Watch James Rosen's video report above. Rosen is the Chief Washington Correspondent for Fox News.

A legal showdown between the Texas and the Justice Department begins on Monday but before anyone steps into the courtroom, one lawmaker is already cross-examining the Attorney General's star witness.

Attorney General Eric Holder is fighting in court to block Texas from enacting the voter law the Lone Star state passed last year which requires registered voters to possess a valid state-issued Photo ID.

Proponents say it'll help curb voter fraud. The nation's top law enforcement officer disagrees.

"We objected to a photo ID requirement in Texas because it would have had a disproportionate impact on Hispanic voters," Holder said.

To prove that, the Department of Justice retained Harvard University Professor of Government Stephen Ansolabehere as an expert witness, who in turn relied on a data firm called Catalist to supply statistical information.

The company provided data such as the percentages of whites and minorities in Texas who are registered to vote.

"I had previously conducted several studies using the Catalist data," the professor testified. "And, I found that nearly all persons identified by Catalist as Black, White or Hispanic identified themselves in the survey as being of the same race projected by Catalist."

Simon Rosenberg, New Democrat Network President, defended Catalist.

"If there was a perception that Catalist was cooking the books in any way, you know, they would lose all their clients, right? I mean this is -- they're all about integrity," Rosenberg said.

Perhaps, but a look at Catalist's website shows they are also all about ideology.

The firm says its' mission is: "To provide progressive organizations with the data and services needed to better identify, understand, and communicate with the people they need to persuade and mobilize."

Its' clients include every major power player on the left, from the Democratic House and Senate campaign committees to Emily's List and Planned Parenthood to the 2008 Obama Campaign.

House Judiciary Committee Chariman Lamar Smith, a Republican from Texas, wrote to Attorney General Holder asking why such an openly partisan firm was engaged to provide data for an ostensibly non-partisan agency.

"If the tables were turned, and this was a Republican company with Republican parties and Republican candidates, Republican organizations, as clients, they'd never get away with being an interpreter of the data," Smith wrote. "The appearance is one of impropriety, which is not something that should be allowed to occur. The results are going to be tainted, no matter what."

Catalist declined to comment. The Justice Department said it is reviewing Smith's letter. The lawsuit that will decide the dispute over this voter ID law -- the case of Texas v. Holder -- is set to begin in federal district court, in Washington, on Monday.

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