The Star Imperium was originally intended to govern the known universe with a hereditary monarchy centered in the position of Imperial Sovereign, but in practice, the monarchy was in fact vacant for much of the time the intergalactic Imperium existed, with a regency exercised by the Council of Twelve. A Monarchist Party, at least nominally supportive of a monarchy, nominally had a parliamentary majority, but in practice it was sometimes bitterly divided into Blue Monarchists and Red Monarchists. Plus the fact there was a relatively strong minority party, the Freethinkers that could combine with dissident Monarchists to form a majority in the Imperial Parliament, so in practice there often was lacking a majority to elect a new Sovereign.
The Fundamental Laws made it difficult for there to be a hereditary monarchy within the Star Imperium because they stated only the oldest surviving child of a Sovereign could inherit the position, although a judicial ruling by the Council of Twelve stated that such a child could be an adopted son or daughter instead of a biological offspring. Nevertheless, no spouse, sibling, parent, grandchild, cousin, or other relative of a Sovereign could inherit the position.
Moreover, the Fundamental Law stated that only a member of the race of Star Elves could serve as Imperial Sovereign.
Attempts to amend the Fundamental Laws to allow non-Star Elf races to serve as Imperial Sovereign or to expand the number of relatives of a ruling Sovereign that possibly might inherit the position almost always failed. First of all, in the early millennia of the Imperium, Star Elves outnumbered Asguardian Dwarfs and Aliens. Second of all, Star Elves within the Monarchist Party combined with anti-monarchist Freethinkers to prevent any amending of the Fundamental Laws to strengthen the monarchy.
Historians and political scientists during the time of the United Planets noted that a nominal monarchy in practice had a lack of a true monarch. In other words, a de facto republic.