 -- During a one-on-one interview with ABC News today in Columbia, South Carolina, Bernie Sanders stood by and escalated his attacks on Hillary Clinton over her taking speaking fees from corporate banks.

The Vermont senator had just finished arguing he was “not into personal attacks and mean-spirited attacks.” When challenged about whether bringing up Clinton’s speaking fees at the debate constituted personal attacks, Sanders replied: “Well, I think it is a fact. A fact. Anyone disagree with me? She has received $600,000 in one year as speaking fees from Goldman Sachs. If that’s not true, I will apologize. It’s true.”

The past few weeks have been the most combative yet in the tightening Democratic race, and Sunday night’s NBC debate was no exception. Sanders and Clinton, now neck-and-neck in early voting states, tangled over health care policy, taxes and Wall Street reform. Sanders showed no hesitation to hit his opponent for being too cozy with the big banks, and today the progressive superstar said he was not nervous that the battle for the nomination was getting too negative. He stood by his campaign’s heated rhetoric.

“If you looked at the Republican debate, I would think it was not very personal at all. I think what I have been trying to do, and the point that I made last night, was I can’t walk down the street, as you well know, without media saying, you know, criticize Hillary Clinton, say something terrible about Hillary Clinton. It’s not my style of politics. What I have tried to do from day one is to run an issue-oriented campaign, and the reason you run is to show the differences of opinion -- you know, you have different opinions. Hillary Clinton and I have differences of opinion. It’s called democracy. But I’m not into personal attacks and mean-spirited attacks,” he said.

All three Democratic candidates spent the weekend in Charleston, South Carolina, where the debate was held. It is a state where President Obama is immensely popular and Clinton is still light years ahead of Sanders in the polls. The former secretary of state evoked the president several times during the debate, arguing in essence, that she was the rightful heir of his presidency or at least the most qualified to defend and build upon the work that he has done.

During the interview with ABC News, Sanders laughed and smiled sheepishly when asked if he felt if Clinton had hid behind Obama during the debate.

“Well, you know, you might say that,” he replied. “Look, at the end of the day, Secretary Clinton and I respect enormously the work that President Obama has done. ... What we have got to do is present our ideas to the American people, and my view is yes, we have accomplished a lot, but there is a lot yet that remains to be done."

“I would build on the good things, but you know, it is no secret. Barack Obama and I are friends. We’ve worked together on many, many issues. We have disagreements. That’s called democracy. So we will expand the good things that he has done, and we will work on areas that he has not done as much I think as needs to be done,” he added, citing income inequality, corporate tax loopholes, and overturning Citizens United ruling, which deemed that organizations and corporations also have a First Amendment right to free speech.