Democrats use a proportional system to nominate their candidate for president.
But “Republicans tend to use winner-take-all systems that reward candidates who win by even the slimmest margins.” Kamarck at 88-92. This “means that Democratic contests that make it past the early states can go on much longer than Republican contests.”
Proportional systems were favored by “early twentieth-century progressive reformers who saw proportional representation as a way to break the power of big-city political machines.” Proportionality was revived by Democrats in their presidential nominating process in the wake of the divisive 1968 nominating experience.
The result is that Democrats typically attract more attention, more voters register Democratic, Democrats build a big tent and a big base, and Republicans hope that greater control by an elite over the process gives them a candidate who aligns with the wishes of the elite.
In 2015-16 the winner-take-all system greatly helped Donald Trump’s take-over of the Republican party. If the Republicans had used proportionality to choose delegates, Trump would have had a much more difficult time getting so many delegates so early. He might well have won the nomination anyhow, but the theory of an elite controlling the process is now debunked.
By contrast, while using the equitable proportional system almost exclusively since the 1990s, the Democrats have nominated candidates who won the national popular plurality in every general election from 1992 to 2016, with the sole exception of 2004. That is six wins out of seven.
One person, one vote builds a bigger, better, reliable base for a national party.