This is quite a long shot, but it is a remote possibility. Timing would be critical.
Imagine this:
- Republicans continue to avoid doing anything substantial regarding Trump’s actions and behavior.
- Largely because of Trump and Republican inaction, the 2018 elections become a stunning reversal of fortune in Congress.
- Pence becomes ensnared in his own conduct issues, related to supporting Trump.
- Articles of Impeachment are brought against both Trump and Pence by the House. Either through conviction and/or resignation, both leave office prior a new Vice-President being named and approved by the Senate.
- The new Speaker of the House is next in the line of Presidential succession. That likely means Nancy Pelosi becomes 46.