The US is “closer to civil war than any of us would like to believe”, a member of a key CIA advisory panel has said.

The analysis by Barbara F Walter, a political science professor at the University of California at San Diego who sits on the Political Instability Task Force, is contained in a book due out next year and first reported by the Washington Post.

It comes amid growing concern about jagged political divisions deepened by former president Donald Trump’s refusal to accept defeat in the 2020 election.

Trump’s lie that his defeat by Joe Biden was caused by mass electoral fraud stoked the deadly attack on the US Capitol on 6 January, over which Trump was impeached and acquitted a second time, leaving him free to run for office again.

The “big lie” is also fueling moves among Republicans to restrict voting by groups that lean Democratic and to make it easier to overturn election results.

Such moves remain without counter from Democrats seeking a federal response but stymied by the filibuster, the Senate rule that demands supermajorities for most legislation.

In addition, though Republican presidential nominees have won the popular vote only once since 1988, the GOP has by playing political hardball stocked the supreme court with conservatives, who outnumber liberals 6-3.

All such factors and more – including a pandemic which has stoked resistance to government – have contributed to the divide Walter has studied.

Last month, she tweeted: “The CIA actually has a taskforce designed to try to predict where and when political instability and conflict is likely to break out around the world. It’s just not legally allowed to look at the US. That means we are blind to the risk factors that are rapidly emerging here.”

The book in which Walter looks at those risk factors in the US, How Civil Wars Start, will be published in January. According to the Post, Walter writes: “No one wants to believe that their beloved democracy is in decline, or headed toward war.

But “if you were an analyst in a foreign country looking at events in America – the same way you’d look at events in Ukraine or Ivory Coast or Venezuela – you would go down a checklist, assessing each of the conditions that make civil war likely”.

“And what you would find is that the United States, a democracy founded more than two centuries ago, has entered very dangerous territory.”

Walter, the Post said, concludes that the US has passed through stages of “pre-insurgency” and “incipient conflict” and may now be in “open conflict”, beginning with the Capitol riot.

Citing analytics used by the Center for Systemic Peace, Walter also says the US has become an “anocracy” – “somewhere between a democracy and an autocratic state”.

The US has fought a civil war, from 1861 to 1865 and against states which seceded in an attempt to maintain slavery.

Estimates of the death toll vary. The American Battlefield Trust puts it at 620,000 and says: “Taken as a percentage of today’s population, the toll would have risen as high as 6 million souls.”

On Sunday, Sidney Blumenthal, a former Clinton adviser turned biographer of Abraham Lincoln and Guardian contributor, said: “The secessionists in 1861 accepted Lincoln’s election as fair and legitimate.”

The current situation, he said, “is the opposite. Trump’s questioning of the election, which was at first rejected by Republican leaders after the attack on the Capitol, has led to a crisis a genuine crisis of legitimacy.”

With Republicans’ hold on the levers of power while in the electoral minority a contributing factor, Blumenthal said, “This crisis metastasises, throughout the system over time, so that it’s possible any close election will be claimed to be false and fraudulent.”

Blumenthal said he did not expect the US to pitch into outright civil war, “section against section” and involving the fielding of armies.

If rightwing militia groups were to seek to mimic the secessionists of the 1860s and attempt to “seize federal forts and offices by force”, he said, “I think you’d have quite a confidence it would be over very, very quickly [given] a very strong and firm sense at the top of the US military of its constitutional, non-political role.

“… But given the proliferation of guns, there could be any number of seemingly random acts of violence that come from these organised militias, which are really vigilantes and with partisan agendas, and we haven’t entered that phase.

“The real nightmare would be that kind of low-intensity conflict.”

Among academics, Walter is not alone in diagnosing severe problems with US democracy. In November, the International IDEA thinktank, based in Sweden, added the US to a list of “backsliding” democracies, thanks to a “visible deterioration” it dated to 2019.

It also identified “a historic turning point … in 2020-21 when former president Donald Trump questioned the legitimacy of the 2020 election results”.

Polling has revealed similar worries – and warnings. In November, the Public Religion Research Institute asked voters if they agreed with a statement: “Because things have gotten so far off track, true American patriots may have to resort to violence in order to save our country.”

The poll found that 18% of respondents agreed. Among Republicans, however, the figure was 30%.

On Twitter, Walter thanked the Post for covering her book. She also said: “I wish I had better news for the world but I couldn’t stay silent knowing what I know.”