House Judiciary Committee Chairman Jerry Nadler, D-N.Y., suggested on Thursday that the White House is secretly instructing the Justice Department to not let special counsel Robert Mueller testify before Congress.

Attorney General William Barr told the Wall Street Journal that it will be Mueller's "call" on whether to testify. Nadler told MSNBC his committee is negotiating with the Justice Department over a hearing date, but has struggled to come to an agreement.

"They simply are dragging their feet at fixing a date so we'll continue to try to fix a date," he said, adding that "if it goes too long then we'll have to consider a subpoena."

MSNBC's Kasie Hunt asked Nadler if he believed timing was the only sticking point.

"The issue has been timing for a while, but whether they are being honest that it's only timing — I don't know. Perhaps the White House is telling them 'don't let him appear and use any excuse,'" he said.

Barr previously said he had no objection to Mueller testifying, but Trump gave mixed signals about it over the past couple weeks. Trump tweeted on May 5 that Mueller should not testify because it could amount to further investigations against him. "No redos for the Dems!" he said. On Thursday, Trump reversed course, saying he would leave it to Barr to decide.

This could represent a breakthrough in the fight being waged between Congress and the Trump administration over access to Mueller's full, unredacted report, underlying documents, and testimony of some of the key players in the 22 month long investigation, including Mueller himself.

Mueller's report, released last month by the Justice Department with redactions, shows his team was unable to find sufficient evidence to establish criminal conspiracy between the Trump campaign and the Kremlin. Mueller also details 10 instances in which Trump might have obstructed justice, but he declines to say either way whether Trump committed a crime, citing a Justice Department guideline that sitting presidents cannot be indicted. Barr and Deputy Attorney General Rod Rosenstein determined there was not enough evidence to establish an obstruction offense. Although Trump and his allies say he is exonerated, Democrats have called for more investigations.

Both the judiciary and intelligence panels have subpoenaed for access to the full 448-page Mueller report after the Justice Department released a redacted version last month. After the Justice Department defied to comply with Nadler's subpoena, the House Judiciary Committee voted to cite Barr for contempt of Congress, but did so along party lines. Meanwhile, the White House asserted executive privilege over Mueller's findings.