A politically paralyzed France has had four prime ministers in the past 20 months and appears on the verge of adding a fifth, a degree of instability suggestive of institutional crisis and unknown since the establishment of the Fifth Republic in 1958.

François Bayrou, a centrist prime minister who has been in office for about nine months, three times as long as his predecessor, has called a vote of confidence for Monday in what appears to be a suicidal move. The vote centers on his unpopular austerity budget proposal, designed to confront a severe deficit and a worsening national debt, in part by freezing welfare payments at their current levels.

Both the far-right National Rally party led by Marine Le Pen and a group of left and far-left parties have said they will oppose Mr. Bayrou. This would ensure the fall of the government, a political void and renewed pressure on President Emmanuel Macron, who has become an isolated figure.

The far right and the left hold 330 seats in the 577-seat National Assembly, the lower house of Parliament. A majority of votes cast in opposition to Mr. Bayrou would be enough to bring his government down.

With Parliament divided into far-right, centrist and left-wing blocs, each large enough to create an impasse, France has been in a state of ominous drift since Mr. Macron called snap parliamentary elections in June 2024, another apparently capricious gesture that upended French politics.