Democratic front-runner 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 on Monday brushed aside 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’s (I-Vt.) earlier comment that her campaign is in “serious trouble,” saying she always expected a close race ahead of the Iowa caucuses.

“I always thought that this would be a close race. I’ve been around the track a few times,” Clinton said in a phone interview from Iowa on MSNBC’s “All In with Chris Hayes.” “It always closes. Iowa’s famously unpredictable.”

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Clinton’s remarks come one day after a new NBC News-Wall Street Journal-Marist poll found Sanders maintaining his New Hampshire lead and closing the gap in Iowa.

"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 said earlier Monday.

The former secretary of State knocked Sanders for not releasing more details on his tax plan. On Monday, Clinton rolled out more components of hers, calling for an additional 4 percent surcharge on people earning more than $5 million per year.

“I have been laying out specific proposals over the course of my campaign, and I always said I’d be laying out additional elements of my tax proposal which I have,” Clinton said. “Sen. Sanders promised to be providing his tax plan before the Iowa caucuses yet I don’t think we’ve seen those.”

While Clinton wouldn’t weigh in on Sanders’s electability directly, the Democratic front-runner touted that she’s a “battle-tested” candidate.

“I know how to go up against the Republicans. There’s hardly anything that you and the voters don’t know about me, so there won’t be a lot of new information coming out, or new labeling coming out, and I think that's a big advantage,” Clinton added.

Jeff Weaver, Sanders campaign manager, responded to Clinton’s comments about the Vermont senator’s electability. “He has really been put through the ringer, so the suggestion that people don’t know who Sen. Sanders is is really not fair,” Weaver said on MSNBC.

Weaver pointed to recent polls that show Sanders faring better with independents and performing better in head-to-head match-ups with GOP candidates than Clinton.

“If Democrats want to win, you have to be able to win Democrats and you have to be able to win the lion’s share of independents,” Weaver added. “If you look at polling, he has consistently does better with independents than Secretary Clinton does.”

Weaver also argued that the Vermont senator’s campaign has attracted young voters and other groups who are less likely to vote. “We got to bring out a large amount of people who don’t traditionally come out,” Weaver said.

When asked about Sanders’s tax proposals, Weaver defended the Vermont senator’s tax plan, saying he’ll unveil more details before the Iowa caucuses, which will be held on Feb. 1.

“He has offered major pieces of his tax plan, and he will be offering up the part that deals with individual income taxes before the Iowa caucuses,” Weaver said.