Using a role-playing game that is a fixture of military and national security planning, the group envisioned a dark 11 weeks between Election Day and Inauguration Day, one in which Trump and his Republican allies used every apparatus of government — the Postal Service, state lawmakers, the Justice Department, federal agents, and the military — to hold onto power, and Democrats took to the courts and the streets to try to stop it.

If it sounds paranoid or outlandish — a war room of seasoned politicos and constitutional experts playing a Washington version of Dungeons and Dragons in which the future of the republic hangs in the balance — they get it. But, as they finalize a report on what they learned and begin briefing elected officials and others, they insist their warning is serious: A close election this fall is likely to be contested, and there are few guardrails to stop a constitutional crisis, particularly if Trump flexes the considerable tools at his disposal to give himself an advantage.

“He doesn’t have to win the election,” said Nils Gilman, a historian who leads research at a think tank called the Berggruen Institute and was an organizer of the exercise. “He just has to create a plausible narrative that he didn’t lose.”

The very existence of a group like this one, which was formed late last year, underscores the extent of the fear in Washington’s political circles — and beyond — that Trump will take the same hammer he has used to fracture the norms of executive governance over the past three years and upend the nation’s delicate tradition of orderly political transitions of power by refusing to concede if he loses.

“We have norms in our transition, rather than laws,” said Rachel Kleinfeld, a senior fellow in the Democracy, Conflict and Governance Program at the Carnegie Foundation, who was not part of the game. “This entire election season is something a democracy expert would worry about.”

It is a fear that has been stoked by the president himself, who has repeatedly warned, without offering evidence, of widespread fraud involving mail-in ballots — which voters are likely to use at unprecedented levels because the pandemic has made in-person voting a potential health risk — to cast doubt on the results of November’s election.

“I think mail-in voting is going to rig the election, I really do,” he  told Fox News’  Chris Wallace  last Sunday. When asked if he would accept the election results, he said: “I’ll have to see.”

Former vice president Joe Biden, the presumptive Democratic nominee, has taken to issuing foreboding warnings of his own. “This president is going to try to indirectly steal the election by arguing that mail-in ballots don’t work — they’re not real, they’re not fair,” he said at a fund-raiser on Thursday night. He has also  mused  publicly about Trump having to be escorted, forcibly if need be, from the White House.

That happened in one of the four scenarios the Transition Integrity Project gamed out, according to summaries of the exercises provided to The Boston Globe. But constitutional experts — and the game play — was less focused on the possibility of a cinematic, militarized intervention on Inauguration Day, which is a possibility many still consider remote, than the room the Constitution appears to leave for a disastrous and difficult transition if the incumbent does not accept a loss.

“How well is our constitutional legal system designed to deal with an incumbent president who insists that he won an election but for the presence of fraud?” said Lawrence Douglas, a professor at Amherst College who has written a book on what would happen if Trump took such a stand. “And I think the rather unfortunate answer is our system is not well designed at all to deal with that problem,” said Douglas, who was not involved in the game.

Brooks got the seed of the idea for the Transition Integrity Project after a dinner where a federal judge and a corporate lawyer each told her they were convinced the military or the Secret Service would have to escort Trump out of office if he lost the election and would not concede. Brooks wasn’t so sure. She and Gilman decided to turn the Washington parlor game into an actual exercise; they held an early meeting in Washington, with about 25 people, in December.

“When we started talking about this we got a lot of reactions — oh, you guys are so paranoid, don’t be ridiculous, this isn’t going to happen,” Brooks said.

Two things have happened since then: Trump has displayed increased willingness to challenge mail-in ballots, and his administration has deployed federal forces to quell protests in front of the White House and in Portland, Ore., and has threatened to do so in other cities.

“That has really shaken people,” Brooks said. “What was really a fringe idea has now become an anxiety that’s pretty widely shared.”

