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