The Vermont senator said Clinton is lashing out -- attacking him on guns, taxes and more -- because polls show their race having tightened in Iowa while Sanders has maintained a narrow lead in New Hampshire.
"Secretary Clinton and her campaign is in serious trouble," Sanders told reporters Monday after a campaign stop in Pleasantville, Iowa.
"And I think a candidate who was originally thought to be the anointed candidate, the inevitable candidate, is now locked in a very difficult race here in Iowa and in New Hampshire," Sanders said. "So obviously in that scenario what people do is start attacking. 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 offered a similar take later at the Iowa Brown and Black Forum. When Sanders was asked if he has noticed that Clinton is attacking him harder and more often, he offered a mischievous "yes."
"It could be that the inevitable candidate for the Democratic nomination may not be so inevitable today," he said.
It's Sanders' antidote to Clinton's "electability" argument. His comment bring to mind her failure to capture the Democratic nomination in 2008, and allude to polls that show Sanders with a real shot at winning in Iowa and New Hampshire.
Sanders is benefiting from a base of supporters that is more enthusiastic than Clinton's -- and he got an unexpected boost from Vice President Joe Biden, who said Sanders has credibility on the issue of income inequality.
"Bernie is speaking to a yearning that is deep and real. And he has credibility on it," Biden said during an interview with CNN chief political analyst Gloria Borger.
"It's relatively new for Hillary to talk about that," Biden continued, acknowledging that Clinton has "come forward with some really thoughtful approaches to deal with the issue" of income inequality.
"Hillary's focus has been other things up to now, and that's been Bernie's -- no one questions Bernie's authenticity on those issues," he said.
Sanders' remarks come after an NBC News/Wall Street Journal/Marist University poll showed Clinton with just a 48% to 45% lead among likely Iowa caucus-goers.
"Our major problem has been I am running against a candidate who was perceived to be the inevitable nominee, right?" Sanders told CNN's Brianna Keilar in between campaign stops in Iowa. "And her name recognition is phenomenal, almost everybody knows who she is, that wasn't the case with me. So we started off in national polls at 3 percent. Well, we've come a very long way."
Sanders said he was proud of the work his team is doing in both Iowa and New Hampshire.
"I think we have a real chance of winning both of those states," he said. "I think we're doing a lot better in Nevada and in South Carolina than people think and if we do well in all of those states, I think we have a real path to victory for the Democratic nomination and I think then we can win the general election."
Clinton, meanwhile, has attacked Sanders on guns in recent days, blasting him for voting for a bill -- while Sanders, Clinton and President Barack Obama were all still in the Senate -- that protected gun manufacturers from liability when their firearms are used in crimes.
"I was there, I voted against it. Sen. Obama was there, he voted against it. And Sanders voted for it," Clinton said of the liability bill in a meeting with the Des Moines Register's editorial board Monday. "And it was the biggest request by the NRA and related gun lobby actor to the Congress and I think and I know the President wants to revisit that as well."
Sanders stood by his vote Sunday on ABC's "This Week" but said he would "revise" parts of the bill.
The debate gun debate continued on the campaign trail Monday and Tuesday.
At the Iowa Brown and Black Forum, Sanders wouldn't back away from that vote, saying: "It's not a mistake. Like many pieces of legislation, it is complicated."
Clinton's campaign and allies were quick to pounce, accusing Sanders of doubling down on his support for a bill that was championed at the time by the National Rifle Association.
"I think most Americans think it was a mistake and wish he would admit it," said Steve Benjamin, mayor of Columbia, South Carolina, and a Clinton supporter, after the debate.
In Tuesday in Ames, Clinton hammered Sanders, saying she has a long history of working -- with success -- to take on issues like income inequality and gun control.
"Don't talk to me about standing up to corporate interests and big powers," she said, in a comment directed at Sanders. "I've got the scars to show for it -- and I am proud of every single one."