"The House of the King" has sent a statement to explain the purpose of the third voting round. In order to clear the will of "The King" to propose a presidential candidate based on the list of nominees presented by political parties, that would be later chosen in trusting/confidence by the Congress of Deputies.
In a strict level of formality the sentence "la disposición que le trasladen los representantes" means the delivered decisions from the representatives.
1) Disposición = Wish, will, decision
2) Trasladen = Conjugation of the verb "Trasladar" In a delivery meaning you could also use these conjugations -----> "Entreguen", "Emitan", "envíen"
3) "si" <---- after ----> "constatar" works as conditional because the context of the paragraph explain a disagreement situation between The King and the political parts. In this sense he needs the list of nominees due to make his proposal, otherwise the presidential election can't go on.
Most understable way to write this:
La Casa del Rey ha emitido un comunicado en el aclara que la finalidad de ir a una tercera ronda de consultas es asegurar que los representantes de grupos políticos entreguen la lista de candidatos necesarios para que El Rey pueda proponer un aspirante, que cuente con los apoyos necesarios para que el Congreso le de la confianza de presidir la Presidencia del Gobierno.