All kings were elected in the Commonwealth, which also had a strong parliament (Sejm) and weak king system. All members of the noble estate had the right to vote. Since the nobility constituted some ten percent of the population on narrow suffrage grounds you could say the Commonwealth was more democratic than the UK before the Second Reform Act. More broadly that wasn't the case because of indirect voting (the nobles elected provincial 'sejmiki' which then elected the Sejm), open voting, and most importantly the hierarchical patron-client relationships of what was still a pretty feudal society.