With early and mail-in voting in the 2020 election under way in many states, Republicans and Democrats are already battling each other over how to collect and count ballots. Lawsuits challenging signatures, drop boxes and deadline extensions have littered the federal courts for months, demonstrating just how many Americans are concerned about a contested or unclear result.

Republican senators have raised these stakes by rushing through the confirmation of Amy Coney Barrett to the Supreme Court, whom President Donald Trump has indicated he expects to be the critical vote to secure his re-election — although Coney Barrett herself disclaimed such a prospect.

Legal challenges and a partisan Supreme Court deciding the outcome may feel like déjà vu, and some observers have directly compared this year to the 2000 Bush-Gore race. But the details of the 2020 contest actually mirror another election much more closely.

In 1876, the very same problems we are facing today – extreme partisanship in a divided government, voter suppression, legal challenges and Republican calls to involve the Supreme Court – combined to form a perfect storm that led to an unprecedented electoral crisis. Without a landslide victory on election night next month, we may be headed for a similar result.

Like today, partisanship and polarisation were consistent features of 19th-century politics. 1876 was no different, with a Republican president in the White House in Ulysses S. Grant, a Democratic House of Representatives and a Republican Senate. Divided government was not unique to Washington, either. Outside the nation's capital, state legislatures and governor's mansions were frequently split between the two parties.

The election of 1876 was also the first time after the Civil War that many former Confederates returned to the polls since the federal government had barred them from political participation in the immediate postwar years. The result was a resurgence of the Democratic Party in the South, while at the same time Black men, recently enfranchised under the 15th Amendment, eagerly voted for the Republicans. With each party at full strength for the first time after the war, a highly competitive and partisan atmosphere set the stage for an extremely close election.

This extreme partisanship and polarisation produced exceptionally high levels of interest and engagement. Both political parties worked exceedingly hard to mobilise their base, and the election of 1876 yielded the highest voter turnout in US history, at 81.8 per cent. (Although, unlike this year, there was also clear evidence of voter fraud. In South Carolina, for example, the turnout of eligible voters was an impossible 101 per cent.)

High turnout also led to massive efforts to suppress the vote. In the South, Black voters risked their lives to cast ballots in support of Republican governments that would continue to secure the civil rights gained during Reconstruction. In response, Southern Democrats and racist vigilante groups such as the Ku Klux Klan used violence, intimidation and voter disenfranchisement to suppress the black vote whenever possible – in many ways anticipating attempts by the modern Republican Party to implement discriminatory measures such as voter ID laws, purging voter rolls and the targeted reduction of polling places primarily serving minority voters. This year, a 2018 federal court ruling will also allow Republicans to use a specific form of voter-intimidation tactic at the polls for the first time in nearly 40 years.

In 1876, this combination of partisanship, divided government, high turnout and voter suppression produced contested results in three states: Florida, Louisiana and South Carolina. When both political parties pursued extensive legal challenges over those results, an unprecedented electoral crisis emerged.

Immediately after Election Day on November. 7, many observers assumed that Democratic nominee Samuel J. Tilden had won. Tilden had 184 of the 185 electoral votes needed, while Republican nominee Rutherford B. Hayes had 165. In the ensuing weeks, it became clear that results in states representing the remaining 20 votes were contested. Reports of fraud and violence against black voters threw the results into doubt.

Individual state results were certified by state election boards and governors, all controlled by political parties. Unsurprisingly, each party produced its own accounting of the outcomes. In Florida, for example, both the outgoing Republican governor and the newly elected Democratic governor certified a separate set of returns for their party. The result was a severe constitutional crisis, and one that some election experts have warned could occur again this year.

Since 1887, the electoral process has been governed by the convoluted Electoral Count Act, but in 1876, the Constitution provided even less clarity. The 12th Amendment, ratified in 1804 to help guide the counting process, offered no clear solution. It stipulated only that once states submitted their certified results, "the votes shall then be counted" in the presence of both houses of Congress. But which set of votes, and counted by whom?

Committees in both the Democratic House and the Republican Senate considered various solutions that inevitably favoured their partisan predilections. In another eerie parallel, some Senate Republicans suggested passing the responsibility to the Supreme Court to decide the 1876 returns. Democrats refused to hand the matter over to the court alone – there was a majority of Republican-appointed justices – but the idea became part of the eventual solution: an electoral commission with five members of the Democratic House, five members of the Republican Senate and five members of the Supreme Court – including two Democrats, two Republicans, and a fifth justice chosen by the other four. The other justices ultimately selected Joseph Bradley, another Republican, which led the commission to award the returns to Hayes in a series of 8-7 party-line votes in mid-February 1877.

Congressional Democrats furiously protested the decision, threatening to use all manner of parliamentary delays to prevent Hayes from taking office. Outside Washington, there was a palpable sense of anger, especially among Southern Democrats, fuelled by the lingering animosities of the Civil War. Tilden received scores of letters from supporters declaring their willingness to use violence to challenge the results by force and install him as president. Restarting the Civil War seemed a real possibility.

Yet, although he believed he was the rightful winner, Tilden remained silent. With increasing political polarisation between a majority Democratic South and Republican North, he was unwilling to risk further sectional violence. As a result, Tilden refused to challenge the results via the courts even as advisers urged him to do so, allowing instead for a peaceful transfer of power.

Given President Trump's persistent refusal to offer similar assurances, this year's outcome has the potential to produce an even greater crisis than 1876. And if the election is either close or contested, the Supreme Court will include a majority of justices from the party that has sought to restrict voter access, especially among black Americans.

Nineteenth-century Americans were no strangers to the kind of divisive partisanship featured in this election. But unlike Americans in 1876, we do not have the recent memory of 750,000 Civil War dead serving as a warning for what happens when one side refuses to accept the outcome of a presidential election. Without the basic commitment of the president to maintaining the democratic process, and with a Republican Party increasingly promoting minority rule, the 2020 election could produce a constitutional crisis even more like 1860 than 1876.

• Rachel Shelden is an associate professor of history at Penn State and director of the George and Ann Richards Civil War Era Centre. Erik B. Alexander is an associate professor in the department of historical studies at Southern Illinois University, Edwardsville