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"Your actions in recent months have demonstrated a disturbing double standard for the treatment of sensitive information, with what appears to be a clear intent to aid one political party over another," Reid wrote to Comey in a letter published on the senator's website.
Comey announced in a letter to congressional leaders Friday that the FBI would reopen the Clinton email investigation after finding potentially new evidence amd its investigation of former congressman Anthony Weiner.
Top Clinton campaign staffers have blasted Comey for breaking with department guidelines for informing the public about ongoing investigations, and for what they've described as a perceived attempt to influence the results of the presidential election.
But Reid's comments may be the fiercest condemnation yet of Comey. The outgoing Senate minority leader said Comey's actions may have violated the Hatch Act, a federal law that ensures "that federal programs are administered in a nonpartisan fashion."
Reid argued that Comey has demonstrated a double standard, making an explosive claim that the FBI director was withholding information about Republican presidential candidate Donald Trump's alleged ties to Russia.
"In my communications with you and other top officials in the national security community, it has become clear that you possess explosive information about close ties and coordination between Donald Trump, his top advisors, and the Russian government - a foreign interest openly hostile to the United States, which Trump praises at every opportunity," Reid said.
He continued:
"The public has a right to know this information. I wrote to you months ago calling for this information to be released to the public. There is no danger to American interests from releasing it. And yet, you continue to resist calls to inform the public of this critical information."
"By contrast, as soon as you came into possession of the slightest innuendo related to Secretary Clinton, you rushed to publicize it in the most negative light possible."
Reid echoed the frustrations of several of Clinton's surrogates, who have insisted that the newly discovered messages may very well be duplicates of ones already reviewed by the FBI.
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Comey was a registered Republican for most of his life, although he said in July that he is no longer registered with any party.
Reid, who is not seeking reelection in November, ended his letter to Comey bluntly.
"Please keep in mind that I have been a supporter of yours in the past. When Republicans filibustered your nomination and delayed your confirmation longer than any previous nominee to your position, I led the fight to get you confirmed because I believed you to be a principled public servant," he said.
"With the deepest regret, I now see that I was wrong."
Reid the full letter below:
Reid Letter to Comey by Brett LoGiurato on Scribd