For the fourth time in American history, the House has embarked on the road to impeachment. Not in more than 150 years have the proceedings unfolded in the middle of a presidential election.

No one knows how this will play out, or what its collateral effects will be. Recent precedent offers no guide. Impeachment inquiries were launched against Richard M. Nixon and Bill Clinton during their second terms, when both were all-but-lame ducks. In 2020, we will see our democratic processes and traditions stretched close to the breaking point.

Maybe all of this was preordained anyway, given the recklessness of the current occupant of the White House. As Joe Lockhart, who was Clinton’s White House press secretary, said to me: “If Donald Trump is president, impeach him or not, he’s going to be the news every day. He has blotted out the sun.”

It is easy to forget that Democrats won back the majority in the House last year by pressing their case on issues that shape the everyday lives of families. With impeachment proceedings underway, there will be little oxygen left for any discussion of health care or infrastructure or college costs or anything beyond the question of Trump’s fitness for office. And if the investigation turns up anything short of a slam-dunk case to remove him, swing voters might punish a party they see as determined to overturn the result of the last presidential election.

All of this helps explain why House Speaker Nancy Pelosi (D-Calif.) was so reluctant to set the formal process into motion, and why she would like to see it come to a conclusion as expeditiously as possible. Senior Democratic lawmakers and top leadership aides have told The Post that a House vote on articles charging President Trump with “high crimes and misdemeanors” could happen within weeks.

But that seems unrealistic. Rushing the process would only give credence to Trump’s near-hourly declarations that all of this is a “witch hunt.” And his White House is certain to resist congressional demands for documents and witnesses upon which the House could build its case, which means battles in the courts that could take months.

Meanwhile, Trump will galvanize his ever-loyal base, and his fundraising will skyrocket. The near-certainty that he would be acquitted by the Republican-held Senate will be spun as an exoneration.

An impeachment investigation will also shape the Democrats’ choice of a nominee, potentially foreshortening the race and freezing the current standings in place. Those who are struggling to break from the rear of a 19-person field will find it even harder — and maybe impossible — to make their voices heard in the din. “Candidates who have advantages now may see those advantages multiplied,” says David Axelrod, who was Barack Obama’s chief strategist.

One of the biggest unknowns is how all of this will affect the prospects of poll-leading Joe Biden. Trump’s brazen and improper play to enlist a foreign government to scuff up Biden assures that baseless allegations about him will be in the water. But even if Trump’s pressure on Ukraine failed, he continues to insist that Biden should be investigated and to spread evidence-free allegations about him. How Biden deals with it will likely be his greatest test as a candidate.

Trump has also created a paradox for himself. As longtime Democratic operative Joe Trippi notes of the president: “The question may not be whether Biden can take him down. It may be his fear of Biden that takes him down.”

The possible effects on Sen. Elizabeth Warren (D-Mass.) are also double-edged. Her campaign, after a rocky start, has been executed more skilfully than any other to date, and she is the only one who has shown consistently upward momentum.

It may or may not help her to have Biden at center stage now. She was the first 2020 candidate to call for Trump’s impeachment, and the relentless focus on that this fall and beyond could dampen scrutiny surrounding the details and cost of some of her more far-reaching policy proposals.

But the most consequential unanswerables are the ones that transcend how the impeachment inquiry will affect the outcome of a single presidential election.

When the country comes out on the other side of whatever terrifying, uncharted territory we are about to cross, what kind of nation will we be? One that is even more deeply and irreconcilably divided than we are now? Or maybe — just maybe — one that is finally ready to demand something better than the ghastliness that has gotten us to this.

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