Let's walk through this.
1. Trump has been repeatedly claiming "Presidential harassment!!!" for over a year. And has already made it clear that this is going to be one of the pillars of his campaign strategy in 2020. This was true before the impeachment announcement. If Democrats were going to be accused of this, anyway, there is less risk to going all in and exciting their own base in the process.
2. The polling numbers on impeachment have been highly volatile, with swings by as much as 20% in either direction depending on what's in the news. A Fox poll in June, for example, found 50% in favor and 48% opposed, so you cannot simply claim that the majority are opposed. The real answer here is it depends on when you ask and how you ask and about what you ask.
3. That last point is even more true today, as the public still has not had the chance to absorb the latest revelations. There will undoubtedly be polls in the coming weeks that reevaluate public support for impeachment. If those polls show strong opposition to impeachment, then the naysayers will then have some data to back up their claims.
4. People who insist that the Republican base will be energized are missing the fact that the impeachment process will also energize the Democratic base. All of the available data from the last few decades show the same thing: when Democrats are energized, when they turn out, they win. Both parties were enthusiastic and energized in 2018 and we still had a 7-point blue wave.
5. On that last point, the Democratic base has been growing increasingly restless: "We elected you, gave you ownership of the House, and yet nothing has been passed and Trump is still running wild and doing enormous damage! Why should we vote for you again in 2020?!" That's a bit of an exaggeration, of course, but not by much. People need to feel that the people they are voting for will fight for what they believe.
6. Formal impeachment hearings will give House Democrats a stronger case in the federal courts for the documents and witnesses they have been subpoenaing. Trump's attorneys have insisted that there has been "no legislative purpose" to these demands. Weak argument though that is, there was a very real fear that five Justices on the Supreme Court would accept it. They will have a much harder time making that case now.
7. Trump does not handle things like this well. He's been increasingly unhinged of late and that will no doubt continue and get worse. Expect the level of paranoia in the White House to increase and expect Trump to continue to incriminate himself and amp up the noise in his tweets and in his interactions with the media. That will not serve him well.
8. With the flood gates opened and White House stonewalling defeated, there will be new revelations coming to light. I don't expect any single "smoking gun" but rather more of a "death by a thousand cuts."
9. This process will take months rather than days or weeks. Depending on the progress through the courts, there is a very real chance that we won't even have a formal vote of impeachment before the 2020 election.
10. Finally, this is the right thing to do, from a legal standpoint, a Constitutional standpoint, and a "norms" standpoint. Sometimes, you just have to do the right thing, even if there is a political cost.
This is just off the top of my head; I'm sure that there are other arguments that can be made. The bottom line is that anyone predicting absolute certainty about 2020 right now is full of shit. You simply cannot make the case that, "His chances for re-election just went up measurably," anymore than I can make the case that "Pelosi just guaranteed a Trump defeat." Both statements are equally foolish.
