Sen. Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE (I-Vt.) says Democratic presidential rival Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden 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.

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"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.