Republicans have questioned whether the investigations have been hindered because of preferential treatment by Democrats to Corzine. Both Republicans and Democrats have delivered strong inquiries of Corzine at congressional hearings on the firm's demise. 

"The firm's CEO at the time of the firm's bankruptcy was Jon Corzine, a former United States Senator and Governor of New Jersey," the letter continues. "Due to Mr. Corzine's long tenure in both the financial services industry and as an elected official, he naturally developed many relationships with highly placed individuals in the government who are very close to the investigation of the MF Global collapse. "

The letter notes that Corzine was colleagues with Commodity Futures Trading Commission (CTFC) Chairman Gary Gensler.

"Prior to the demise of MF Global, as the result of the direct lobbying efforts of Mr. Corzine and other high level executives, the CFTC delayed certain regulatory changes that would have enhanced customer protections," the letter reads.

It also notes that Corzine has been and continues to be a major bundler for President Obama's reelection campaign. 

"In addition it was revealed on Friday April 21, 2012 that Mr. Corzine was listed as a "first quarter volunteer fundraiser" by President Obama's campaign, having raised over $500,000 for the president's reelection," "Clearly it would not strain credulity for the American people to perceive a conflict of interest when an individual raises such large sums of money for the President's reelection campaign and who is at the same time possibly under investigation by an agency or agencies controlled by the administration for possible criminal wrongdoing."

The letter cautions though that it is not meant to suggest that there's any "direct evidence" that the Department of Justice is giving preferential treatment to Corzine and the case.

"In order to begin the long and difficult process of restoring the trust and confidence of the American people in their government, and to reaffirm the perception that every American is treated equally under the law, we are calling on you to appoint an Independent Counsel for the investigation regarding MF Global."

Read the letter below: