As primary day nears in South Carolina, where President Barack Obama remains popular with the state's large African-American population, Hillary Clinton is stepping up her criticism of Sen. Bernie Sanders (I-Vt.) as insufficiently supportive of the nation's first black president.
In the debate, sponsored by PBS NewsHour and held in Milwaukee, Clinton seized upon Sanders' new interview with MSNBC, in which he said he didn't believe Obama has closed the "gap" that the American people feel with their government.
"This is not the first time he has criticized President Obama," Clinton said in the final and most tense exchange of the night. "In the past, he's called him weak. He's called him a disappointment. He wrote a forward for a book that basically argued voters should have buyer's remorse when it comes to President Obama's leadership and legacy."
"The kind of criticism that we've heard from Sen. Sanders about our president, I expect from Republicans. I do not expect from someone running for the Democratic nomination to succeed President Obama," she added.
Sanders called Clinton's accusations "a low blow."
"I think it is really unfair to suggest that I have not been supportive of the president. I have been a strong ally of him on virtually every issue," he said, adding that senators are allowed to criticize the commander-in-chief in a democratic society.
"One of us ran against Barack Obama. I was not that candidate," Sanders retorted.
Sanders, indeed, did not run against Obama. But he did think about it -- or at least encouraged other progressives to think about it.
"I think one of the reasons the president has been able to move so far to the right is that there is no primary opposition to him and I think it would do this country a good deal of service if people started thinking about candidates out there to begin contrasting what is a progressive agenda as opposed to what Obama is doing," Sanders said in 2011.
The book Clinton referenced, by liberal commentator Bill Press, is called Buyer's Remorse: How Obama Let Progressives Down.
What Sanders wrote was more of a promotional blurb than a forward: "Bill Press makes the case why, long after taking the oath of office, the next president of the United States must keep rallying the people who elected him or her on behalf of progressive causes. That is the only way real change will happen. Read this book."
Reached for comment by email, Press said the Clinton campaign must be "desperate" to keep bringing up the blurb, which it had previously done in a press release.
"All Bernie says is that the next president must rally the American people to get Congress to act. He says that in every speech," Press said. "My book is critical of President Obama's failure to deliver on several key progressive issues. Senator Sanders' blurb is not."
In preparation for South Carolina's Feb. 27 Democratic primary, Clinton has increasingly been highlighting her ties to Obama. Her first ad in the state featured Eric Holder, Obama's former attorney general and the first black man to hold that position. Holder argued that Clinton was the person best-positioned to carry on the president's legacy.
And on Thursday, her campaign tweeted out a response to Sanders' MSNBC interview, making fun of Sen. Marco Rubio (R-Fla.) in the same breath for his much-repeated line in the last debate.
Obama has not endorsed in the Democratic presidential primary, although some believe he has dropped hints that he is backing Clinton.