It's because of what happened to Clinton when he was impeached.  He only got more popular. The difference is, is Clinton kept doing his job passing legislation, etc. whereas the Orange one is going to hold his breath until he turns Blue until he gets what he wants.

The sample is too small to make anything of it.  Also, the fact the incumbent president has someone running against him in his own party is (so far) a disastrous sign for him, but again--small sample.

Mind you, if something truly damning could be found; something that would actually be abhorrent even to his unshockable base, say, perhaps that rape trial that went away in 2016 when the victim received death threats, could be proven beyond a reasonable doubt, the Dems should proceed regardless. It would be their job to take out the Criminal-In-Chief should they be able to do so cleanly. It would be the Republicans job to take him down as well, although they would never do so. Politics is a team sport and Trump is their golden boy. Party above all, including Country.

But a dirty, protracted, uphill battle of nuances, shadings, and debates over whether a crime is a crime if those committing it never looked up if it was a crime beforehand would largely benefit Trump and portray his accusers as limp and weak.

Part of me wants to see the dirty laundry show, no question, but from what we know of the evidence against Trump at this point, it wouldn't do any good. 

Did Mueller mention in his statement that he prepared his report for Congress and, as of yet, Congress has not been allowed to read the full version? He may have said all he had to say in the report, but our elected representatives have not been allowed to read all of what he had to say.

And considering the subject is the potential criminality of the highest official in the land, that's something that needs to be dealt with in its entirety. We know he's a braying ass, a liar, a punk, a cheap shot artist, a jerk to women, someone who doesn't pay his contractors, a serial adulterer, and a dweeb in constant need of financial bailout from daddy, but is he a flat-out traitor to his country? It shouldn't be an open question, and smokescreens, victory tweets, & FoxNews brown-nosing aside, it still very much is.

In the '98 midterms, the GOP actually lost 5 seats in the House, the Senate stayed even.  Newt Gingrich's prediction of gaining 30 seats because of the impeachment failed rather dramatically.

Trump doesn't need MORE voters. The number he has was already enough. He only needs a few folks here and there to stay home on Election Day, and waging a losing battle every day for months would discourage a great many already ambivalent Americans from participating further in our dog-and-pony-show political process. 

America loves a winner. Trump gets a lot of love from his loser fanbase by claiming to be one and playing the part. The Democrats would be Losers from day one of an impeachment investigation and every day after that, especially come the end when the Republicans assure us all that the whole gory process was all a big buncha' hooey and award their boy a solid-gold Get Out Of Jail Free card. 

Do you impress a lot of the people you meet by picking fights you can't win and then dragging your bloodied carcass across the pavement, tugging on their pant legs, and asking them for their support?

The Democrats need something with which they can win decisively. As of yet, they don't have it.