History. 3 of 5: the vote went to a third round or more in 1995 (five rounds), 2000 (six) and 2002 (five); it ended in the first round in 1965 and the second in 1993.
Hunch. In four of the last six elections that filled a Social Democrat or Swiss People's Party seat, most recently in December 2023, parliament needed a third round or more.
The third round market resolves on the round-by-round results on the parliament page: YES if no one wins more than half of the valid votes in the first two rounds on 9 December and a third round is held, and VOID if there is no vote that day. A repeat vote after a winner says no does not count, and RFI and swissinfo are the backups.
We looked for cases where a minister from the Social Democrats on the left or the Swiss People's Party on the right left the government in the middle of a term, and parliament filled only that one seat that day. We found five such elections since the seats were first shared out in 1959. Each time the party of the minister who left kept the seat. (Swiss Parliament; Swiss Parliament; swissinfo; RFI)