Using an online portal that links the various groups and their contact lists, more than 140,000 people have registered to begin protesting within hours of Trump’s decision, at predetermined locations in more than 600 cities.

The protests are designed to harness what organisers predict would be a wave of nationwide outrage that would follow Trump’s action, which Democrats in Congress warn could trigger a constitutional crisis.

“It’s pretty rare that people would commit to mobilise around something that we don’t even know will happen, so it shows the energy and passion out there,” said Lisa Gilbert, vice-president of legislative affairs for Public Citizen, a Washington, D.C.-based environmental and government reform organisation. “The reaction will be swift.”

The threats of mass protests appear also to have broad support within the Democratic Party. Last week, former Attorney General Eric Holder wrote on Twitter that Mueller’s sacking would be an “ABSOLUTE RED LINE”.

“The American people must be seen and heard – they will ultimately be determinative,” wrote Holder, who served as attorney general from 2009 until 2015 under Barack Obama.

Planning for the coordinated protests – even if such plans remain unlikely to be activated – is once again helping to unite the activist left amid recent divisions over how best to use direct action in expressing their disgust with Trump and his policies.

Within hours of Trump’s election in November 2016, anguished liberals began consoling themselves through large, often impromptu and disorganised protests in major cities.

In January, that activism further gelled when several million people marked Trump’s inauguration by taking part in the Women’s March, either in Washington or at one of more than 600 satellite demonstrations nationwide.

Protests also erupted throughout the year after Trump pushed to enact a ban on travellers from seven majority-Muslim nations, tried to bar transgender troops from the military, pulled out of the Paris climate accord and rescinded protections for undocumented immigrants brought to the United States by their parents, to name just a few actions.

But for months, activists have struggled to agree on whether such demonstrations are effective when compared with other forms of action, such as making calls to Congress or working to elect like-minded candidates.

David Sievers, campaign director for MoveOn.org, one of the groups helping to organise the potential demonstrations, said the firing of Mueller would far outweigh concerns that demonstrations could distract from progressives’ electoral mission.

“This is different than disagreeing with policy – it’s threatening the idea there are checks on presidential power,” Sievers said. “The role of these protests is to show from Grand Forks, North Dakota, to Houston, Texas, and everywhere in between, people are going to demand an immediate reaction from Congress.”

After people sign up through the TrumpIsNotAboveTheLaw.org website, they are given the location of the protest in their city.

If Trump fires Mueller before 2pm, the demonstrations would begin at 5pm that day. They would begin at noon the following day if Trump were to act after 2pm. Organisers in New York already have stockpiled loudhailers in flats near Times Square, the would-be location of the New York City demonstration, the Los Angeles Times reported.

But some scholars who have been closely watching the anti-Trump protest movement remain sceptical that the firing of Mueller would trigger mass protests.

Rachel Einwohner, a professor of sociology at Purdue University, said activists and the broader public may already have outrage fatigue.

“The potential firing of Mueller would be outrageous, but we have already seen so many outrageous things,” she said. “I don’t know this one would create any more resistance and outcry than what we’ve already seen.”

Todd Gitlin, a professor of journalism and sociology at Columbia University who has closely followed protest movements since the 1960s, said predicting public reaction to the firing of Mueller is difficult because “historical parallels really don’t work”.

Although tens of thousands of protesters hit the streets within hours after President Richard M. Nixon announced the bombing of Cambodia in 1969, the protest movement was largely deflated by the time he fired Watergate special prosecutor Archibald Cox in 1973, Gitlin noted.

The effectiveness of electronic organising for mass protests vs. person-to-person mobilisation on college campuses, as was the case in the 1960s, is also still largely unknown, he added.

But should Trump act, Gitlin said it would clearly represent a “high-noon moment” for progressives.

Even if Mueller remains, Gitlin and Einwohner said the activist movement will confront major questions for the remainder of the Trump presidency about how it should use demonstrations.

The key to a successful demonstration, they say, is making sure that it is sustained, remains newsworthy and continues to draw new participants.

“Mass demonstrations are useful when they are bigger than expected, and that is what happened in January,” Gitlin said. “Now if you try to organise big marches in 2018, you are asking for journalists to shoot you down by saying, ‘Oh, you only have half the numbers.’ ”

That test could come next month, when the Women’s March and other groups are planning protests to mark the first anniversary of Trump’s inauguration.

Women’s March organisers have decided to hold their main “Power to the Polls” event in Las Vegas on January 21, a nod to Nevada’s importance in the midterm elections. Dean Heller of Nevada is widely viewed as one of the most vulnerable Republican senators up for re-election next year.

“We still see [marches] as very important, but we also need to pivot to electoral organising,” said Linda Sarsour, co-chair of the Women’s March, which also is supporting satellite events in several other major cities.

Other activists hope to keep the focus on Washington. They are organising the “People’s March on Washington” on January 27 to call for Trump’s removal from office.

Lawrence Nathaniel, a 24-year-old lead organiser, said he hopes to pull off the event without the support of other organisations, even though he estimates the event on the Mall will cost US$50,000.

“We want to try to get back to the old days when normal Americans just sort of organised themselves together,” said Nathaniel, a native of Columbia, South Carolina.