I’ve played quite a bit of MTG in the past and I think this would be a really entertaining twist on the genre. 
The way I see it though, buildings like the Pentagon, IRS office, White House, etc. should probably be analogous to artifacts (colorless) since any of the groups would be able to take advantage of them (socialists, greens, libertarians, etc.).
As for Mana, it seems to me that it could be analogous to something like the amount of supporters you have, or voters. Building up support, would allow you to “play” other types of cards, such as Ayn Rand, Karl Marx, Alan Greenspan, Greenpeace, etc., each of which would be associated with a specific color identified by its associated group. Then “artifacts” would consist of the buildings as mentioned above.
Other types of cards, such as “instants” or “sorcery” would be other kinds of effects. For example, the card “Counterspell” in MTG nullifies another players attempted play – in this case, it could be replaced with something similar to “presidential veto” or something. Other types of spells in MTG would need to be replaced by relevant “events” in this game, for example, cards like “Declaration of War” or “Stock Market Crash”. You could effectively “remove” other cards from play by using a “Corruption Scandal” card or a “Recall” card.
“Enchantments”, in MTG, introduce long-lasting effects that essentially inject additional rules onto all players. In this situation, they could be replaced by “Laws” that are passed by various players, enabling or restricting certain areas of play. “Enchantments” in MTG can be disenchanted and, likewise, “Laws” in this game could be “Repealed”. Perhaps a “Filibuster” card could be used to counter an attempted “Law” card.
However, I still wonder what the endgame will be. In MTG, the idea is to inflict 20 points of damage on your opponent. What would the situation be here? Elect a majority to congress? Establish your ideology over the country? (Play until the electoral gridlock destroyed the country? :P)