John Costigan

John Costigan had served as a Conservative MP for over 30 years and had been a cabinet minister under four Conservative prime ministers — John A. Macdonald, John Abbott, John Thompson, and Mackenzie Bowell. An Irish-Catholic champion of separate school rights, Costigan grew increasingly disillusioned with the Conservative Party's handling of Catholic education issues and the Manitoba Schools Question in the 1890s. By 1900, he had crossed to Wilfrid Laurier's Liberals, seeing in the Franco-Catholic Laurier a more natural ally for his community's interests.

Laurier appointed Costigan to the Senate in 1907, where he served as a Liberal until his death in 1916. His defection after three decades of Conservative service was a powerful statement about how the Manitoba Schools Question had fractured the old Conservative coalition. His record of crossing after serving in four Conservative cabinets makes him possibly the most senior Conservative ever to defect to the Liberals.