Sanders: Clinton campaign in 'serious trouble'

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Sen. Bernie SandersBernie SandersSanders wins over W.Va. superdelegate Boxer: Sanders appeals to young voters with grandpa effect The Trail 2016: Interleague play MORE (I-Vt.) says Democratic presidential rival Hillary ClintonHillary Rodham ClintonEmbattled pharma CEO endorses Trump RNC launches ad against 'Crooked Hillary' Sanders wins over W.Va. superdelegate MORE is in "serious trouble," citing her stepped-up attacks against him.

"Secretary Clinton and her campaign is in serious trouble," Sanders told reporters after a campaign stop in Iowa, according to multiple reports on Monday. "So obviously in that scenario what people do is start attacking."

Sanders noted that Clinton was seen as the presumptive Democratic nominee months ago but that polls now show them "locked in a very difficult race" in Iowa and New Hampshire.

"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," he added, according to reports.

Clinton has stepped up criticism of Sanders on the issue of guns, which is expected to be a focus in President Obama's final State of the Union address on Tuesday.

Clinton on Monday cast herself as on the side of Obama on the issue, firing back after Sanders's campaign manager said earlier that "Clinton has been all over the map on guns."

An NBC/Wall Street Journal/Marist survey released over the weekend found Clinton and Sanders in a close race in Iowa and New Hampshire, where ballots will be cast next month.