Sen. Patty Murray, D-Wash., a member of an appropriations subcommittee with jurisdiction over the IRS, said the practice was an “outrageous violation of the public trust” that could undermine the agency’s credibility.

IRS officials acknowledged that party affiliation information was routinely collected by a vendor for several months. They told the vendor last month to screen the information out.

“The bottom line is that we have never used this information,” said John Lipold, an IRS spokesman. “There are strict laws in place that forbid it.”

Washington state residents do not express a party preference when they register to vote. Residents of 20 other states and the District of Columbia have to provide a party affiliation when registering. Voter registration information is publicly available.

Murray said she learned about the problem from the president of the National Treasury Employees Union, Colleen Kelly. The IRS is part of the Treasury Department.

“This agency should not have that type of information,” Murray said in a telephone interview from Seattle. “No one should question whether they are being audited because of party affiliation.”

Kelly said Thursday that several IRS employees had complained to the union about the practice. She said IRS officials weren’t even aware of it until she wrote them in late December.

In a letter to Kelly, Deputy IRS Commissioner John Dalrymple said the party identification information was automatically collected through a “database platform” supplied by an outside contractor that targeted voter registration rolls among other things as it searched for people who aren’t paying their taxes.

“This information is appropriately used to locate information on taxpayers whose accounts are delinquent,” he said.

Murray and Kelly, however, remained skeptical. Kelly said the collection of such data was even more troubling because the IRS intends to start using private collection agencies later this year to go after back taxes.

“We think Congress should suspend IRS plans to use private collections agencies until these questions have been resolved,” she said.

According to Murray’s office, the 20 states in which the IRS collected party affiliation information were Alaska, Arkansas, Colorado, Connecticut, Delaware, Florida, Louisiana, Massachusetts, Michigan, Nevada, New Jersey, New York, North Carolina, Ohio, Oklahoma, Rhode Island, South Carolina, Texas, Utah and Wisconsin.

Les Blumenthal: 202-383-0008

lblumenthal@mcclatchydc.com