"He's a relatively new Democrat, and, in fact, I'm not even sure he is one," Clinton told Politico's Glenn Thrush in an interview published on Wednesday morning
. "He's running as one. So I don't know quite how to characterize him. I'll leave that to him."
The critique was a step further than she usually goes on the subject, which she typically brings up to tout her own long history with the party.
Clinton continued that line of attack on MSNBC's "Morning Joe" Wednesday morning.
Asked if she thinks Sanders is a Democrat, Clinton replied, "I think he himself doesn't consider himself to be a Democrat."
"Look, he's raised a lot of important issues that the Democratic Party agrees with, income inequality first and foremost," she said. "But it's up to the Democratic primary voters to make that assessment. I've been in the trenches for a long time."
Throughout this presidential campaign, Clinton has routinely touted her ties to the Democratic Party, a jab at Sanders, a politician who has caucused with Democrats since 1991 but has never been a member of the party.
Clinton grew up in a Republican household and was briefly a Republican when she went to Wellesley College in 1965. Shortly after arriving on campus, however, Clinton grew more liberal and has been a member of the Democratic Party since her graduation in 1969.
Sanders' adviser Tad Devine defended the Vermont senator's ties to the Democratic Party Wednesday.
"She's right he is a new Democrat, he was elected as an independent," Devine told CNN's John Berman and Kate Bolduan. "When we were in New Hampshire they were attacking Bernie for attending fundraisers for Democratic senators because there were lobbyists there. Bernie Sanders can pull together and unify the Democrats, and he has brought thousands of new voters to the party."
Sanders unsuccessfully ran for the U.S. Senate in 1971 as a member of Liberty Union party and, in 1981, when he successfully ran for mayor of Burlington, Vermont, he did so as an independent. At times, the senator has identified as a democratic socialist.
Sanders, however, listed the Democratic Party as his party affiliation in his statement of candidacy for his presidential run. He also declared as a Democrat during the New Hampshire primary.
Clinton also hit Sanders over an interview he did with the New York Daily News
. The interview prompted heavy criticism
because of Sanders' difficulty answering questions about a core campaign proposal, breaking up the big banks, as well as foreign policy.
"I think the interview raised a lot of really serious questions," Clinton said. "I look at it this way -- the core of his campaign has been break up the banks, and it didn't seem in his answers he understood how that would work under Dodd-Frank."
She continued, "I'd think he hadn't done his homework and he has been talking for more than a year about (having) done things that he obviously hadn't really studied or understood, and that does raise a lot of questions."
"Voters," she said, "ask themselves, can he deliver what he is talking about, can he really help people?"