Is this the beginning of the end of the Trump administration?

[pbrower2a]

No. This will take years to play out, rather than 4 weeks or even several months. Trump will stabilize in a while and that will keep him above water for a year or so, I think. I think however the other shoe drops and then the real music starts where Trump meets his downfall.

The real question is when President Trump will recognize how poor a job he is doing. Who knows -- he might go to the mirror and say

YOU'RE FIRED!

[pbrower2a]

~Six months from now, will there be any Democratic Senators calling on Trump to resign, and who will they be?

I think you left out the word "not" here.

I think people are getting ahead of themselves here.  Right now, AFAIK, no prominent Democrats are calling on Trump to resign.  They're calling for an investigation.  "Investigation" can last for months/years, so it's not clear to me how far things will have actually advanced by the fall of this year.

Things move quickly in the Trump era

See also explosions, cave-ins, collapses, flash floods, tornadoes, and earthquakes.

The VP is now connected.

Bad as Trump is, I would be satisfied with a Romney/Hagel Presidency about now. I am a very partisan Democrat, but I am more concerned with the Presidency being a worthy quest in 2020 than with the partisan identity of the President.Just promise to redo the 2016 election while doing the 2018 election (Trump and Pence disqualified, of course).