"I wonder if he ever considered that," he added. "Like I said, it's anybody's right to speak, but you have to wonder about responsibility, about being responsible to the body and to the public at large."

Harkin estimated that the cost of running the Senate during Cruz's 21-hour filibuster was probably "hundreds of thousands of dollars at least."

Harkin said Cruz's speech was especially confusing because after at the filibuster, Cruz voted along with every other senator in favor of advancing the House-passed continuing resolution.

"Some things are really hard to grasp around this place, you know," Harkin said. However, Harkin acknowledged that he didn't spend much time listening to Cruz.

"You'll excuse me if I didn't pay a lot of attention to him," Harkin said.

During his filibuster, Cruz said he hopes Republicans vote against the spending bill later in the week, after Senate Democrats try to strip out language in the bill that defunds ObamaCare. In its current form, the House-passed spending bill does defund ObamaCare, which is partly why every Republican voted to advance the bill today.