Politico reports today that Democrats are beginning to take Donald Trump’s efforts to delegitimize a Hillary Clinton victory in advance very seriously — they believe he’s preparing to “poison” a Clinton presidency by pushing the notion that the “election was stolen,” as part of a “Clinton delegitimization project.”

This blog has made a similar case, as have many others: Trump’s repeated suggestions that the election could be “rigged” suggest he may be hoping to leverage his following into a political force or monetized media audience after a loss, and the idea that Clinton is not a legitimate president might serve as a focal point for such an effort. Democrats tell Politico that they are aware that this could hamper her ability to govern. As one puts it: “When you see Trump and his forces at best trying to delegitimize her, at worst trying to delegitimize the entire democratic process, we’re heading down a very dangerous path.”

Presidential historians I spoke to agree with this — and they also suggest that Trump’s effort to sow doubts about the integrity of the election well in advance of its outcome may be without parallel in American history.

“The idea that he’s now claiming that this might be a crooked election, I don’t know if there’s a historical precedent,” Doris Kearns Goodwin, who has written numerous books about American presidents, told me.

Goodwin pointed out that Theodore Roosevelt did castigate the primary process as rigged by party leaders during the four-way 1912 election, enabling William Howard Taft to win the Republican nomination. (Roosevelt subsequently ran as a Progressive.) She also noted that Andrew Jackson’s supporters railed against the outcome of the 1824 election as a “corrupt bargain” after he won a plurality of the electoral college vote, only to watch the House of Representatives deliver the presidency to John Quincy Adams. She also cited the disputed 1876 election.

But Goodwin noted that what Trump is doing may be unprecedented.

“It seems to me that this is different from what has happened before,” Goodwin said. “What he’s doing is something really serious, which is questioning ahead of time the legitimacy of our democracy.”

Jon Meacham, who has also authored numerous books about American presidents, including one about Andrew Jackson, agreed that there may be something unique about what Trump is doing.

“Jackson referred to his losing after he won the popular vote as a ‘corrupt bargain,’ but even there, he was reacting to events that had already taken place,” Meacham told me. “He was not undermining trust in institutions without evidence.”

Both Goodwin and Meacham agreed that this could have untold consequences for a Clinton presidency — and, more broadly, the country’s civic health.

“If he persuades a large group of his voters that somehow they weren’t fairly represented in this election, that’s a really troublesome thing for what has always been the touchstone of our democracy — that once the election happens, however difficult the transition is, we move on, and the people who lost mobilize themselves to try and win next time,” Goodwin said.

“If you pre-delegitimize the results of a democratic election,” Meacham added, “then you’re laying the groundwork for four to eight years of constant hostility towards the winner of that election.”

Both Goodwin and Meacham cited the 2000 election, noting that after the Supreme Court ruling that delivered the presidency to George W. Bush, Al Gore gave a speech in which he called on the public to accept the outcome as legitimate. Gore pledged to do all he could to help unify the country, to send a message to the world about the continued and undiluted “strength of American democracy.”

Trump appears to be laying the groundwork to do precisely the opposite of that. It’s yet another thing that raises questions about his fitness for the job.

“American presidents are the custodians of cultural and political order,” Meacham said. “There have been political forces in American life that have been disrupted and reform minded. This is less about disruption and really close to destruction.”