Missouri Sen. Claire McCaskill said Thursday that she's "worried about" Sen. Bernie Sanders' recent comment that Hillary Clinton isn't qualified to be president.

Calling Clinton unqualified "is like fingernails on a blackboard to many women across this country," McCaskill said on MSNBC, and added that Sanders "probably knows that."

McCaskill, who has already endorsed Clinton, said she worries that Sanders' remarks could be divisive in the Democratic Party. But she stopped short of saying Sanders should exit the race.

"His supporters are passionate, for a good reason," McCaskill said when asked if he should abandon the race. She added that the presidential campaigns of Clinton and Sanders need to "stay by-and-large very positive about the things we agree on, and don't go into negative attacks."

When asked if she thought gender was a reason behind Sanders' comments, McCaskill answered, "Well, I just think it's one of those things that maybe it's not right. But, I think women who have succeeded in male-dominated fields have been used to being marginalized about whether or not they're truly qualified."

"And this is an example where we have a candidate for president, who is the most qualified, and happens to be a woman," she added. "And I think there are many women that are excited about that, and for anybody to call her unqualified is hard for those women to hear. I don't think Bernie really believes that Hillary Clinton isn't qualified to be president."