The upcoming special election in Wisconsin’s 7th congressional district could be the most critical House election in U.S. history.

Since Rep. Sean Duffy Sean DuffyCNN's Ana Navarro to host Biden roundtable on making 'Trump a one-term president' Bottom line McCarthy blasts Pelosi's comments on Trump's weight MORE (R-Wisc.) retired three months ago, House Republicans have held a 4-3 seat lead in the state — one of 26 state delegations the party controls.

Since Democrats govern the House, this fact is largely irrelevant on all counts except one: the 12th Amendment to the Constitution states that if a presidential election yields no winner, “the House of Representatives shall choose immediately, by ballot, the president.

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But in choosing the president, the votes shall be taken by states, the representation from each state having one vote.” Support from a majority of states is then required to become president.

So if a Republican wins the WI-7 special election in May, the GOP will maintain leads in a majority of state delegations, giving President Donald Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE a definite advantage if no presidential candidate wins 270 electoral votes this year. And if a Democrat wins the Wisconsin seat, that state’s delegation would be evenly split, giving Republicans control of only 25 states — not enough to select Trump on a party-line vote.

The Republican-controlled Senate, meanwhile, would be mandated to pick the vice president — assuredly Vice President Pence. If the House cannot agree on a choice for president before noon on Jan. 20, 2021, Pence would become president until, according to the 20th Amendment, a president “shall have qualified.”

All of this would be uncharted territory for a country that has seen its share of contested elections. That is not to say presidential elections have never tested the Constitution. In 1800, Thomas Jefferson prevailed over running mate Aaron Burr on the 36th ballot after allegedly cutting a deal with Federalists, led by Delaware Representative James Bayard, who acknowledged betraying his party out of an "obligation” to not “hazard the Constitution upon which the political existence of the State depends.”

In 1824, John Quincy Adams triumphed over Andrew Jackson and William Crawford with support from 13 of 24 state delegations (and notably, Henry Clay). A single switched vote by Adams-supporting representatives from Illinois, Louisiana, Missouri, or Rhode Island would have denied Adams the presidency on the first ballot. At that point, anything could have happened, including Adams running mate John Calhoun temporarily (or perhaps not temporarily) ascending to the presidency.

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At other times, the House has come close to choosing a president. The 1876 election between Rutherford Hayes and Samuel Tilden produced disputed vote tallies in Florida, Louisiana, and South Carolina, prompting Congress to sidestep constitutional precedent and instead create a commission to decide the outcome, resulting in the destructive Compromise of 1877.

And while the 2000 election between George W. Bush and Al Gore Albert (Al) Arnold GoreFox's Napolitano: 2000 election will look like 'child's play' compared to 2020 legal battles Who calls an election? Why we need patience and nonpartisanship this time Universal mail-in voting jeopardizes the equal right to vote, but absentee voting protects it MORE is known for hanging chads and a Supreme Court ruling along ideological lines, neither candidate would have clinched 270 electoral votes had Gore pulled out a win in his home state of Tennessee and added about 7,000 votes in New Hampshire, and if Bush had picked up about 4,200 votes in Iowa, 400 in New Mexico, and roughly 6,000 in Maine’s 2nd congressional district.

That year the House GOP controlled only 25 state delegations, meaning if no members broke ranks by Janu. 20, 2001, Dick Cheney likely would have temporarily (or again, perhaps not temporarily) assumed the presidency.

Yet 2020 is different. For much of recent U.S. history, each party’s elected officials have included a fair share of swing voters, such as conservative Democrats and liberal Republicans. From a deadlocked House in 1960 or even 2000, a consensus commander-in-chief might have emerged through negotiations, not unlike the bipartisan wrangling required in policymaking.

But these days, it is difficult to conceive of a deadlocked House finding common ground. And that is what makes the Wisconsin 7th special election a uniquely compelling event, and a seat that could be up for grabs.

While Sean Duffy was a safe incumbent, and while Trump defeated Hillary Clinton Hillary Diane Rodham ClintonBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Virginia Democrat blasts Trump's 'appalling' remark about COVID-19 deaths in 'blue states' The Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally MORE there by 20 points in 2016, four years earlier, Mitt Romney Willard (Mitt) Mitt RomneyThe Memo: Warning signs flash for Trump on debates GOP votes to authorize subpoenas, depositions in Obama-era probe Overnight Defense: Trump hosts Israel, UAE, Bahrain for historic signing l Air Force reveals it secretly built and flew new fighter jet l Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' MORE beat President Barack Obama Barack Hussein ObamaGOP senator blocks Schumer resolution aimed at Biden probe as tensions run high D-Day for Trump: September 29 Obama says making a voting plan is part of 'how to quarantine successfully' MORE by only three points. Meanwhile, 2018 Democratic House candidates in the state earned only 12,500 fewer votes than they did in 2016, while their Republican counterparts had 97,000 fewer votes during the same period.

With seven electors voting for someone other than Trump or Clinton in 2016, and with the most politically polarizing president in modern times up for re-election, the race to 270 electoral votes realistically might not be won. Whoever becomes president in January 2021 might owe her or his fortunes to whoever’s elected to fill Duffy’s old seat.

B.J. Rudell is associate director of POLIS: Duke University’s Center for Political Leadership, Innovation, and Service, part of the Sanford School of Public Policy. In a career encompassing stints on Capitol Hill, on a presidential campaign, in a newsroom, in classrooms, and for a consulting firm, he has authored three books. He has shared political insights across all media platforms, including for CNN and Fox News.