An over-view of the "worst case senario" (short of Trump losing his re-election bid) is given definition in this post:

BREAKING NEWS: Schumer’s proposal to McConnell. If he rejects these reasonable ground rules & insists on a non-trial, the House should consider treating that as a breach of the Senate’s oath & withholding the Articles until the Senate reconsiders. 

Editor:  In addition to this extension of idiotcy, Pelosi is threatening to hold up sending the impeachment bill from the House to Seante until the Senate moves to protect "due process" as relates to the House charges of impeachment.  

McConnell says,  "No deal."  And that should be fine with all of us.  Pelosi really has very little leverage, here.  There will be no more debates about impeachment or at least none that any Republican member should attend.  I mean,  the case for impeachment is over.  If she hopes to delay sending the House Impeachment Bill until after the election,  in the hopes of recapturing the Senate,  that changes absolutely nothing.  Why?  

Well,  lets say the Dems retake the Senate.  It still takes 67 votes to find the President "guilty," and that would require a Senate vote of 67 including approximately 13 to 15  Republicans    . . . .    the same number of GOP defections required in the GOP controlled Senate, today.  In other words,  without a clear cut crime,  there is simply no way the Dems can remove Trump except via an election.

Of course,  without the Senate and the House, Trump's presidency would be stymied.  Understand the only point I am making is this:  Impeachment is not the solution to the Dems' desire to get rid of Trump.  Never was.

You should know that a President without either house of Congress,  is not entirely helpless.   He can prevent the Dems from reversing his judical appointments, prevent them from packing the courts with One World Socialist judges,  keep his regulation agenda in place,  and keep a reasonable tax policy in place as well as get get things done via executive order.  More than this,  the President can utilize a thing called a "signing statement" (under existing presidential powers) to refuse to enforce any number of sections in a bill,  should he sign that bill.  He could not not add to the bill, that would be legislation.  But he certainly could release a signing statement detailing what parts of the bill he intends not to enforce.  

Finally, you should know that the last time the Dems controlled the House and the Senate (2009 - 2010),  that reality only lasted two years.  In other words,  should the worst happen,  it is extremely possible for the President to own one of the two houses in the last half of his second term.   

Now you know.