The presidential primary season presents a lot of important angles for understanding electoral rules, as detailed in our primary focus series -- particularly involving the impact of using a plurality voting system instead of ranked choice voting and using winner-take-all delegate rules instead of proportional representation ones.
The primaries also help demonstrates the limitations of the Top Two primary system as compared to the improvement provided by the Top Four Primary with ranked choice voting. Top Two has benefits like allowing all voters to participate on an equal basis in primaries and creating incentives for winners to reach out to more voters than they would under the status quo in order to win, but it can be an awkward instrument. As we sometimes see happen in actual Top Two elections, vote fractures can result in unrepresentative outcomes -- ones that, if translated into a Top Two primary, can deny one major party a place in the general election even when its candidates collectively earn the most votes.
Take recent presidential primaries in Michigan and North Carolina. The Republican and Democratic primaries took place on the same day, with -- as explained at FairVote's useful resource on primary rules in states -- a fully open primary in Michigan in which any registered voter could choose either party's ballot, and a semi-open primary ballot in North Carolina where unaffiliated voters could take either major party ballot. Although the presidential candidates weren't running against one another across party lines, the results are still reasonably indicative of what might have happened if they were.
In both primaries, a majority of voters chose to participate in the Republican contest, but the Democratic candidates were the two highest vote-getters. Here are the results, as reported in the New York Times, and with independent Bernie Sanders listed as a Democrat given his choice to participate in its primary.
North Carolina
Total votes for Republicans - 1,138,718
Total votes for Democrats - 1,129,241
Hillary Clinton (D) - 616,383
Bernie Sanders (D - 460,316
Donald Trump (R) - 458,151
Ted Cruz (R) - 418,740
John Kasich (R) - 144,299
Marco Rubio (R) - 87,858
Other (D) - 52,542
Other (R) - 29,670
Michigan
Total votes for Republicans - 1,324,621
Total votes for Democrats - 1,194,643
Bernie Sanders (D) -595,222
Hillary Clinton (D) - 576,795
Donald Trump (R) - 483,751
Ted Cruz (R) - 330,015
John Kasich (R) - 321,655
Marco Rubio (R) -123,673
Other (D) - 22,626
Other (R) - 65,527
In other words, voters in the general election in November in two battleground states (with Michigan leaning toward Democrats and North Carolina toward Republicans) would have only had Democrats to choose using Top Two. Clinton and Sanders also would have been the only candidates to advance under Top Two in primaries that took place in Massachusetts and Illinois, states where they also collectively won more votes than Republicans. Clinton and Sanders nearly would have done so in two battleground states where Republicans won the most primary votes and the Democratic contests were too lopsided to take advantage of the split votes among Republicans -- New Hampshire and Virginia.
Although Republicans generally have earned more votes than Democrats in primaries held on the same day, their fracturing of the vote has meant that they only would have shut out Democrats from the general election in the swing state of Ohio.
Top Two still is fairer than a one-round plurality vote election where the single candidate on the top of the heap is the winner, however. For example, in Hawaiian congressional vacancy elections, all candidates for office appear on the same ballot (so all Democrats and all Republicans and all others), and the candidate with the most votes wins. If this were used in the presidential primaries, it would have some truly bizarre results. Not only would Sanders have won Michigan and Clinton won in North Carolina -- despite Republicans collectively earning the most votes -- but Clinton would have carried South Carolina despite Republican candidates winning nearly twice as many votes as Democrats.
This year's elections are showcasing the increasingly problematic nature of our current electoral system. They also underscore the value of incorporating the instant runoff form of ranked choice voting to achieve results that better respect voters and uphold representative democracy.