In the relationship between kingship and emperorship in medieval Germany, the entire concept of anti-kings stands out as a political oddity. Anti-kings were dukes that were elevated by a pope, or anti-pope, to the kingship of Germany in order to challenge the existing overlord of Germany (king or emperor) and, as such, were relatively recent phenomenon in German history. The first anti-king appeared during the Investiture Controversy. Previously, there may have been multiple contenders to the royal throne, but once a king was elected, the issue was settled. Whether the king was strong or weak, loved or hated, he was the elected and crowned king, heir to the imperial throne.
This changed dramatically with the Investiture Controversy. Because of the actions of the papacy, then King Henry IV was deposed and excommunicated. So, theoretically, since the kingdom was without a king, a new one had to be chosen. A group of princes for the first time in German history elected an anti-king, Rudolf of Swabia, under the condition that he not name his son as his successor. But Henry IV refused to relinquish his claim to the throne, thus more than one king vied for the royal throne of Germany. This return of the elective principle continued even after the death of Rudolf. The magnates chose another anti-king, Herman of Salm, so pitifully weak that he could barely keep himself in the field. After his death in 1088, the princes gave up on the practice of contesting royal elections.
The problem returned during the lifetime of Frederick II of Hohenstaufen in the conflicts between Philip of Swabia and Otto of Brunswick. Both men, Crowned kings of Germany, fought to see which would be victorious and thus able to claim the imperial throne. During the imperial reign of Frederick II, an anti-king reappeared in the person of Henry Raspe, Landgrave of Thuringia. He was elected king by the magnates in 1246. he died n 1247 having accomplished almost nothing.
Since 1075, there had been these few instance of an anti-king arising in Germany, but there had never been a single case of anti-emperor. There had been kings of Germany who had challenged the emperor for the kingship, emperorship or even over policy, but never since Otto the Great's coronation of Otto II as joint-emperor in 967 had there been more than one emperor of the Holy Roman Empire at any one time. This is a most unexpected discovery, especially when examining the reign of Frederick II.
By 1214, King Frederick II's forces had routed these of his enemy, the Emperor Otto IV. Yet it was not until after Otto's death in 1219 that Frederick II sought the imperial crown, even thought he had obtained the royal insignia in the summer of 1217, further legitimizing his claim and position. Frederick's armies were victorious, no opposition remained, and yet, unexpectedly, he did not move to claim his imperial inheritance.
Despite the fact that there is no extant document that states it, I maintain that thee was a belief of tradition that just as there was only one Augustus, one Constantine, so too should thee be only one Holy Roman emperor. The Holy Roman emperor claimed descent from Augustus Caesar and Constantine who in their time dominated the world, both in the secular and spiritual aspects. In particular, Constantine saw himself not as an heir to the apostles but as an apostle himself. It is in their imperial name that the Holy Roman emperors fought and battled with the Papacy. Otto of Freising recorded Frederic k Barbarossa's pronouncements on this belief:
"By virtue of my office and dignity of the empire, the authority to convoke councils (especially in such great perils of the church) is vested in me - for it is recorded that the emperors Constantine and Theodosius and Justinian also, and in more recent time Charles the Great and Otto did so.."
By invoking such personages, the emperors attempted to lift the stature of the empire above, or at last equal, to that of the Papacy. On the surface it appeared that the Holy Roman Empire failed in this task. After 1250, the Empire had lost its struggle with the Papacy. But the Empire was never racked by the king of internal battles, with the anti-popes, that shredded the dignity of the papacy and might have eroded the imperial position. The Holy Roman Empire maintained some of its dignity because it was possible after 1075 or there to be several kings fighting for sole authority in the kingdom and possible for a king to challenge the emperor. But even should the king emerge victorious over the emperor as Frederick II did over Otto IV, the king waited until the death of the reigning emperor before moving to have himself crowned.
This mystique about the singularity of the emperorship is most clearly described in Dante's De Monarchia. Dante, the last defender of the Holy Roman Empire, wrote that the Empire had existed long before the church and that the basis for the authority of the empire partly lay in the fact that Christ chose to be born "during the perfect monarchy of the immortal Augustus." The institution of the Holy Roman Empire had the support of tradition dating back past Charlemagne to Augustus himself.
Another factor reinforcing this idea is that during the many conflicts between emperor and pope, even anti-popes in opposition to the emperor did not try to crown their candidate for king as emperor. Even the great magnates did not push for this. It appears to be one thing to raise up and anti-king to oppose the emperor, quite another to push for hat king's imperial coronation while the previous emperor still lived. There was no provision, no law, not even a tradition that prevented the creation of rival emperors. Indeed, in the later period of the ancient Roman Empire, such rivals were not uncommon. The fact that such a rival was never created, even when the opportunity presented itself, implied that the German ruling class perceived the position of Holy Roman emperor as something beyond a simple political office.
 The Deeds of Frederick Barbarossa, 308.
 Dante Alighieri. Monarchy. Translated by D. Nicoll. (London, Weidenfeld and Nicolson, 1954), 26.