Trump's Arpaio pardon could be used by Robert Mueller in his Russia investigation.

Courtesy of Business Insider:  

The Washington Post's report on Saturday that President Donald Trump asked Attorney General Jeff Sessions about dropping the case against former Arizona sheriff Joe Arpaio offers a critical window into Trump's thinking. 

Trump's alleged conversation with Sessions and subsequent decision to pardon Arpaio could shed new light on Trump's motives when he spoke to former FBI director James Comey about ending the investigation into former national security adviser Michael Flynn one day after Flynn resigned.

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Specifically, Trump's decision to pardon Arpaio is key to determining his intent when he had the February conversation with Comey about dropping the Russia investigation before ultimately firing Comey after he refused to do so, said Renato Mariotti, a former federal prosecutor.

According to Comey's testimony before the Senate Intelligence Committee in June, Trump privately told Comey, who was spearheading the FBI's Russia probe at the time, he "hoped" Comey would "let this go," referring to the investigation.

Trump supporters and GOP lawmakers have made the argument that Trump was not  pressuring Comey to drop the case, but rather expressing his hope that it would be dropped.

But his conversation with Jeff Sessions, followed by his pardon of Arpaio, now suggests his intention of protecting those he considers allies at all cost.

"[Trump's] defense would be that he thought it was appropriate to end the Flynn investigation because it was meritless and that there was nothing wrong with him, as president, making that determination," Mariotti told Business Insider. 

But the president's decision to pardon Arpaio demonstrates that "this has become a pattern of activity where he tries to end investigations of his friends," he added. "Everything he said, did, and was told as to Arpaio is relevant to help us understand what he was thinking when he tried to end the Flynn investigation." 

Moreover, despite statements from his allies and administration officials who painted Trump's comments to Comey as musings and not a direct order, Arpaio's pardon suggests "that he was serious about ending investigations as to his friends" and it wasn't "just idle talk."

What's more it establishes a pattern of behavior, and THAT might be very helpful in establishing grounds for a charge of obstruction of justice.