To "become a politician" means to enter into or pursue a career in politics, typically by seeking elected office or working as a government official. Full definition
He left his profession and became a politician in order to bring quality to governance. (nigeriapoliticsonline.com)
«I think you all know that [US] president [Barack] Obama spent some time as a community organiser before becoming a politician,» he told delegates. (politics.co.uk)
Stripping out lawyers, who have a disproportionately nasty habit of becoming politicians, the three most tax - dodging professions account for about half the votes among Greek MPs. (economist.com)