Why? Two years of pissing off right wingers who have spent two years pissing off them. There is nothing to lose. You no longer win at the ballot box through civility and compromise. They serve their party, not yours. You can't exactly be pissed about party over country at this point. This is what you wanted. This is the country you have demanded by electing Trump and pushing out your moderates. You have won. You now have political opponents who will go lower when you go low. And we both know you will go lower still.
Fact is my friend, both parties have pushed out their moderates. This is why you see the rise in independents from 30% in 2006 up to 43% today. Neither party has anyone left willing to work across the aisle with the other party to keep this nation moving forward. Those who identify as Democrats has seen a drop from 36% of the electorate down to 28%, Republicans from 31% down to 26% since 2006. Both parties today more or less have litmus tests.
Neither represents middle America anymore, those who happen to be on the political spectrum of being center, center left and center right. Today both party either represent the extreme right or the extreme left. This shows in the rise of independents and both major parties shrinking.
You're probably right, the Democrats will go ahead with impeachment of Kavanaugh and probably Trump also. Call it political vindictiveness from the ultra, extreme, far left. As you point out, why not? Trump has just played to his base ignoring everyone else that wasn't an avid, pro-Trumper.
But keep this in mind, the strength of independents is growing and perhaps, ten or fifteen years will top the 50% mark in their share of the total electorate. Today only 20% of independents see Democrats in congress in a positive or favorable light, 48% unfavorable which includes 28% very unfavorable. Republicans in congress are viewed by independents as 22% positive or favorable, 47% unfavorable. no love there for either congressional party. Questions 90A and 90B
https://d25d2506sfb94s.cloudfront.net/cumulus_uploads/document/m6ng12ro4y/econTabReport.pdf
Now independents don't like Trump much, hence they are willing to vote for the Democratic congressional candidates this time around. It's not that they want the Democrats to take over the house, it's they want to show their disdain for Trump's obnoxious, uncouth, unpresidential behavior and his raunchy personality more fit for the WWE than the Oval Office.
But let the democrats show a lot of political vindictiveness, independents are liable to return the house to the Republicans in the next election. What choice do they have? Republicans and Democrats control, have a monopoly on our two party system, our electoral process, so voting for one party in one election, then voting them out and voting in the other party is the only thing left. Especially when neither party represents anything of value to middle America. Only their base.