Sen. Carl Levin (D., Mich.) repeatedly pressed the Internal Revenue Service to investigate the tax-exempt status of specific conservative nonprofit organizations in letters to then-IRS commissioner Doug Shulman and director Lois Lerner in 2012.

Levin said he was concerned nonprofit organizations were abusing their tax-exempt status and engaging in partisan politics and requested information from the IRS on 12 organizations.

"Organizations are using Internal Revenue Code section 501(c)(4) to gain tax exempt status while engaging in partisan political campaigns," wrote Levin in one letter on July 27, 2012. "Making the problem worse is that the IRS knows there is a problem because of the public nature of the activity but has failed to address it."

He asked whether the 12 organizations "applied for [tax-exempt status]; and if so … received the described exemption for political activity from the IRS."

Levin’s list contained nine conservative groups, including Club for Growth, Americans for Tax Reform, and Americans for Prosperity. It also included two liberal groups and one centrist group.

The IRS officials were reportedly already aware that the agency had been targeting conservative groups for special scrutiny during the time Levin was corresponding with Shulman and Lerner.

The IRS acknowledged on Friday that agency officials singled out conservative groups applying for tax-exempt status.

Levin is a prominent supporter of the DISCLOSE Act, which would require the disclosure of corporate donors to tax-exempt organizations. He was an original cosponsor of the legislation.

He told New York Times columnist Joe Nocera he would take on groups he believed were abusing their tax-exempt status.

"Tax-exempt 501(c)(4)s are not supposed to be engaged in politics," he said. "We’re going to go after them."

Levin, who chairs the Senate Permanent Subcommittee on Investigations, said the subcommittee will be investigating the IRS targeting of conservative groups.

"After Friday’s announcement that the IRS, to the extent it has been enforcing the law, may have done so in ways that singled out some groups for special scrutiny, we have determined that the subcommittee should investigate that additional issue as well," Levin and Sen. John McCain (R., Ariz.) said in a statement.

Levin’s office did not respond to requests for comment.

Bradley Smith, chairman of the Center for Competitive Politics, said Levin should not be surprised that the IRS was caught targeting Tea Party groups when senators are sending these kinds of letters.

"And he wonders why the IRS gets caught using partisan criteria to investigate Americans," said Smith.

Democratic Sens. Jeanne Shaheen (N.H.), Tom Udall (N.M.), and Al Franken (Minn.) sent a similar letter to Shulman in February 2012, asking for the IRS to investigate tax-exempt groups they believed were engaged in political activities.

Similar letters were also sent to the IRS by Sen. Max Baucus (D., Mont.) in 2010 and House Democrats in 2012, the Atlantic reported on Monday.

David Keating, president of the Center for Competitive Politics, said requests from members of Congress for the IRS to single out specific groups for scrutiny were inappropriate.

"There seems to be a consensus that members of Congress should not pressure government officials to reward friends with contracts or special treatment," said Keating. "Neither should they demand that government agencies punish political opponents."

The director of the IRS Rulings and Agreements division, Holly Paz, said in June 2012 that the agency would make it a priority to investigate the tax-exempt status of political nonprofits, the Wall Street Journal reported at the time.

Paz said the IRS was developing a questionnaire to send to these organizations, according to the Wall Street Journal.

The rulings and agreements division was apparently aware that IRS officials were targeting groups with words like "tea party" in their names as early as June 2011, according to a timeline compiled by the IRS inspector general.

Paz, who is responsible for developing the IRS exempt organization division’s guidance and rulings, contributed $2,000 to President Barack Obama’s campaign in 2008.