Hillary Clinton Calls for 'Spirited Debate' With Bernie Sanders as Race Heats Up In snowy Iowa, things are getting heated.

WATERLOO, IOWA -- In snowy Iowa, the race is heating up.

With just three weeks until the caucus, Hillary Clinton took the gloves off to go after her Democratic challenger, Bernie Sanders -— who is now neck-and-neck with her in the polls.

“I think it’s time for us to have the kind of spirited debate that you deserve us to have. We’re so much better than the Republicans, but we do have differences and you deserve to know what those differences are,” Clinton said at a campaign event in Waterloo, Iowa today, after taking on Sanders for his position on health care.

"I just have a difference with Senator Sanders. He has a different plan,” Clinton said.

She said that where she and Sanders “part” on health care is that her plan would not raise taxes on the middle class, while his plan would.

Michael Briggs, a spokesman for Sanders's campaign, responded to Clinton’s attack by defending Sanders’s Medicare single-payer health care plan, saying it "would save the average family thousands of dollars a year in health care costs.” He did not say whether or not the plan would raise taxes on the middle class.

Later in her event on Monday, Clinton also went after Sanders on gun control -— pointing out again that she voted against immunity for gun manufacturers in lawsuits in the “Protection of Lawful Commerce in Arms Act," while he voted for it. Sanders has since said that he is willing to reconsider liability for gun manufacturers.

“I believe those are one of the significant differences between me and Senator Sanders,” Clinton said about her gun policies.

During a campaign event in Iowa later on Monday, Sanders responded to Clinton’s attacks against him, saying it’s a sign her campaign is in “serious trouble.”

It’s a rare move for Clinton to call Sanders out by name during her prepared remarks, and is yet another sign her campaign is taking the Vermont Senator seriously as the race ratchets up.

On Sunday, a new NBC/WSJ/Marist poll showed Clinton ahead of Sanders by just 3 points in Iowa. The same poll showed Sanders ahead of Clinton by 4 points in New Hampshire.

Clinton's campaign has acknowledged that she is in for a tough fight in the early voting states. Last week, the campaign sent a series of fundraising emails about the tight polls, including one from their campaign manager Robby Mook with the subject line "nervous.”

“There’s a situation developing in Iowa and New Hampshire that could change the course of this election,” Mook wrote in an e-mail, noting that Sanders’s campaign is outspending Clinton in TV ads in both states.

On Monday, just hours before Clinton and Sanders will appear together at the Iowa Brown & Black Presidential Forum, the Democratic candidate repeatedly asked voters at her campaign event to come out and caucus for her on Feb. 1.

But when asked by ABC News if she was feeling nervous about Iowa, Clinton kept optimistic.

“I’m feeling great,” she said, “You always have to work hard and be running hard.”