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The chairman of the House Judiciary Committee on Thursday accused the Justice Department of using data from an “agent of the Democratic Party” to bolster its case for blocking Texas' controversial voter ID law.

Republican Rep. Lamar Smith, who represents Texas, said he’s "disappointed" and concerned by the "unacceptable" move, demanding an explanation in a letter sent to Attorney General Eric Holder.

The letter notes that the Justice Department is using data compiled by a company whose client list has included President Obama’s own election campaign.

"The Department of Justice has a responsibility to enforce and uphold the laws of the land without the influence of partisan politics," Smith wrote. "When the Department goes to court, it represents the United States of America, not the Attorney General’s political party."

Smith's letter comes just days before a federal judge in Washington is set to hear arguments over whether to overrule the Justice Department, which concluded in March that the Texas law requiring voters to have state ID to cast a ballot is illegal under federal law because it disproportionately disenfranchises Hispanic voters.

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"(A) Hispanic registered voter is at least 45.6 percent, and potentially 120.0 percent, more likely than a non-Hispanic registered voter to lack (state-issued) identification," the head of the Justice Department's Civil Rights Division, Tom Perez, told Texas officials in a letter in March. "Even assuming the data most favorable to the state, Hispanics disproportionately lack either a driver's license or a personal identification card issued by (the state), and that disparity is statistically significant."

According to Perez's letter, the Justice Department based its decision to block the law on "the state's own data" -- data handed over by Texas months earlier after the department requested more information. But hoping to bolster its upcoming case in court, the Justice Department recently submitted a report from Harvard University political science professor Stephen Ansolabehere – who used the Washington-based firm Catalist for information.

In his letter to Holder on Thursday, Smith took direct aim at that firm, citing a "conflict of interest."

"Though Catalist is technically a private, for-profit company, it is really an agent of the Democratic Party," Smith wrote. "There is at least the appearance that, rather than election laws that protect Texans' right to vote in a secure and fair election, Catalist might prefer that Texas’s election laws favor Democratic Party candidates."

In fact, Catalist's client list posted online features prominent liberal and Democratic causes, including Obama's 2008 election campaign, the Democratic Governors Association, the Democratic Senatorial Campaign Committee, the American Civil Liberties Union, NARAL - Pro-Choice America Foundation, and the Texas Democratic Trust.

In his letter, Smith called Catalist "an explicitly partisan operation founded, run, and staffed by dedicated Democratic activists," noting that Catalist’s president is "a key player in the Democratic Party."

Smith asked Holder to "vouch for Catalist's non-partisan independence and reliability," and if Holder can't do so "then I request that you drop the Department’s objections to Texas’s voter identification law."

In his report, Ansolabehere determined that 11 percent of white registered voters don't have a state ID, 18 percent of Hispanic registered voters don't have a state ID, and 21 percent of black registered voters don't have a state ID. He came to that conclusion by first matching voter registration lists with names of those registered for a driver's license or firearms license. He found that 1.9 million people are registered to vote but don't have those state IDs, so he then turned to Catalist to determine the likely races of those 1.9 million people.

"The voter identification requirement will affect Whites and Minorities differently, and the difference is substantial," Ansolabehere wrote.

Ansolabehere described use of Catalist data as "appropriate," noting that "independent confirmation of the quality of the Catalist data files, their record matching algorithms, and their methodology for identifying races based on names and Census data come from recent industry-wide competitions." He also said he independently tested their data several times to validate its reliability, and he "found that nearly all persons identified by Catalist as Black, White or Hispanic identified themselves" the same way in his own survey of them.

Ansolabehere's report said Catalist "specializes in voter registration data" by matching voter registration records to commercial lists, such as National Change of Address, Axciom and InfoUSA, and then determining "the likely racial identity" of registered voters.

A legal aide to Smith acknowledged that Catalist's data is "more in-depth" and "more refined" than what the Justice Department was likely looking at when it first objected to the voter ID law, but the aide said the issue now really comes down to "whether Catalist is providing the data in good faith" to Ansolabehere and whether the information was manipulated at all.

On Thursday, Smith also asked Holder for information on how it came to choose and pay Catalist for its data.

"Equally troubling, nothing in the record indicates that the Department conducted an open bidding process when it obtained Catalist's data services," Smith wrote in his letter. "If the Justice Department intentionally chose an organization with a left-leaning bias to review Texas’ voter ID law, this would be a disturbing misuse of taxpayer dollars and undermines the credibility of the Department’s challenge to the law."

A spokeswoman for Catalist declined to comment for this article. The Justice Department said it's reviewing Smith's letter.

Texas lawmakers passed the voter ID law more than a year ago, touting it as a way to help deter and detect election fraud. In January, the state filed suit in Washington, asking a federal judge to green-light implementation of the law. Under the Voting Rights Act, Texas must get pre-approval from the Justice Department or a federal judge before making any changes to state voting laws.

Texas Attorney General Greg Abbott has said the lawsuit in January was "in anticipation" of opposition from the Justice Department, which came two months later. A “bench trial” begins in Washington on Monday.