Simply put, popular vote is a disaster.
Our two-plus system came about because of Plurality elections and a division of ideals. There are basically only two parties because they represent two opposing ideals: Liberalism and Conservatism. The center isn’t a moderate version of these, but rather a break-over point; and the two ideals are practiced with varying intensity.
With a popular vote, the party with the most votes wins. If there are more Conservatives, you get the Republican; and if there are more Liberals, you get the Democrat. Whichever party has slightly-more population or can simply maintain greater turn-out gains permanent, unyielding control.
Were that not bad enough, we nominate by party primary: there will be a Democrat, a Republican, maybe a Libertarian, and an Independent if they can get enough signatures on a petition. Of course, only the Democrat or Republican will win. If people don’t all register to the winning party and vote in their primary, then they have no say in who gets elected, and the winner need only represent that party.
We see these problems in the division between “red states” and “blue states”: while local elections may define the state as “purple”, the dominating party holds both Senators and the Governor’s house, exclusively reflecting the ideology of the voters registered to that party. States with a near-even population split regularly experience upheavals, and can come within a fraction of a percent of electing wildly-different candidates.
By extension, a national popular vote would either change nothing or would permanently define America as a red nation or a blue nation. This excludes representation for nearly half the voters and, at best, would convert the United States to a single-party system.
The Unified Majority system replaces State elections and even the Electoral College with a process of selecting representatives of the voters as a diverse body, then selecting from those a representative of the voters as a whole.
Rather than representing parties, the election then represents voters. If the voters change…
…then so do the candidates selected in the primary—potentially several candidates from the same party, spanning the variations in voter’s views. Rather than pinning the center to the break between the two parties—between Liberalism and Conservatism—the center represents the average of all voters.
This system doesn’t just elect a winner, but a representative: the candidate ultimately agreed upon by the preferences of the voters as a whole becomes the winner. A candidate who offends the ideals of a great many voters cannot win, and the variation between candidates of the same party matters a great deal more than the party to which they affiliate.
Under a Unified Majority system, the electorate comes together in unity to select the candidate best representing us all.