Complex sentence: The leadership remained active during the 1917-1918 rule of Sidónio Pais ("Sidonismo"), to whom it showed its support, and it backed the Ditadura Nacional, which was established by the 28 May 1926 coup d'état.
Split into simpler sentences: The leadership remained active during the 1917-1918 rule when it supported the leadership of Sidónio Pais. But, it also backed the Ditadura Nacional ("National Dictatorship"), established following the 28 May 1926 coup d'état.