There are a few variations owed to how congressional reapportionment works, but in general, all things being perfectly equal to the population; a majority in a winner takes all federalist system is ... Anyone guessed it? 25%+ How? If the 25%+ majority of ~51% of the electorate, wins the representation for that ~51%, then it constitutes a majority in Congress. This means that political parties ought to be able to work some interesting strategies to better position themselves against other parties...and I think this concludes why the two party system in the United States is inevitable. The only time more than two parties can survive in the US is when there's such a population disparity between the ideologies that it is below the 25% margin, which is tough for any political party to do, most parliamentary governments have only 3-5 major parties meaning 33%-20% of the electorates. I think the Republican party conservative wing therefore has a tremendous advantage (30%+ of the electorate) to the Democratic liberal wing which is (25%+) and the Liberal wing of the Democratic party is approaching a critical barrier that suggests fragmentation and collapse of that party. If the Democrats have fewer than 25% of the partisan electorate it will be increasingly difficult for the Democrats to hold the "center" (moderates) because there are not enough partisans distributed through the apportioned congressional districts to win enough seats to be viable without them meaning that moderates will trend toward the most viable political party, in this case the Republican party. The Republican party has been beaten back, therefore, by a very potent and well organized 28% or so liberal partisan group, but this group cannot compete with the nearly 38% conservatives, and thus the Republicans require less competitiveness to actually win the elections. The Republicans compared to Democrats were rather unorganized, underfunded, and demoralized and still pulled 46% of the vote for President. Time will tell but this is a pretty solid case against the diminishing Democratic Party.