(CNN) Democratic presidential candidate Bernie Sanders has an explanation for rival Hillary Clinton's increasing attacks on guns, taxes and more: Clinton is "in serious trouble."

The Vermont senator said Clinton is lashing out -- attacking him on guns, taxes and more -- because polls show their race having tightened in Iowa while Sanders has maintained a narrow lead in New Hampshire.

"Secretary Clinton and her campaign is in serious trouble," Sanders told reporters Monday after a campaign stop in Pleasantville, Iowa.

"And I think a candidate who was originally thought to be the anointed candidate, the inevitable candidate, is now locked in a very difficult race here in Iowa and in New Hampshire," Sanders said. "So obviously in that scenario what people do is start attacking. Suddenly Bernie Sanders is not a nice guy. That is not surprising when you have a Clinton campaign that is now in trouble and now understands that they can lose."

Sanders offered a similar take later at the Iowa Brown and Black Forum. When Sanders was asked if he has noticed that Clinton is attacking him harder and more often, he offered a mischievous "yes."

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