Mitt Romney refused to release returns before 2010.

In her letter, Dill told King and Summers that there’s little that can be done to stop the negative attack ads on TV because they’re protected by the First Amendment.

But she said candidates themselves can do more to be transparent, and she proposed an Oct. 1 date for releasing the returns. “Releasing our federal tax returns would provide that antidote of sunshine, which is so crucial to the functioning of a healthy, robust democracy,” she wrote.

Three other independents in the race — Steve Woods of Yarmouth, Danny Dalton of Brunswick and Andrew Ian Dodge of Harpswell — were not included in Dill’s proposal.