Washington (CNN)Hillary Clinton on Thursday sought to turn Sen. Bernie Sanders' "establishment" attack right back on him -- saying he served in Washington much longer than she did.

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.

Washington (CNN)The Democratic presidential hopefuls will face voters in a CNN town hall on Monday in Des Moines -- one week before the highly anticipated Iowa caucuses.

Former Secretary of State Hillary Clinton, former Maryland Gov. Martin O'Malley and Vermont Sen. Bernie Sanders will field questions from Iowa Democrats in this prime-time event hosted by the Iowa Democratic Party and Drake University.

"We are honored to partner with CNN on their town hall with our three fantastic Democratic candidates," said Dr. Andy McGuire, chair of the Iowa Democratic Party. "With this event airing just one week before the first-in-the-nation Iowa caucuses, it's an incredible opportunity for Iowans to see our candidates detail their plans to move our country forward and their vision for Iowa and the nation."

The town hall, which will be moderated by CNN anchor Chris Cuomo, will air from 9 p.m.-11 p.m. ET, the network announced. A CNN spokesperson added that it will make the town hall available to its Iowa affiliates to air live.