Imagine a presidential election in a tumultuous year, when the country has never seemed more divided.

Supporters of both parties are unwilling to accept defeat, with some even threatening civil war if their candidate of choice doesn’t take the White House. The economy is in shambles, disgruntled citizens — some armed with guns — protest on the streets, and there are accusations of voter fraud and intimidation.

They might sound like headlines from 2020, but every one of these things happened in 1876. As journalist and historian Richard Kreitner explains in his new book, “Break It Up: Secession, Division, and the Secret History of America’s Imperfect Union” (Little, Brown), the political strife that shook this country 144 years ago feels oddly familiar.

The 1876 election was contentious. Though Democratic candidate Samuel Tilden won the popular vote and the Electoral College, several Southern states, including Florida, suspected voter fraud and invalidated Democratic votes — enough to give Republican Rutherford B. Hayes the lead.

Both parties and their voters threatened violence. Plans were made for a counter-inauguration for Tilden in Manhattan, in which he would “seize the federal treasury building and fund a shadow government through customs revenues.” Democratic state militias would invade Washington and chase Hayes out of the White House. As one supporter suggested publicly, “a little bit of war to inaugurate Mr. Tilden would do us no harm.”

Hayes supporters were no less ready for battle. One Texas voter pledged “hundreds + thousands” of ex-soldiers to fight for the Republican candidate.

The only difference between that political battle and the tensions leading up to the 2020 election is that, in 1876, nobody had Twitter.

It’s just one example of how we’re not necessarily living through extraordinary times, or at least not as extraordinary as we’re led to believe. While our country may seem more politically divided and polarized than ever, Kreitner says that the bipartisan nostalgia for a golden age of national unity longs for a time that never existed.

“I didn’t come along and divide this country,” President Trump said at his first presidential press conference in 2017. “This country was seriously divided before I got here.”

He’s not wrong, says Kreitner.

“Our most powerful myth, that the many ever melded into one, is just that: a myth,” he writes. “Our refusal to recognize this, like patients who insist, against all evidence, that they are not ill, has been a major cause of our political dysfunction.”

According to a 2019 Georgetown poll, 67 percent of Americans think we’re heading toward an inevitable civil war. Media on both sides have stoked the flames, with CNN talking heads and Rush Limbaugh alike predicting the country’s dissolution. A Washington Post headline from last year gave this chilling assessment: “In America, talk turns to something not spoken of for 150 years: Civil war.”

But they’re wrong. It hasn’t been 150 years since civil war has been seriously contemplated. Not even close. “Disunion — the possibility that it all might go to pieces — is a hidden thread through our entire history,” writes Kreitner. “From the colonial era to the early republic, through the fabled American Century and up to our own volatile moment.”

The only difference is we don’t say as much explicitly. “Have you seen the Lincoln Project ads, ‘Trump or America’?” Kreitner asks. “What do they mean by that? If Trump is re-elected, there’s no more America? The idea of disunion is a topic that comes up a lot, we’re just not always aware that we’re doing it.”

The Broadway musical “Hamilton” got a lot of things right about the Founding Fathers. But there is one conspicuous omission from Lin-Manuel Miranda’s hip-hop history lesson. The feud between Aaron Burr, the ambitious vice president, and Alexander Hamilton wasn’t just about personal animosity.

It was, Kreitner writes, an “explosive debate [about] whether the United States should remain a single nation or break in two.”

Burr had been approached by a New England cabal, who were fearful of losing power in the rapidly growing Union and had decided to secede and start their own “northern confederacy.”

They wanted Burr to take the helm of their new nation, and maybe even bring New York into the fold. When Hamilton caught wind of the plot, he denounced Burr as a threat and a traitor.

But even Hamilton wasn’t so sure about whether the country should stick together. In his final letter before meeting Burr for their infamous 1804 duel, Hamilton suggested that “breaking up the United States would spark a rebirth of American democracy.”

Thomas Jefferson, our nation’s third president, hoped the states would stay united “if it be for their good. But separate them if it be better.” He was OK with the country being peacefully broken apart “as long as the principles on which it had been founded endured,” Kreitner writes.

“Forming a single sea-to-sea nation wasn’t so important.”

A lot of things that feel unique to 2020 have actually been constants in our nation’s history. Like fake news: During the failed impeachment of Andrew Johnson in 1868, one Texas newspaper got ahead of itself, declaring “War! War!! War!! The president impeached! The first gun fired! Grant declares himself dictator! General war inevitable!” As Kreitner points out, “None of it except impeachment was true.”

There were accusations of foreign collusion. President Jefferson’s critics accused him of “taking orders” from the French emperor Napoleon, conspiring to impose sanctions on foreign commerce as a way of “helping France at England’s (and New England’s) expense.”

