When the United Nations pushed into space with the First Great Colonisation Rush it was still a semi-fractured collective of individual nations forced into a single entity by necessity. It attempted to deal with the vast political, economic, and cultural differences between the various nation states and zones of the planet the same way many governments have done in the past: It ignored them.
This worked well while profits continued to help the United Nations bring Earth to greater heights, but as the First Great Colonisation Rush drained the planet of people and resources - causing the worldwide economic collapse that became known as the Great Depression - the cracks in this system began to magnify.
On Omega V, the loss of the regular resupply transports and the end of direct contact with Earth and Centauri was troubling, but not something that ended their almost self-sufficient colony - but since they were so newly created, they had no resources to find out why Earth had stopped resupplying them. Several of the first colonies died out after this loss of contact, but others endured and gradually inched their ways to self-sufficiency; when Omega V (now christened New Earth) reached out to find those other colonies, they banded together in order to ensure their survival. A provisional government was set up that would oversee all of the surviving colonies; New Earth (as the most self-sufficient world) became the seat of government but each world would have a say in the newly formed congress.
As the years went by this new collective of extrasolar worlds felt secure enough in its identity to christen itself a united singular entity: The Alliance of ExtraSolar Unions.
With this new identity, the allied worlds worked to ensure the government was set up to treat the worlds as equally as possible - each colony would have representatives based on its population.
The new government retained the concept of the President as ultimate head of state, but instituted a Senate system where the leaders of the respective colony worlds would advise the President. Over the years, these Senate positions would move away from 'leaders' of colony worlds as populations grew and bigger elections made single 'leaders' less realistic, though the Senate positions themselves increased in political power. As the population grew further the Senate became the 'Upper House' of government, while a 'Lower House' was formed to hold representatives who could more closely relate to their constituent voters.
The system remained mostly unchanged aside from increased elector regions as population growth has continued, though attempts over the years to keep the colonies with equal representation have always been an issue.
Originally, the Senate held more power than the President - and technically still does - but the President is able to overrule Senate concensus if approved of by at least one of the Houses, and his power has only grown as time has passed.
The earliest political parties for the Union formed relatively quickly after contact was re-established with the United Nations; the first party to officially name itself being the Union Action Party, a party that was rabidly against reconnecting with the peoples who abandoned them in their time of need (ignoring the fact that at the time, the United Nations was itself in great need). However, the war initiated by the United Nations galvanized the population of the fledgling AEU and though both sides were drained by the costs of interstellar war, the United Nations - still not fully recovered from the Great Depression - faltered first and surrendered, giving the AEU official legitimacy in human records.
Today, while the Senate is technically more powerful than the President, the President has veto power over decisions made and in general is able to wield that power more effectively. Each House, if they can gather a majority vote, can also overrule the President; but realistically this is rare.
The Senate is comprised of 92 senators, one from each elector region. The House of Representatives is currently comprised of 460 representative ministers who won their respective constituency in their elector region. The President is the leader of the victorious political party (IE, the one which gained more than 47 elector votes in the electoral college during a general election).
