An unidentified source told NBC News that Clovis was questioned last week by investigators connected to the Federal Bureau of Investigation probe.
Sessions previously came under fire for failing to disclose two contacts with Russian ambassador Sergey Kislyak - in May and September 2016 - when he said during his Senate confirmation hearing that he had not met with Russian officials while part of the Trump campaign.
"The political climate inside Washington has made it impossible for me to receive balanced and fair consideration for this position", Clovis wrote. "I don't remember much about it".
Clovis, who possesses a bachelor's degree in political science, an MBA degree and a doctorate in public administration, repeatedly acknowledged his lack of background in the hard sciences when responding to Stabenow. "He has laid out his reasons in the attached letter", a spokesperson for Agriculture Secretary Sonny Perdue said in an email.
According to court documents filed by Special Counsel Robert Mueller, Papadopoulos, a minor Trump campaign adviser said at the meeting of foreign policy advisers in March 2016 "that he had connections that could help arrange a meeting between then-candidate Trump and President Putin". Also there were Trump and his son-in-law and White House senior adviser Jared Kushner.
Court documents released along with Mr Papadopoulos's plea reveal numerous people with whom he corresponded. The FBI did not identify Clovis.More news: Blizzcon 2017: Hearthstone's Kobolds and Catacombs Expansion Detailed
Donald Trump and Sam Clovis. "However, if a volunteer made suggestions on any foreign policy matter, Dr. Clovis, a polite gentleman from Iowa, would have expressed courtesy and appreciation". Clovis could not stop an American citizen from traveling overseas "in his personal capacity", she said ...
Roberts has not issued a statement on Clovis as of early November 2, but Stabenow said in an email, "Sam Clovis' decision to withdraw his nomination as chief scientist is a victory for science and our farmers who rely on agricultural research".
Sam Clovis said on Thursday that he would not go forward in trying to become the USDA's undersecretary for research, education and economics.
"The emerging information about his role in the Trump campaign's interactions with Russian Federation raises serious concerns", said Michigan Sen. "As we consider his nomination, I will be looking into these facts, along with his questionable qualifications and long history of divisive and outrageous statements". He quickly rose through the ranks of the campaign after Trump's strong finish in the Iowa caucuses, taking a national role in the organization as officials scrambled to build a campaign team.
Trump and his aides have sought to diminish Papadopoulos's standing on the campaign.
White House spokeswoman Sarah Huckabee Sanders told the Des Moines Register that officials respect his decision to withdraw.
"There's no way he would fight this or cause trouble for the White House", a Republican official and longtime associate of Clovis said.