There were race-baiting politicians like “Pitchfork Ben” Tillman, who crashed the 1896 Democratic National Convention in Chicago and told the crowd he would fight for “the emancipation of the white slaves.”

But the real reason we’ve always been on the brink of breaking apart, says Kreitner, is our size.

“Joining people together across vast systems is inherently incompatible with democratic self-government,” he told The Post. “You have to give up too much power in order to do that.”

Napoleon, the pre-Putin dictator suspected of sowing chaos in the US, agreed with the theory that America’s size would be its undoing.

The French emperor only consented to the Louisiana Purchase in 1803, a territory sale that doubled the size of the United States, not as an act of generosity but because he believed “America’s ungainly size would lead to its demise,” writes Kreitner. “The new acquisition would be too much for the United States to swallow, Napoleon thought.” Expanding the United States wouldn’t heal its divisions and differences of opinion but only make them worse.

“Eventually,” Napoleon reasoned, “the swollen Union would collapse under the weight of its own contradictions.”

So why hasn’t it? It’s been a combination of chance and accident, says Kreitner. “This idea that we’re bound to survive and have to stay together, history shows that’s not really the case.”

In many cases, it’s come down to compromise. In the 1876 standoff between presidential candidates Tilden and Hayes, both of whom had supporters ready to wage war on their behalf, the dispute was settled not by protests or violent clashes or even investigations into voter fraud.

It all happened behind closed doors, in a “smoke-filled room in Washington.”

The Democrats agreed to accept Hayes’ inauguration in exchange for the withdrawal of federal troops from Louisiana and South Carolina. It was a handshake deal that had nothing to do with voters and certainly didn’t address any of the underlying resentments and disagreements that made both parties so willing to take up arms to keep the other out of power.

It was the beginning of the end for Reconstruction — or, more specifically for the still-embittered South, the end of “federal tyranny and black suffrage,” writes Kreitner.

Secession movements aren’t relics of another era. There are groups that even today are plotting how to make a formal split from the United States.

The Texas Nationalist Movement, formed in 2005, claims to have around 385,000 members (or 1.3 percent of the Texas population). And the idea of seceding has enough weight in the Lone Star State that it was debated in earnest at the 2016 Texas Republican Convention.

Yes California, a separatist group founded in 2015, filed a proposed initiative last summer demanding a popular vote in 2022 on whether to leave the Union. The arguments for leaving range from anti-Trump sentiments to economic resentments. If California became an independent nation, it would be the fifth-largest economy in the world.

Neither “Texit” nor “Calexit,” as their movements have been dubbed, have much of chance. While the US Constitution has procedures for admitting new states, there is nothing suggesting how states might leave, and it even explicitly states that the Union is “perpetual.”

But it’s not just the constitutional roadblocks that keep secession from happening. It’s ultimately fear that keeps us together, says Kreitner.

“It’s a fear of uncertainty, of what would happen if the Union broke apart,” he tells The Post. “For colonists, it was the fear of Indian raids that made them stick with the Union. Today, national dissolution seems to be something Americans both desire and fear.”

The push and pull between those two instincts has been evident throughout our history. In 1961, an attempt to commemorate the 100th anniversary of the Civil War fell apart when a New Jersey delegation of the Civil War Centennial Commission was banned from the South Carolina hotel where the event was being hosted. The reason? They were black.

President John F. Kennedy moved the festivities to a nearby naval base, but the South Carolina commission refused to go along with the new plan. Instead, they decided to … wait for it … secede from the national commemoration.

A hundred years after the Civil War, the United States was still embroiled in the same disagreements.

“That day, the Confederate battle flag was raised over the South Carolina capitol building,” Kreitner writes. “It would remain there for more than fifty years.”

But the recent Georgetown poll shows that we haven’t given up hope. Despite grim predictions of civil war, more than 80 percent of voters also told pollsters that “compromise and common ground should be the goal for political leaders.”

“It’s a productive ambiguity,” says Kreitner. “I don’t think it’s healthy to look at history and decide we’ve been through worse before so we’ll probably be fine now. We need to be having those arguments out in the open. Is the Union worth preserving? And if so, why?”

He personally goes back and forth on whether the United States should break apart or stay together.

“Does living in such an incomprehensibly massive country make each of us more or less significant, our lives more or less meaningful, our rights to life, liberty, and the pursuit of happiness more or less secure?” Kreitner asks. He isn’t sure of the answer.

But he’d like to live long enough to see the country’s tricentennial in 2076.

“I’ve joked with my grandmother that I want to live to 86 to see the country celebrate 300 years,” he says. “But honestly, I think there’s a fifty-fifty chance we won’t make it that far.”