What would happen if Trump lost and invalidated the election?

Patrick Leahy would become President, there would be no one in the House of Representatives, and 2/3 of the Senate (with a Democratic majority) would still be seated.

I found this on our very own Hal Brown's blog.
“You never change things by fighting the existing reality.
To change something, build a new model that makes the old model obsolete.”
– R. Buckminster Fuller