@OhHolyJesus
Can I ask those in the know...
I realise that if a president does the VP becomes default president - but for how long, the remainder of the term? Would there be another election? Obvs the U.K. is different as there can be a call for a lack of confidence that leads to a snap election.
I probably need to check the Constitution as I'm just thinking back to Bartlett getting shot at in the West Wing so my knowledge of this is based on a fictional character!
The procedure for replacing the President with the VP is under the 25th Amendment of the Constitution. Whether it is reversible depends on the reasons it was invoked in the first place. It's pretty complicated and involves agreement between the VP and the executive:
Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President.
You can have a voluntary situation - say, if Biden needed two have an operation requiring a general anaesthetic - he could voluntarily sign a declaration temporarily handing over powers to the VP. Or it could be hostile, say he went a bit doolally, the VP would have to get together with the House leaders and cabinet members to oust him. Or it could be pragmatic, if he dies mid term! The appointment lasts as long as the declaration stays in place, up to the election of a new president, or re-election of the existing one.
If the VP is not fit to take the place of the President for whatever reason, then there is a line of succession. The next in line I think is the leader of Congress (currently Nancy Pelosi) and then so on down the ranks. I don't think you have a presidential election though until the 4 year term is up.