Susan Rice, Barack Obama's longtime national security adviser, declined Wednesday to testify before a Senate subcommittee about Russian activities during the 2016 election campaign.

Sen. Lindsey Graham, R-S.C. had requested that Rice appear before the Senate Judiciary Subcommittee on Crime and Terrorism on Monday.

Rice's refusal to testify was first reported by CNN.

In a letter addressed to Graham and ranking member Sen. Sheldon Whitehouse, D-R.I., Rice's attorney Kathryn Ruemmler said that her client opted not to appear because Whitehouse had said he did not agree with Graham that Rice should testify.

Ruemmler called Graham's unilateral invitation "a significant departure from the bipartisan invitations extended to other witnesses."

Had she appeared, Rice would have likely faced questions about the so-called "unmasking" of American citizens caught up in conversations with foreign targets of surveillance by the intelligence community. The most prominent figure to be "unmasked" was retired Lt. Gen. Michael Flynn, who resigned as Trump's national security adviser in February.

Rice became a central part of the Russia investigation when President Donald Trump said she may have committed a crime when she asked intelligence analysts to disclose the name of a Trump associate mentioned in an intelligence report. Rice has said she did nothing improper.

In a statement of his own, Graham noted Rice's previous denials and said, "I expect we will continue down this path. I hope Ms. Rice will come before the committee – and not just the press."

Judiciary Committee Chairman Sen. Chuck Grassley, R-Iowa, said he was "deeply disappointed" at Rice's refusal and added, "Declining to attend because you didn't get an invite from a member of your party is a poor excuse and makes it appear as though she’s hiding something. No investigation will be complete until her role is understood."

Monday's hearing is scheduled to include testimony from former Director of National Intelligence James Clapper and former Deputy Attorney General Sally Yates.

Yates' and Clapper's upcoming public testimony is much-anticipated, as they were both scheduled to speak before the House intelligence committee in March. But that hearing was cancelled, some Democrats believe, because the White House wanted to limit what Yates could say. The House intelligence committee has yet to reschedule the public hearing.

Yates is expected to give senators details about her Jan. 26 conversation with the White House counsel about Flynn. She is expected to say that she saw discrepancies between the administration's public statements about Flynn's contacts with the Russian ambassador to the U.S. and what really transpired, a person familiar with that discussion and knowledge of Yates' upcoming testimony told The Associated Press. The person spoke on condition of anonymity ahead the hearing.

Yates is expected to say that she told White House counsel Don McGahn that she was concerned Flynn's communications with the Russian ambassador could leave Flynn in a compromised position as a result of the contradictions between the public depictions of the calls and what intelligence officials knew to be true, the person said. White House officials have said publicly that Yates merely wanted to give them a "heads-up" about Flynn's Russian contacts, but Yates is likely to testify that she approached the White House with alarm, according to the person. The White House has said Flynn was later fired because of misleading the vice president on the content of Flynn's discussions with the ambassador.

The Associated Press contributed to this report.