President Donald Trump attempted to undermine former acting Attorney General Sally Yates by attacking her in a Monday morning tweet ahead of her Senate testimony on Russian hacking in the 2016 election and the extent of the Trump campaign’s ties to Russian officials.

Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. Counsel. — Donald J. Trump (@realDonaldTrump) May 8, 2017

Yates is scheduled to testify before a Senate subcommittee on Monday afternoon, after Trump administration officials tried to stop Yates from testifying at a House Intelligence Committee hearing in March. She reportedly sounded the alarm about former national security adviser Michael Flynn’s communications with Russian officials before he was forced to resign in February.

Rep. Devin Nunes (R-Calif.), chairman of the House committee, abruptly canceled the March hearing, after the Justice Department unsuccessfully sought to block Yates from testifying, according to letters obtained by The Washington Post. Nunes later stepped down from the investigation after he was called out for questionably handling classified information and appearing to collude with the Trump administration.

Trump fired Yates in January, after she refused to enforce his ban on travel and immigration from seven majority-Muslim countries, which was quickly halted by multiple courts.

In another tweet Monday morning, Trump blamed former President Barack Obama for Flynn’s downfall because Flynn’s security clearance was approved under the Obama administration, an argument that White House press secretary Sean Spicer also used last month in attempting to explain Trump’s poor vetting of Flynn.

When asked about Trump’s attack against Yates at the daily White House briefing, press secretary Sean Spicer said multiple times that “the tweet speaks for itself.”

This article has been updated with Spicer’s comments from Monday’s press briefing.