House Speaker Nancy Pelosi on Tuesday will formally back an impeachment inquiry — a move that paves the way for Donald Trump to become the third president to be impeached. There are two ways to read this:

A. Pelosi (D-Calif.) is being dragged into something she badly wanted to avoid for fear of making President Trump a martyr and motivating his base for his 2020 reelection bid.

B. She judiciously and wisely withheld her impeachment support until the evidence became super compelling — specifically thanks to new developments involving Ukraine — and she feels now that her party can successfully litigate the case against Trump.

Wherever the truth lies, it’s a momentous decision — and one that carries with it all kinds of unintended consequences that even a smart tactician such as Pelosi can’t possibly foresee. Here are a few things we can say:

First, we should all challenge the conventional wisdom that impeachment will probably help Trump. This prevailing belief stems from Democrats’ surprising success in the 1998 election shortly after Republicans launched their impeachment proceedings against President Bill Clinton.

  • Clinton’s impeachment straddled the 1998 election, meaning there was no time for it to fade as an issue. By contrast, today this process could wrap up before the calendar ever hits 2020 or before we even have a Democratic nominee, judging by the length of past impeachment proceedings. The Post’s Paul Kane has suggested this could all be an afterthought by November 2020, and I tend to agree.
  • Impeachment was actually thought to have hurt the Democrats in the 2000 election, particularly when it comes to then-Vice President Al Gore. People wanted to move beyond the controversies of the Clinton administration. Gore tried to distance himself, meaning he could not utilize Clinton’s support in the election. (While Gore lost the presidential race, Democrats picked up a few seats in the House and Senate.)
  • Clinton, unlike Trump, was popular when he was impeached — with an approval rating in the 60s. That’s when you probably expect to have a successful election. Trump, by contrast, is generally in the low 40s today.
  • Clinton’s impeachment was built on conduct that was widely viewed as a private matter — an affair — and Americans didn’t really view it as being their business. Trump’s impending impeachment, though, has to do with public actions that he took as president — including potential obstruction and corrupt dealings with Ukraine.

On the flip side, though, is the unpredictability. Let’s say this is simply an unknown or even a 50-50 proposition — a 50 percent chance it helps Democrats and a 50 percent chance it hurts. Right now, polls suggest that Democrats are favored in 2020, including with Joe Biden leading by as much as double digits nationally and in key states. At the very least, impeachment would seem to add a major variable in an election in which Democrats were starting to look very good.

There is also the fact that Americans haven’t seemed to be on board with impeachment. Polls generally show around one-third favor impeachment, though that has hit as high as about 40 percent at given junctures.

But there is also the possibility that it will rise once Democrats go down this road. A June poll from Quinnipiac University is instructive. While it found that Americans opposed impeachment 61 percent to 33 percent, it also asked whether Trump deserved to be impeached. In that case, it was much closer, with 50 percent saying Trump didn’t deserve it but 44 percent saying he did. Independents were split 45-45.

The limited impeachment polling since the Ukraine situation blew up also suggests support could rise. YouGov asked Tuesday, “If President Donald Trump suspended military aid to Ukraine in order to incentivize the country’s officials to investigate his political rival, Joe Biden, and his son, would you support or oppose impeachment?” In that case, 55 percent supported impeachment -- including 44 percent “strongly" -- while just 36 percent opposed it.

It all suggests strongly that there are many people in this country who maybe don’t want to see Trump impeached, for political reasons or because they don’t really like the idea, but who do think his actions could warrant exactly that step. It’s not difficult to see them coming around, just as it’s not difficult to see House Democrats who have resisted this idea rallying to the cause now that their leader is committed.

In an ideal world, would Pelosi want to do this? Her past comments suggest not, and the unpredictability of impeachment has to heavily weigh on that decision. There’s also the fact that we’re still awaiting key details on the Ukraine situation — details that could support impeachment or undercut it.

House Speaker Nancy Pelosi (D-Calif.) called for a formal impeachment inquiry of President Trump on Sept. 24 after a history of quelling impeachment calls. (The Washington Post)

But Pelosi’s impending announcement feels a lot like Democrats crossing the Rubicon. Once they start going down the road of impeachment, it will be difficult to pull it all back, no matter the evidence, because the Democratic base is behind it and doing so might depress base voters.

The next 13 months just got even more interesting.