Brooks, Gilman, and others recruited a slate of players including a former swing state governor, a former White House chief of staff, and a former head of the Department of Homeland Security. They invited both Democrats and Republicans who they knew had concerns about Trump’s comments on the election; nearly 80 people in all were involved. The Republicans were described by participants as “never Trump” or “not Trump Republicans.”

They played using the so-called Chatham House Rules — in which participants can discuss what was said, but not who was there; some participants were willing to be named. They included Republicans Trey Grayson, the former Kentucky secretary of state, and conservative commentator Bill Kristol, as well as Democrats Leah Daughtry, who was CEO of the 2008 and 2016 Democratic National Convention Committees, former White House ethics czar Norm Eisen, and progressive Democratic strategist Adam Jentleson.

The game was elaborate. The participants took on the roles of the Trump campaign, the Biden campaign, relevant government officials, and the media —generally, Democrats played Democrats and Republicans played Republicans — and used a 10-sided die to determine whether a team succeeded in its attempted moves. The games are not meant to be predictive; rather, they are supposed to give people a sense of possible consequences in complex scenarios.

Each scenario involved a different election outcome: An unclear result on Election Day that looked increasingly like a Biden win as more ballots were counted; a clear Biden win in the popular vote and the Electoral College; an Electoral College win for Trump with Biden winning the popular vote by 5 percentage points; and a narrow Electoral College and popular vote victory for Biden.

In the scenarios, the team playing the Trump campaign often questioned the legitimacy of mail-in ballots, which often boosted Biden as they came in — shutting down post offices, pursuing litigation, and using right-wing media to amplify narratives about a stolen election.

To some participants, the game was a stark reminder of the power of incumbency.

“The more demonstrations there were, the more demands for recounts, the more legal challenges there were, the more funerals for democracy were held, the more Trump came across as the candidate of stability,” said Edward Luce, the US editor of the Financial Times, who played the role of a mainstream media reporter during one of the simulations. “Possession is nine-tenths of the law.”

In multiple scenarios, officials on both sides homed in on narrowly decided swing states with divided governments, such Wisconsin, Michigan, and North Carolina, hoping to persuade officials there to essentially send two different results to Congress. If a state’s election is disputed, a legislature controlled by one party and governor of another each could send competing slates of electors backing their party’s candidate.

Both sides turned out massive street protests that Trump sought to control — in one scenario he invoked the Insurrection Act, which allows the president to use military forces to quell unrest. The scenario that began with a narrow Biden win ended with Trump refusing to leave the White House, burning government documents, and having to be escorted out by the Secret Service. (The team playing Biden in that scenario, meanwhile, sought to patch things up with Republicans by appointing moderate Republican governors, including Charlie Baker of Massachusetts, to Cabinet positions.)

The scenario that produced the most contentious dynamics, however, was the one in which Trump won the Electoral College — and thus, the election — but Biden won the popular vote by 5 percentage points. Biden’s team retracted his Election Night concession, fueled by Democrats angry at losing yet another election despite capturing the popular vote, as happened in 2000 and 2016. In the mock election, Trump sought to divide Democrats — at one point giving an interview to The Intercept, a left-leaning news outlet, saying Senator Bernie Sanders would have won if Democrats had nominated him. Meanwhile, Biden’s team sought to encourage large Western states to secede unless pro-Democracy reforms were made.

That scenario seemed highly far-fetched, but it envisioned a situation in which both sides may have incentives to contest the election.

“There is a narrative among activists in both parties that the loss must be illegitimate,” he said.

According to the Constitution, the presidency ends at noon on Jan. 20, at which point the newly inaugurated president becomes the commander in chief.

The games, ultimately, were designed to explore how difficult it could be to get there.

“The Constitution really has been a workable document in many respects because we have had people who more or less adhered to a code of conduct,” said retired Army Colonel Larry Wilkerson, a Republican and former chief of staff to Colin Powell who participated in games as an observer. “That seems to no longer to be the case. That changes everything.”