Hillary Clinton Questions Whether Bernie Sanders Is a Democrat: 'I'm Not Even Sure He Is One'

"I'm not even sure he is one," Hillary Clinton said of Bernie Sanders.

“He’s a relatively new Democrat,” Clinton said. “I’m not even sure he is one. He's running as one. So I don't know quite how to characterize him. I'll leave that to him.”

Clinton has increasingly been using this line of attack in the past few days. In an interview on "Morning Joe" this morning, Clinton said she was unsure if Sanders should be running against her, because “he himself doesn’t consider himself to be a Democrat.”

“He’s raised a lot of important issues that the Democratic Party agrees with, income inequality first and foremost,” Clinton acknowledged, but argued whether Sanders has “done his homework.”

“He’s been talking for more than a year about doing things he, obviously, hadn’t really studied or understood, and that does raise a lot of questions," Clinton said.

And in an interview that aired on “Good Morning America” on Monday, Clinton told ABC News' George Stephanopoulos that there would be “no indication” that Sanders would help elect more Democrats to the 115th Congress.

Clinton also stressed during a campaign event in Eau Claire, Wisconsin, on Saturday that she believes “it's kind of important if we're selecting somebody to be the Democratic nominee of the Democratic Party."

Sanders is the sitting independent U.S. senator of Vermont. Appearing on ABC News' “This Week” on Sunday, Sanders said the accusations from Clinton that he’s not truly a Democrat is simply a sign that she’s “getting very nervous.”

“I believe I am the strongest candidate to take on the Republicans and the fact that I have been the longest serving Independent in the history of the United States makes my candidacy even strong,” Sanders told Stephanopoulos.