Former Obama national security adviser Susan Rice has refused an invitation to testify next week on Russian meddling in the U.S. election.

In a letter obtained by CNN, Rice's lawyer, Kathryn Ruemmler, said the decision was made because Sen. Lindsey Graham, R-S.C., chairman of the judiciary subcommittee on crime and terrorism that invited her to testify, didn't have the support of the panel's top Democrat, Sen. Sheldon Whitehouse, D-R.I.

"Senator Whitehouse has informed us by letter that he did not agree to Chairman Graham's invitation to Ambassador Rice, a significant departure from the bipartisan invitations extended to other witnesses," Ruemmler wrote. "Under these circumstances, Ambassador Rice respectfully declines Senator Graham's invitation to testify."

An unnamed source told CNN that Rice initially thought the invitation had bipartisan backing, adding that Rice saw the invitation as a "diversionary play" to distract from ongoing investigations into Russian interference in 2016.

Graham told CNN on Tuesday that he wanted to ask Rice questions because he had seen reports that she was involved in the "unmasking" of an American citizen who was incidentally surveilled.

"I believe Russia tried to interfere in our election," he said. "Did the Obama administration try to politicize intel? I don't know. I'd like to get the answer to both."

Despite refusing to testify in an open setting, in the letter, Ruemmler does indicate that Rice is "prepared to assist congressional inquiries into Russian election interference because of the important national interests at stake, provided they are conducted in a bipartisan manner, and, as appropriate, in classified session."