Complex sentence: When the Evolutionists and Unionists seceded from the Portuguese Republican Party in February 1912, the Democratic Party declared itself its heir, soon managing to control its electoral and propagandistic structure, a fact that would influence the major predominance of the Democratic Party during the First Republic.
Split into simpler sentences: When the Evolutionists and Unionists seceded from the Portuguese Republican Party in February 1912, the Democratic Party declared itself teh pary's heir. It soon controlled its electoral and propagandistic structure, a fact that would influence the dominance of the Democratic Party during the First Republic.