Last-minute political negotiations were underway in Bern on Tuesday, as parliament prepared to elect a new cabinet member to fill the post being vacated by the defence minister, Adolf Ogi.
The election is one of the most closely watched in recent years.
A joint session of the 246-member parliament will choose a successor on Wednesday. The race is expected to be very close, and go to several ballots.
Traditionally, the post is reserved for the party of the outgoing minister, but there is no guarantee that Ogi's right-wing Swiss People's Party will retain the seat, or see one of its official candidates secure parliament's endorsement.
Both the main candidates, the head of canton Zurich's police department, Rita Fuhrer, and a Thurgau politician, Roland Eberle, have spent the past two days at hearings organised by the three other government parties, trying to convince them of their suitability for the vacant post.
Under the "magic formula" system of government, which has ensured political stability for the past 41 years, one cabinet seat is reserved for the People's Party, and two each for three other main parties.
However, the Social Democrats have warned they might try to force the People's Party out of government because they disapprove of the party's right-wing policies on some issues, and in particular its "opposition" role within the collective responsibility cabinet.
Analysts say the Social Democrats, along with the two centre-right parties in government, may try to elect a non-member of the People's Party, or an unofficial "liberal" representative of the party, such as Samuel Schmid, a member of the Senate for Bern.
The People's Party is divided into two main wings - the hardline Zurich-based section, close to the party's populist figurehead, Christoph Blocher, and the canton Bern-based more liberal wing, of which Ogi and Schmid are members.
Blocher warned last week that if Schmid was elected, the party would adopt an even greater opposition role.
Discussions were expected to continue, particularly in restaurants and hotels around the parliament building, until late on Tuesday as politicians drew up battle plans and a list of possible alternatives to the official People's Party candidates.
The Green Party has already put forward a candidate, Cécile Buhlmann. She may fare well in the first ballot, but is seen as having no chance of winning the cabinet post.
by Ron Popper