(CNN) Former President Donald Trump's attorneys are fighting a secret court battle to block a federal grand jury from gathering information from an expanding circle of close Trump aides about his efforts to overturn the 2020 election, people briefed on the matter told CNN.
The high-stakes legal dispute -- which included the appearance of three attorneys representing Trump at the Washington, DC, federal courthouse on Thursday afternoon -- is the most aggressive step taken by the former President to assert executive and attorney-client privileges in order to prevent some witnesses from sharing information in the criminal investigation events surrounding January 6, 2021.
The court fight over privilege, which has not been previously reported and is under seal, is a turning point for Trump's post-presidency legal woes.
How the fight is resolved could determine whether prosecutors can tear down the firewall Trump has tried to keep around his conversations in the West Wing and with attorneys he spoke to as he sought to overturn the 2020 election and they worked to help him hold onto the presidency.
This dispute came to light as former Trump White House adviser and lawyer Eric Herschmann received a grand jury subpoena seeking testimony, the people briefed said.
Other former senior Trump White House officials, including former White House counsel Pat Cipollone and his deputy Patrick Philbin, appeared before the grand jury in recent weeks, after negotiating specific subjects they would decline to answer question about, because of Trump's privilege claims.
Herschmann himself is not in court fighting the subpoena. Instead, Trump's lawyers are asking a judge to recognize the former President's privilege claims and the right to confidentiality around his dealings. Herschmann's grand jury testimony has been postponed.
It's still unknown if prosecutors want to use the information for possible cases against Trump or others.
Trump's lawyers have expected the Justice Department to eventually seek a judge's order to compel additional testimony from White House witnesses, CNN has previously reported.
The Justice Department didn't respond to a request for comment.
Under grand jury secrecy rules, the legal dispute is under seal, with no public documents to show the state of play.
The Justice Department has been girding for a legal challenge along these lines for months, CNN previously reported.
In addition to Cipollone and Philbin, former vice presidential aides Greg Jacob and Marc Short have appeared before the grand jury in the DC courthouse and declined to answer some questions because of Trump's executive privilege claims, CNN previously reported.
On Thursday afternoon, Evan Corcoran, Tim Parlatore and John Rowley, who work together representing Trump in the January 6 probe, exited the courthouse accompanied by a law clerk.
Parlatore told reporters he was there "representing a client" but would provide no further details. The other lawyers declined to comment.
The Trump legal team's push to broadly assert privilege has been subject of disagreement among its lawyers over legal strategy, people briefed on the matter said.
Herschmann received a grand jury subpoena for testimony and documents related to January 6 weeks ago. But he was irked before his court date by what he saw as vague guidance from Trump lawyers not to share information, people briefed on the matter say.
Herschmann pushed Trump lawyers to provide him with more detailed instructions for which topics to assert privilege over, according to emails reviewed by CNN and first reported by The New York Times.
"A letter directive from President Trump without a court order would not be sufficient. I don't understand your statement that the chief judge will decide the issue," Herschmann wrote. He then raised concerns about DOJ seeking to compel his testimony if he refused to testify to certain questions.
Herschmann previously testified to the House committee about what he saw at the White House around January 6.
The outspoken lawyer expressed concerns that the Trump team's approach potentially put him at risk for grand jury contempt, according to people briefed on the matter. He pushed back when Trump's lawyers sent him a letter with instructions that he cite executive or attorney-client privileges to the grand jury.
Other former Trump aides have expressed similar frustration at the vagueness of the Trump privilege claim, people briefed on the matter tell CNN.
Attorney-client privilege claims can be overcome for a number of reasons, including if any information is shared outside of the attorney-client pipeline and if the communication relates to possible wrongdoing. In the January 6 situation, a federal judge in California has already found email exchanges to and from John Eastman, Trump's election attorney, wouldn't be covered by this confidentiality, providing the records to House investigators and allowing the Justice Department to access those and other similar exchanges.
Executive privilege is a tougher pursuit for investigators, though not impossible to overcome. The Justice Department gained access to Nixon's Watergate tapes for a federal grand jury in the 1970s because of a Supreme Court ruling that the criminal investigation needed the materials. But courts haven't hammered out exactly where the lines would be drawn in this investigation, or for a former President who may try to keep secret advice he was given while leading the country.
The dispute is separate from privilege protections Trump has tried to claim in the separate investigation of handling of federal records and national security information after his presidency. That investigation prompted the FBI to seize classified documents from Trump's Mar-a-Lago resort, and a judge acting as a special master is now working through the more than 10,000 non-classified records to determine if Trump can block them from investigators.