The document, an 18-page 1973 investigative memorandum from the Watergate Special Prosecution Force, sheds new light on how prosecutors were investigating attempts at domestic political violence by Nixon aides, an extremely serious charge.
NBC News is publishing the memo, and an accompanying memo about an interview prosecutors conducted with GOP operative Roger Stone, as part of special coverage for the 45th anniversary of the Watergate break-in.
READ: Prosecutors' Memo on "Investigation Into The Assault On Anti-War Demonstrators On May 3, 1972"
READ: Prosecutors' Memo on "Interview with Roger Stone, re: May 3rd Incident"
A plot to physically attack Ellsberg is notable because the former Pentagon official has long alleged that Nixon operatives did more than steal his medical files, the most well-known effort to discredit him.
In his memoirs, Ellsberg wrote that in May 1972, the White House had flown "Cuban-American CIA 'assets' from Miami to Washington to disrupt a rally that I and others were addressing on the steps of the Capitol," with orders "to incapacitate [me] totally."
Nixon officials denied that account, however, and there were never any indictments related to the accusation.
The memo, written on June 5, 1975, by Watergate special prosecutor Nick Akerman, provides some contemporaneous support for Ellsberg's allegation that he was targeted.
The attack would be on "long-haired demonstrators, in particular Ellsberg," the prosecutors' memo states, with the objectives of impugning Ellsberg for protesting near to Hoover lying in state and "simply having Ellsberg beaten up."
Prosecutors concluded that White House counsel Charles Colson directed the operation, which Colson denied.
Prosecutors initially responded to newspaper reports that Bernard Barker, a Cuba-born Watergate burglar, and a group of nine Miami associates "had engaged antiwar demonstrators in a fight" and that Colson was behind it.
Colson pleaded guilty to obstruction of justice related to stealing Ellsberg's medical files, but was never charged for conduct related to this plot. The memo addresses that distinction, noting that while prosecutors concluded Colson was involved, the evidence they had "would not be sufficient to indict Colson."
"There is still no clear way to link Colson to the assault which is muddled by his efforts to organize a lawful counterdemonstration," the memo concludes. "This melding of the counterdemonstration and the assault had been a problem throughout this investigation in charging anybody with a crime."
The memo also suggests that Nixon was briefed on aspects of the plot, because aides John Ehrlichman and Bob Haldeman said the story "might someday hurt the president" if links to Nixon operatives were revealed. Ehrlichman had discussed "bringing the Cubans up to rough-up the demonstrators," and Nixon responded, "Campaign activities — I got that."
The conversation, captured on the Watergate tapes, continues as the talk turns to how the incident might ultimately be handled by Congressional investigators and the Watergate prosecutors.
"Probably, [it] will get in some way logged into the grand jury business because of the money," Haldeman says.
While that possibility apparently concerned the White House, the document shows prosecutors never had quite enough evidence to build the plot into their case against Nixon and his operatives.