WASHINGTON -- A group of civic watchdogs and the Service Employees International Union have asked the U.S. Office of Government Ethics to examine Mitt Romney's financial disclosures, ratcheting up scrutiny of the Republican presidential candidate's finances.

The group wrote in a letter to the ethics office that the review is warranted because of concerns that Romney's disclosures "are not in compliance" with federal regulations:

"As you know, the Act requires candidates for federal office to disclose their financial holdings so that the public can identify potential conflicts of interest and personal economic priorities of federal officials and candidates. However, candidate Romney has refused to disclose most of his stock holdings. This lack of disclosure undermines public confidence in government, denies voters the facts they need to make an informed decision in the 2012 presidential election, and violates both the spirit and the letter of the law."

Romney faces serious questions about tax returns he has released and pressure to produce more of those returns. Rep. Sandy Levin (D-Mich.) has proposed legislation that would require presidential candidates to release 10 years worth of tax returns and other financial data.

“Gov. Romney refusal to release a broad set of tax returns only serves to highlight the unanswered questions about his finances and the extent to which he has benefited from offshore tax havens and other tax loopholes," Levin said in a statement to The Huffington Post. "He should follow the standard set by his father decades ago because if he has nothing to hide, he has nothing to hide. He should be proud to fully inform the American people."

Rep. Brad Miller (D-N.C.) said the legislation may be necessary. "It is the kind of information Americans expect to get from presidential candidates," he said. So far, he added, the punishment for Romney's failure to disclose financial infomation has been political -- not legal.

The watchdog groups lobbying the ethics office include Citizens for Responsibility and Ethics in Washington, People for the American Way, Public Campaign, Public Citizen and The Social Equity Group. With the SEIU, their letter, the groups argued that Romney is required by law to provide a more meaningful disclosure of his assets and investments. They wrote:

"What little information is public about candidate Romney's holdings establish that he owns a significant -- if not a controlling -- interest in a wide variety of operating companies that directly do business with the United States government or that would directly benefit or be harmed by regulatory policies of the executive branch. For example, based on press reports and publicly available information, Romney's funds may include companies such as Hospital Corporation of America, Clear Channel Communications, and Sensata Technologies. Ownership of each of these companies would present a clear conflict of interest for any high government official, let alone the President. ... Accordingly, it is of the utmost importance that candidate Romney comply with the Act to the fullest degree, and that your Office assure strict compliance with the candidate disclosure rules."

The Office of Government Ethics hasn't responded to the letter, an SEIU spokesman said.

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