Casdon
There is a precedent, Boris Johnson did not resign as mayor of London until he won his seat as an MP.
It isn't a question of precedent, it's the rules
From Parliamentary briefing on the 2024 Metro Mayor elections
A Member of Parliament can stand to be a metro-mayor, and vice versa. Dan Jarvis served both as mayor of South Yorkshire and MP for Barnsley betwee2018 and 2022. One sitting MP, Ben Bradley (Con, Mansfield), is standing on 2 May, to become mayor of the East Midlands.
A Member of Parliament cannot take office as a Police and Crime Commissioner (PCC), and therefore a sitting MP cannot take office as a mayor with PCC responsibilities. Tracy Brabin had to resign her seat as an MP in 2021 before accepting the office of mayor of West Yorkshire, as that mayoralty has PCC responsibilities.
Nothing about having to resign before standing for Parliament.
Burnham can't do both jobs because the Mayoralty of Manchester includes Police and Crime Commissioner duties. Otherwise, it seems he could do both,,
The PCC is being abolished in 2028.
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