So the battle is joined. Finally, after pressure that has been building since before Donald Trump was elected president, the Democratic Speaker of the House Nancy Pelosi has decided to open an impeachment inquiry. With only a bit over a year until the 2020 election, that contest will now be defined by the question of the president's criminality and his fitness to serve.

Or perhaps not. It's entirely possible that, far from putting Trump's fitness front and center, the impeachment fight will largely remove it from the public's agenda. Indeed, if after impeachment the Democrats do run in 2020 on Trump's manifest unfitness, they're likely to find they've grasped the rapier by the blade rather than the pommel.

To explain why, let's walk through how the impeachment inquiry could play out. The most optimistic scenario from the Democrats' perspective would be that it becomes obvious that Trump did something so egregious and dangerous that his own party turns against him. He's impeached, he's convicted in the Senate, and Mike Pence becomes president. In this scenario, obviously, running against Trump's unfitness won't be efficacious, not only because he won't be president anymore but because the GOP will have shown itself to be perfectly capable of rising to the occasion and ousting a dangerous leader. The Democrats will have to run on something else.

The most pessimistic scenario, meanwhile, would be that the inquiry fizzles quickly as it becomes clear the evidence is ambiguous. In that case, it will obviously be in the Democrats' best interest to change the subject as swiftly as possible, while Trump will be the one bringing up their failure at every opportunity.

Most scenarios fall somewhere in between these poles, though. What if the Democrats are able to muster a serious case, but the Republicans are largely unmoved and refuse to hold a trial in the Senate? Or if they hold a trial and vote to acquit? Or if the inquiry is still ongoing through the general election? In each of those cases, wouldn't the most important issue be the need to remove an unfit president, and the failure of his own party to do so?

It's possible, but I'm skeptical that such a framing would lead to a Democratic victory.

For one thing, if the Senate held a trial and voted to acquit, then a good portion of the public would see the issue as having been dealt with. Not partisan Democrats, of course — but those folks are going to turn out to vote anyway. Infrequent, unaffiliated, and swing voters might be more likely to interpret the result as evidence that both sides view the matter through partisan lenses and tune it out. And the Democrats likely can't win the electoral college — or the Senate — without those voters.

You might think that if the GOP played blatant partisan defense for the president, that would agitate the Democratic base to a greater degree and give them an electoral edge. But that's not how the Kavanaugh fight played out. The Democrats lost that one, but the Republicans remained supremely mobilized even after victory — and that's what gave the GOP the boost they needed in the Senate. If Susan Collins, Joni Ernst, Cory Gardner, Martha McSally, and Thom Tillis all hold the line in a Senate trial, then they must believe they have more to fear from crossing the boss than they do from crossing anti-Trump voters in their states. That should give Democrats pause before assuming impeachment will be enough to flip those seats.

Then, while the inquiry is ongoing, there's a real possibility that the allegations move in the minds of at least some of the public from the political column to the criminal one. The standards of evidence could move from "do I want to hire this guy again?" to "is he guilty enough to convict?" — and attacks could feel more like prejudging the case than weighing it and finding the president wanting. This dynamic is arguably ridiculous — but it's also exactly what happened in the Kavanaugh hearings. Ironically, starting a process designed to remove the president for his crimes could lead the public to view that process as the sole legitimate venue for adjudicating whether those crimes were committed, and remove them from the electoral calculus. And the GOP is going to pursue all of these lines of argument through official channels and through friendly media, to make sure that the public evolves in the desired fashion.

Finally, and most basically, nobody knows how impeachment is going to play out, politically speaking. That's reason enough to run a campaign on issues that are more certain to pay off. Trump has no choice but to run against impeachment — he's the target, after all. But the Democrats do. In fact, that's just what the Democrats did in the race for the House in 2018, even as the Kavanaugh hearings raged, to great success. In 2020, similarly, they'll have to make a more interest-based and systemic argument — one that indicts Trump, but for failing to deliver rather than for being unfit, and as an exemplar of a politics and culture gone wrong rather than a unique example of personal perfidy.

Does that mean the Democrats made a terrible mistake in launching the impeachment inquiry? Actually, I don't think so. There's a case to be made that the impeachment inquiry is as much a matter of the dignity of the legislative branch as it is about the seriousness of the president's crimes. What moved so many to the impeachment column, after all, was the administration's initial refusal to facilitate congressional oversight by providing the details of the whistleblower's report. For the sake of Congress' constitutional role, that has to have a consequence. But that doesn't make for a great bumper sticker.

Neither, though, does a campaign against the president as unfit, with or without impeachment. While impeachment is indeed unpopular, it's not unpopular because people like or trust the president. It's unpopular because it is viewed as pure political theater: a waste of time and a distraction from the people's business. Inasmuch as that's the case, it's likely that a campaign focused on Trump's unfitness would also be unpopular — as a sign that the Democrats didn't have a better case to make for themselves — and would fail to deliver either the White House or the Senate to Democratic hands.

So if impeachment forces Democrats to center their campaign message elsewhere, that would actually be to their benefit. Meanwhile, the distinct lack of fallout from the deeply unpopular Clinton impeachment for either party suggests that the public can readily forgive and forget in order to move on.

But it does mean that the primary campaign could look a little different going forward. Until now, the leading candidate for the nomination has been making the case for restoration, that if America got rid of Trump we'd be well on our way to being great again. Impeachment will not only sharpen partisan lines, making Biden's dream of cross-partisan comity an even more obvious non-starter, but arguably will take away his major issue.

That, and not the dirt flung at his son, is the biggest threat impeachment poses to the Biden campaign. And it might be one reason why his strongest challenger, who has been making the case for structural change from the beginning, was also one of the earliest advocates of impeachment.

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