Democratic presidential candidate Bernie Sanders speaks at the University of Iowa in Iowa City, Iowa, January 30, 2016, ahead of the Iowa Caucus.   / AFP / Jim WATSON        (Photo credit should read JIM WATSON/AFP/Getty Images)
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(CNN) —  

Democratic presidential candidate Bernie Sanders said Sunday that rival Hillary Clinton’s email controversy is a “very serious issue,” even though he has refused to attack the former secretary of state on that front.

Asked Sunday by CNN’s Jake Tapper on “State of the Union” whether voters should interpret his refusal to engage Clinton on the email issue to mean that Clinton did nothing wrong, Sanders replied with a firm, “No.”

“Nope, nope. That is not, I think, a fair assessment. That is, I think, a very serious issue,” Sanders said. “There is a legal process taking place, I do not want to politicize that issue. It is not my style.”

Sanders famously said in the first Democratic debate that he was tired of hearing about Clinton’s “damn emails,” brushing aside one of the biggest issues Republicans have raised to attack Clinton during the 2016 presidential cycle.

Sanders assessment that Clinton’s use of private email on a private server during her time as secretary of state is a “very serious issue” comes after the State Department announced Friday that it was withholding 22 emails determined to be “top secret” by the intelligence community. The emails were not marked as classified at the time they were sent.

Sanders also would not address reports that the Clinton campaign is training its caucus leaders to potentially throw some support behind long-shot candidate former Maryland Gov. Martin O’Malley in order to blunt potential Sanders gains as a means of exploiting the complicated Iowa caucus rules.

“I can’t keep up with what the Clinton campaign does,” Sanders said, before instead choosing to criticize the Clinton campaign over ads suggesting that Sanders attacked Planned Parenthood – whom Sanders qualified as the “establishment” recently despite his decisively pro-abortion rights record in the Senate.

Sanders also pointed to the huge gains his campaign has made in Iowa, where he is now in a tight race with Clinton months after starting “50 or 60 points behind” in early polls.

Sanders said his campaign has 15,000 volunteers barnstorming the state in the final push to Monday’s caucuses and stressed that his campaign has struck a chord with middle class voters looking to shake up the status quo.

“Our message is resonating,” he said.