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The law is meant to protect the privacy of personal tax information, but critics argue that it is being used to prevent guilty parties in the IRS or elsewhere from being exposed.

“The IRS has been used as a political weapon, something in the campaign arsenal of the higher-ups for so long that of course there are going to be measures put in place to protect the secrecy of what they’re doing,” Ms. O'Donnell said. “I think that is exactly what’s happening here. And this is exactly why there needs to be a hearing.”

For Ms. O'Donnell, the improper accessing of her tax records was more than a breach of privacy. It also carried tangible consequences.

Just as she was announcing her ill-fated 2010 Senate bid, a tax lien was placed on a house purported to be hers. The lien was highly publicized and used to discredit Ms. O'Donnell’s candidacy just as it was getting off the ground, even though she no longer owned the home in question.

The IRS eventually blamed the lien on a computer glitch and withdrew it. Ms. O'Donnell sold the home in 2008, and financial documents from her lender show that her back payments were satisfied in July of that year, long before the IRS initiated the erroneous lien.

Ms. O'Donnell also battled a three-year audit of her personal finances that ultimately ended with her repaying $1,100 to the federal government. She said friends and family also were subjected to intrusive audits, though they were cleared.

Before the ordeal began, Ms. O'Donnell said, she was warned of what would come.

As she pondered a Senate run, she said, she was told by a prominent political figure in Delaware that if she challenged Mr. Castle — a former Delaware governor and congressman and a fixture in the state Republican Party — that the IRS and others would “F with her head.”

The case eventually spawned an inquiry from Sen. Chuck Grassley of Iowa, an influential Republican who serves on the Finance and Judiciary committees.

As a result of his probe, the Treasury Inspector General for Tax Administration revealed that at least four politicians or political donors have had their personal tax records improperly accessed since 2006, including one case described as a willful violation of federal law.

The Justice Department has declined to prosecute in any of the cases, and many of the details remain unclear.

The IRS previously stated that the “willful violation” unearthed by inspector general was not by an IRS employee.