Cohen has already been scheduled to testify before the House Oversight and Reform Committee on Feb. 7.

But Blumenthal, in his letter to Graham, argued that members have a "constitutional duty" to be a "check" on the president.

"The hour to put country before petty partisan differences has come. The American people are beginning to ask whether their president is a criminal, and Mr. Cohen’s testimony can help answer that question," Blumenthal added.