In this scenario being played out....the House would have to draw up an investigation, and the first step would be a list of charges. Right now.....no such list exists. Amount of time to play out the charges game? I would take a wag here and suggest that they'd have to appoint some prosecutor team, and that charges wouldn't occur until after they get their hands onto the tax records. The fact that the President isn't cooperating? This will go before the Supreme Court, and probably end up occurring around October/November timeframe.
If the Court says no.....they are privacy issues here, then the whole impeachment thing likely falls apart. If the Court says yes, then they'd be handed over. Amount of time to read five years of 200-to-300 pages of tax returns for each year? Another weg, but you'd need at least a dozen specialized accountants and likely three months.
So these charges would occur in April/May of 2020. The House (by the letter of the Constitution) would act as grand jury, and process the charges to the Senate.
Here's the interesting thing. If the Senate denies the impeachment managers and the case, then the whole thing is 'dead-in-the-water'. The charges will never be read in the Senate.
My guess is that they expect to happen, and it'll be some colossal item to be used to terminate various GOP senators in November.
However, there are two additional scenarios here.
The Senate might shock everyone and say 'sure'.....come on in and lay out your evidence. If the evidence collected by the House is weak or marginal? The impeachment piece would end in matter of five work-days. No impeachment occurs, and the whole thing backfires upon the Democrats in a massive way.
The second scenario is that we get? This fake Russian collusion stuff, and the dossier. There are all of these Obama figures who might be drawn by mid-summer 2019....into grand jury proceedings and they might eventually force one of them to admit that they might have briefed President Obama on their plan. AG Barr might stand there in a private moment with House Speaker Pelosi and tell her....they aren't just fishing for three or four characters.....they are fishing and calling for President Obama to give a statement in front of a grand jury. Then he might suggest that he know someone briefed Pelosi in the summer of 2016 of this whole plan. It'd be a total lie, but she might get freaked because she's forgotten lot of stuff lately, and maybe one of Obama's people might point her as part of this whole game.
If you ask me.....this whole impeachment talk has weak links attached, and if this backfires.....it's the worst possible time and it might lead to a massive Democratic loss (maybe 45 states going to Trump).
So the key part? This tax return game, and how Justice Roberts might react. Add to the mess.....any day now....RBG might be in a bad health situation and have to retire. Without her.....the whole house of cards collapses, and there is no start-up of this mess.
I come to this last element which no one talks about.....that the Democrats have accomplished virtually nothing so far in 2019, and this would consume the rest of this year, and half of next year. How would you go and face the voters, with nothing accomplished over two years?