Ex-federal prosecutor explains how Jeff Sessions could sink Trump -- and himself -- in Mueller testimony

Attorney General Jeff Sessions had to walk a tightrope during his interview with special counsel Robert Mueller's team, according to one former federal prosecutor.

Renatto Mariotti, who's running for Illinois attorney general as a Democrat, said the attorney general is a crucial witness in the investigation of President Donald Trump's ties to Russia -- and his possible efforts to cover up those connections.

"Based on what we know publicly, the most important testimony Sessions could provide relates to obstruction of justice," Mariotti tweeted Tuesday morning. "He was present during conversations with Trump regarding the firing of Comey, and he wrote a memo providing an alternative explanation for the firing."

He said Mueller likely asked Sessions what the president told him about the firing, and why he and deputy attorney general Rod Rosenstein were asked to prepare those memos -- and why their memos didn't match up with the reasons stated by Trump or in a letter written by White House adviser Stephen Miller.

Mueller no doubt asked the attorney general about the president's reported outbursts following his recusal from overseeing the FBI probe of Trump's campaign ties to Russia, which is a key piece of evidence in an obstruction case, according to Mariotti.

"Those statements provide evidence of Trump’s intent," Mariotti said. "Mueller needs to prove that Trump fired Comey (and asked Comey to kill the Flynn investigation) with 'corrupt' intent."

Mueller must prove Trump wanted to unlawfully end the investigation, Mariotti said, and that could hinge on whether the president knew Mike Flynn had lied to FBI agents before asking Comey to spike the investigation of his national security adviser.

Flynn has since pleaded guilty to lying to FBI agents, and so has George Papadopoulos -- an adviser on the foreign policy team headed by Sessions.

"Sessions also could provide testimony about the campaign," Mariotti said. "What did he tell Papadopoulos about setting up a meeting with Putin? Sessions publicly claims he discouraged Papadopoulos, but later email traffic suggests otherwise."

Mueller also likely questioned the attorney general about his responses to questions from former Sen. Al Franken (D-MN) about his contacts with Russian officials during the campaign.

"Did Sessions tell the truth to Congress?" Mariotti said. "For example, he told Congress that he didn’t discuss election-related matters with the Russian Ambassador."

The attorney general recused himself from the probe in March after the Washington Postrevealed he met twice with Russian Ambassador Sergey Kislyak during the campaign but failed to disclose those contacts under questioning by Franken during his confirmation hearing.

"It’s a crime to lie to Congress, so I expect Sessions to very carefully select his words to ensure that he can’t be charged for his prior statements or his statements to Mueller, because lying to the FBI is also a crime."