Hillary Clinton and Bernie Sanders discuss a point during the second Democratic presidential debate in Des Moines on Saturday. (Photo by Jim Young/Reuters)

A new ad from Vermont Sen. Bernie Sanders prompted a swift reaction from the Clinton campaign which on Thursday called it an "attack ad" and a violation of a Sanders pledge not to run any negative ads.

In an interview with Hillary Clinton on Thursday, MSNBC host Rachel Maddow raised the question: What was so negative about the ad, which implied -- correctly -- that Clinton has accepted money from Wall Street, but also pledges to regulate it?

The 30-second ad, called "Two Visions," never mentions Clinton by name, but it's easy to tell who Sanders is talking about.

"There are two Democratic visions for regulating Wall Street," Sanders says in the ad, speaking directly to the camera. "One says it's OK to take millions from big banks and then tell them what to do. My plan: break up the big banks, close the tax loopholes and make them pay their fair share."

Maddow seemed incredulous: "Honestly, it is not much of an attack," she told Clinton.

Clinton noted that the ad was a "broad assertion that caught up all Democrats," including the man both she and Sanders hope to succeed-- President Obama.

"It's also a very direct criticism of President Obama," Clinton said. "Who, as you recall, took a lot of money from the financial industry when he ran in 2008.

"That didn’t stop him from fighting for the hardest regulations on Wall Street since the Great Depression, signing Dodd-Frank, getting everything he could get out of the Congress at that time," she added.

The Sanders campaign has denied that the ad is negative.

As the race between Sanders and Clinton has gotten closer in the past week, the tone of their once-sleepy campaign has turned combative. New polls show Sanders within striking distance of Clinton in Iowa, and with a lead in New Hampshire. Clinton's national lead over Sanders has also taken a hit.

But Clinton insisted that the two campaigns are sticking to the issues, not resorting to personal attacks.

"It’s a funny kind of charge," Clinton said. "It's sort of a 'pox on all of your houses' for all the Democrats and that's what raised eyebrows."

"I take him at his word," Clinton added. "On anything personal, we don't do that in our side of the debate. We engage on substantive differences, and there are some."

The changing dynamic in the Democratic race has also drawn Clinton out to a slew of interviews -- including the one with Maddow -- aimed at progressive audiences and focused on engaging the Sanders campaign at every turn.

THE TONIGHT SHOW STARRING JIMMY FALLON -- Episode 400 -- Pictured: (l-r) Presidential Candidate Hillary Clinton during an interview with host Jimmy Fallon on January 14, 2016 -- (Photo by: Douglas Gorenstein/NBC) Presidential Candidate Hillary Clinton during an interview with "Tonight Show" host Jimmy Fallon on January 14, 2016. (Douglas Gorenstein/NBC)

Earlier in the night, Clinton taped an interview with "Tonight Show" host Jimmy Fallon -- Snapchat selfies and all. She also addressed the tightening poll numbers against Sanders.

Clinton downplayed the significance of earlier polls in the race that initially showed her with larger leads, calling them "artificial."

"We're in a tight race," Clinton said. "All of those early soundings and polls... once you get into it, this is a Democratic election for our nominee and it gets really close, exciting and it really depends upon who makes the best case that you can be the nominee to beat whoever the Republicans put out."

"I find it exciting," Clinton added. "This is not a job they give away, you really do have to work hard for it. It is the hardest job in the world, so I get up every day and go right at it."

Her full interview with Fallon will air at 11:35 p.m. on Thursday.