She also attacked his experience and fitness to lead the country, criticizing his suggestion that the U.S. should warm its relationship with Iran.
The Democratic front-runner told CNN's Wolf Blitzer on "The Situation Room" that Sanders' charge that Planned Parenthood and the Human Rights Campaign support Clinton because they are all part of the establishment didn't make sense to her.
"I just don't understand what that means. He's been in Congress, he's been elected to office a lot longer than I have," Clinton said.
She noted she only served in the Senate for eight years.
"He's been in the Congress for 25. And so I'll let your viewers make their own judgment," Clinton said.
Clinton focused solely on elected office: She was first lady with President Bill Clinton for eight years in the '90s and served as secretary of state in President Barack Obama's administration for his first four years in office.
Clinton also derided Sanders' criticism of the two groups who endorsed her, saying they are champions of Democratic causes. Planned Parenthood provides health care and is an abortion provider and Human Rights Campaign promotes LGBT rights.
"I don't really understand what he means by that. These are two of the really great human rights, progressive organizations in our country," Clinton said.
Clinton also took issue with comments Sanders made during the last Democratic debate, when he called to "move as aggressively as we can to normalize relations with Iran."
The former secretary of state hammered the senator for the position, using it as an opportunity to highlight her record.
"I think it would be a mistake to offer normalized relations," Clinton said, saying her commitment to the Iran nuclear deal is strong. "I engaged in long, patient diplomacy to put together the sanctions on Iran that finally brought them to the negotiation table and actually began the preliminary negotiations that the President and Secretary (John) Kerry continued to completion."
It was a consistent message for the candidate, who attacked Sanders in a speech earlier Thursday and released a video through her campaign that afternoon criticizing his proposal.
She said the Iranian government still engages in a variety of concerning behaviors, including supporting terror, backing Syrian President Bashar Assad and destabilizing the region.
"We can't rush into normalizing relations," she said. "The President doesn't believe that, I don't believe that, and I think Senator Sanders is wrong about that."
She wouldn't answer whether Sanders was qualified to be commander in chief, but said she was.
"I think, obviously, I would be the better choice. I think I am prepared to be both president and commander in chief," she said. "You have to have both the experience and the judgment."
A new CNN/ORC poll released Thursday
found Sanders leading Clinton among Iowa caucus-goers on economic issues, but showed Clinton had a strong advantage in the area of foreign policy.