Socialist Sen. Bernie Sanders proudly proclaimed that he did not count former Secretary of State Henry Kissinger, who served in the Nixon and Ford administrations, as a friend. Sanders sought to leverage Clinton's previous comments about Kissinger to his advantage during Thursday night's Democratic presidential debate in Milwaukee, Wis.

Clinton said she was "flattered" by Kissinger's praise of her management of the State Department, during MSNBC's Democratic presidential debate. During Thursday's debate on PBS, Sanders suggested that Kissinger was complicit in "one of the worst genocides in the history of the world" in Cambodia.

"In her book and in this last debate she talked about getting the approval or the support or the mentoring of Henry Kissinger. Now I find it rather amazing because I happen to believe Henry Kissinger was one of the most destructive secretaries of state in the modern history of this country," Sanders said. "I am proud to say that Henry Kissinger is not my friend. I will not take advice from Henry Kissinger."

Clinton responded by questioning who the socialist would look to for expertise on foreign policy.

"Well I know journalists have asked who you do listen to on foreign policy and we have yet to know who that is," Clinton replied.

"Well it ain't Henry Kissinger," Sanders interjected.

"That's fine, that's fine. You know, I listen to a wide variety of voices that have expertise in various areas," Clinton said. "I think it is fair to say whatever the complaints that you want to make about him are, that with respect to China — one of the most challenging relationships we have — his opening up China and his ongoing relationships with the leaders of China is an incredibly useful relationship for the United States of America."

Clinton argued that the next president of the United States will need to listen to differing points of view in order to govern effectively. Sanders replied that trade agreements supported by Kissinger have caused Americans to lose their jobs to the Chinese.

Sanders attack at Clinton's purported area of expertise — foreign affairs — indicates how competitive the Democratic presidential race has become headed into the Nevada caucus on Feb. 20.