NBC's Kristen Welker reported that Hillary Clinton's email issue has eroded the trust of voters. And that it's "becoming a big political problem for Secretary Clinton. Everytime she wants to be talking about one of her policies, it's overshadowed by more questions about her use of a private email account while she was secretary of state."

Watch Clinton's exchange with Welker:

"If you look at the polls, though, the polls do show that the email issue has helped to erode some of the public's trust in her."

Clinton is on a swing through Iowa.