The Justice Department refused Sen. Ted Cruz's request for a special prosecutor to investigate the Internal Revenue Service's illegal targeting of conservative groups, a decision the Texas Republican said was hypocritical and politically motivated.

The DOJ is already investigating the tax agency for singling out groups for special scrutiny that included the conservative buzzwords "Tea Party," "patriot" or "9/12" in their names.

But Cruz wants a special prosecutor to lead the case after accusing the Obama administration of slow-walking the probe and falling short of its promise to hold accountable those responsible for wrongdoing.

Principal Deputy Assistant Attorney General Peter Kadzik said a special prosecutor wasn't necessary because the department has faith in its process and the agents handling the case.

"This investigation has been and will continue to be conducted by career prosecutors that are designed to ensure the integrity of an ongoing criminal investigation," wrote Kadzik in a letter to Cruz.

Kadzik added that the department has stayed above politics during the probe. "Any other approach would be inconsistent with the fundamental principles to which this department is dedicated," he said.

But Cruz called Kadzik's decision the "height of hypocrisy" because Barbara Bosserman, the lead attorney in the department's investigation, has donated more than $6,700 in recent years to President Obama's political campaigns and the Democratic National Committee.

"The investigator is a partisan Democrat," the freshman lawmaker said.

"Sadly … Eric Holder has chosen to reject the bipartisan tradition of the Department of Justice of putting rule of law above political allegiance," he added.

Cruz, a Tea Party favorite and possible candidate for the 2016 Republican presidential nomination, said former Presidents Richard Nixon and Bill Clinton appointed special prosecutors and that Holder should do likewise.

"The integrity of the Department of Justice deserves better. The American people deserve better," Cruz said.

The Justice Department has said that removing Bosserman from the probe because of her political contributions would violate department policy and possibly federal equal opportunity law.