The most likely reason is the writers don't have a clue on how to write from the POV of a Republican as the main character. They'd LOVE to have an member of the GOP as the symbol of pure evil, but couldn't figure out to do it. They went with what they were familiar with. Thus, he's a RAT.
The one element that I can't buy is that Spacey's character is a South Carolina DemonRat. If he was from say, New Jersey or Illinois, the scenario would work perfectly. But a white Dem from South Carolina in the 21st century? Even if he's a completely ruthless, power-hunger manipulator who eliminates and destroys people in both parties who oppose him, a South Carolina RAT would become a modern day Huey Long type figure at best. He wouldn't play with the national Dems, especially since he's a white male from the south. Regardless of his actual political views and governing style, a white Dem from South Carolina would also most likely portray himself as a "blue dog" and a "conservative" (or at very least, a "moderate") to rise to power and become a U.S. Congressman from there, which would alienate the national Dems even more.
South Carolina?
I can only conclude he represented the Black district, managing to win primaries despite his whiteness, like Steve Cohen in TN. Probably due to his power and length of term in office.
“The most likely reason is the writers don’t have a clue on how to write from the POV of a Republican as the main character. “
Mmm, makes sense. “The West Wing” tried to do it with Alan Alda as the Republican nominee in the final season, but they just had to make him pro-choice because they can’t respect a pro-life character at all.