WASHINGTON – Cynthia Dill, the Democratic candidate for Maine’s open U.S. Senate seat, is calling on her two leading opponents to publicly release 10 years of tax returns.

In a letter Monday to independent Angus King and Republican Charlie Summers, Dill said the three candidates should release their returns simultaneously through a third party.

Summers’ campaign said he would agree to Dill’s proposal. King’s campaign did not immediately commit to it but didn’t rule it out.

Dill’s proposal could focus additional attention on King, the wealthiest of the three top candidates, at a time when the race to succeed Sen. Olympia Snowe appears to be tightening.

It comes days after Republican presidential nominee Mitt Romney released last year’s tax returns and other tax information after a months-long campaign by Democrats insinuating that the wealthy businessman was trying to hide something.

In her letter, Dill said that releasing the three candidates’ tax returns jointly — through a third party — would bring “more civility, openness and transparency to the U.S. Senate race.”

She said the information would provide an “antidote of sunshine” to a race that has already attracted more than $1.5 million in spending from outside groups, much of that spent on ads targeting King or Dill.

Crystal Canney, King’s spokeswoman, said the campaign’s staff had not had a chance to discuss the proposal thoroughly with the former governor, but she noted that financial disclosure forms filed with the U.S. Senate contained information on King’s financial status.

“I think it is significant that he was the only one who filed his financial disclosure forms on time,” Canney said.

Summers’ campaign indicated Monday afternoon that it would accept the offer.

“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, a Summers campaign spokesman.

As the wealthiest of the three top contenders, King would be the most heavily scrutinized by the other campaigns and groups that hope to continue chipping away at his lead in public opinion polls.

His wealth has already become an issue in the race, as some opponents have claimed that he made large profits from the wind power industry.

Asked whether her proposal was aimed primarily at King, Dill replied no and said “we all know that Angus is rich.” She said her proposal is aimed at “getting to the truth.”

“I’m in the campaign to win because I feel I am the candidate who is best in touch with the day-to-day challenges of everyday Maine people,” said Dill, a lawyer, mother and state senator from Cape Elizabeth.

Voters can get a general sense of the candidates’ finances from disclosure documents filed with the U.S. Senate.

King’s filing, for instance, showed that he earned $256,000 last year as a college lecturer, former Maine governor and consultant, and from his ties to the wind power company Independence Wind.

Summers, meanwhile, earned about $70,000 as Maine secretary of state, and had total assets of between $77,007 and $331,001. Dill reported earning roughly $35,700 in 2011 and to date in 2012, which includes $22,771 as a state senator.

In an interview, Dill did not commit to releasing her tax records if King, Summers or both keep theirs private. Asked why she didn’t simply release her returns Monday, Dill said she feared that her opponents would somehow use them against her.

“If the other candidates don’t want to release theirs, then I will decide what to do,” Dill said.

Staff Writer Kevin Miller can be contacted at 317- 6256 or at:

kmiller@mainetoday.com