Democratic presidential candidate Bernie Sanders speaks at Northstar Elementary School in Knoxville, Iowa, this month. (Andrew Harnik/AP)

PLEASANTVILLE, Iowa — Bernie Sanders declared Monday that the presidential campaign of his Democratic rival Hillary Clinton is in “serious trouble,” suggesting that tightening polls in Iowa have prompted her recent attacks on his positions on guns and health care.

“I think a candidate who was originally thought to be the anointed candidate, to be the inevitable candidate, is now locked in a very difficult race,” the senator from Vermont told reporters. “Obviously what people in that scenario do is start attacking. Suddenly Bernie Sanders is no longer a nice guy, and he’s wrong on this, and he’s wrong on that.… That is not surprising when you have a Clinton campaign that is now in trouble and now understands that they can lose.”

Sanders’s assessment, offered to reporters here after an event at a high school, came shortly after Clinton said on the campaign trail in Iowa that the single-payer health-care plan that Sanders is pushing is a “risky deal” that could be undermined by Republican governors.

[Clinton in Iowa attacks Sanders health-care plan as a ‘risky deal’]

For days before that, Clinton had been aggressively criticizing Sanders on gun control, particularly a 2005 vote to grant legal immunity to gun manufacturers and dealers when their products are used in a crime. Sanders has since said he is open to taking another look at that law, which Clinton opposed while she was a senator representing New York.

Clinton and Sanders were in statistical dead heats in polls of likely voters in Iowa and New Hampshire, the first two nominating states, released Sunday by the Wall Street Journal, NBC News and Marist College.

Previous polls had showed Sanders with a modest lead in New Hampshire and Clinton ahead by a more comfortable margin in Iowa.

[Clinton and Sanders try to erode each other’s strengths in tightening race]

Sanders said Monday that momentum is on his side in Iowa.

“I think the inevitable winner is not quite so inevitable,” he said. “I think we’re going to win this.”

Sanders agreed that he has major differences with Clinton on health care, but he said she was misrepresenting how the kind of plan he is advocating would work.

Sanders supports a move to a single-payer system under which all Americans would be enrolled in Medicare. Earlier Monday, Clinton said she does not want to “rip up” the Affordable Care Act and suggested that Republican governors could undermine the implementation of the plan Sanders is advocating.

Sanders disputed that, saying: “This is, in fact, a national program, no matter what a Republican governor might want to do.”

On the campaign trail Monday, Clinton said she would continue to talk about the differences she has with Sanders.

"I think it's time for us to have the kind of spirited debate that you would wish us to have," she said. "We do have differences, and you deserve to know what those differences are."