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California has a system where two candidates, chosen in a nonpartisan primary, are on the ballot in each congressional or statewide race. It's sometimes called a jungle primary.

This makes it so that some elections have two candidates from the same party; in 2016 seven congressional districts had two Democrats running against each other. The Senate race also had two Democrats. It seems really strange to make there be a chance of limiting the options for voters to one party.

What are the arguments in favor of this system?

It would seem a reasonable assumption that the Democratic-controlled California legislature would have implemented this system in order to help elect more Democrats. There are few things more consistent than politicians favoring changes that benefit their own political interests.

However, both in California and Washington, a top-two system was put in place not by a vote of the legislature but by an initiative. It was, in fact, opposed by the political parties, major and minor. In Washington, the Republican, Democratic, and Libertarian parties went so far as to attempt to use legal action to prevent the system from coming into force. In California the groups favoring the top-two system included the Chambers of Commerce and Republican ex-Governors, suggesting that they saw it as in the Republican's interest. As such, it is not clear that the system was designed to favor Democratic interests.

Under the standard closed primary, many elections are effectively decided at the primary level. The general election is a foregone conclusion. As a consequence, everyone belonging to the minority party has no effective say in who is elected. They cannot influence the nominee of the majority party and their vote is largely ceremonial in the general election. Under top-two they do have a vote in the nomination process and in the general election can choose the lesser of two evils.

Under the standard closed primary, the winner is not necessarily the preferred candidate of the majority of the electorate. Consider the choice of four candidates:

1. A Democratic Socialist 39%


2. A Centrist Democrat 26%


3. A Republican 20%


4. A Libertarian Republican 15%

Results:

• Under a standard primary system, the Democratic Socialist wins the Democratic nomination, and then the general election, assuming party loyalty holds.


• Under the top-two system, the two Democratic candidates proceed to the general election, but the Centrist candidate wins because if he gets the support of the two republican candidates' supporters.

Why nonpartisan blanket primaries

The seats where the Republicans do not have at least one candidate are generally the seats where the Republicans weren't going to win. Taking the example from the other answer, consider a seat where the Republicans only get 35% of the two party vote. The chances of that seat electing a Republican are minuscule. The last time I can recall something like that happening, the incumbent was caught with bribe money literally in his freezer a few weeks before the election. And his challenger still barely won.

With partisan primaries, the Republicans in such a district essentially have no reason to vote in the general election. They have a candidate, but the candidate has no chance. The actual winner is always the Democrat chosen by the Democratic primary.

With the non-partisan primaries, the Republicans can vote for the lesser of two evils. This encourages more moderate winners, as the general election generally has a more moderate electorate than either primary.

The biggest problem that I see with the California system is not that it keeps Republicans from winning. Overall, Republicans have been more likely to benefit from it than to lose as a result of it. Republicans have managed a couple times to run just two candidates who then beat (e.g.) five Democrats in a swing district in the primary. Then the Republicans were guaranteed a win. The biggest problem is that it allows the occasional goofball result like that.

To fix that, the primary should switch to ranked (IRV, Condorcet, etc.) or similar voting such that candidates don't split the vote. In most districts, that will lead to a Democrat and a Republican winning. In a few districts, that will lead to two Democrats and allow the more moderate one to win the general election.

Avoiding wasted votes

Another problem is that in a general election, if there are three candidates, but a voter prefers two to another, the voter may have to vote tactically to avoid the disfavored candidate winning. I.e. the voter may have to vote for a second choice rather than a first choice. With the top two system, there are only two candidates in the general election. That makes voter decisions simpler and more transparent. We don't have to wonder if the result might have been different if there were only two choices, because there are always only two choices.

Why Republicans don't win in California

Another problem with the California system is that its use of geographic districts means that the Democrats win more seats than their share of the vote. To fix that, they should switch to a proportional system, e.g. Schulze Single Transferable Vote. In a proportional system, the Republicans would have won something like eighteen to twenty-two seats rather than seven. We don't know the exact number because of districts where there were no Republican candidates. And of course, a proportional system would allow third parties to win some seats.

With geographic districts, many voters don't get representation of their choice. The more evenly divided the district the fewer voters get their preferred choice. In a proportional system virtually every vote counts. Voters may not always get their first choice, but they can choose how to compromise.

Jungle primaries prevent "sabotage voting". This occurs in many states, even with closed primaries. Jungle primaries should be used in every state.

Arguably the intention is to repress Republican candidates in Democrat - concentrated districts (and in state wide races).

By having an "open primary", the minority party is always disadvantaged because its already small percentage of the vote is further fractured by the primary vote (some Republicans vote for one candidate, others for another).

This gives the majority (Democrats in California) the advantage because although their vote is also split, since they have a larger voter base, the split votes still end up beating the Republican votes.

Hence how the districts end up with two Democrat candidates, which is what was intended to eliminate the possibility of a surprise Republican victory.

So, the argument in favor of such a system is to decrease the power of the minority party (which is something the majority party doubtless wants).

For example,

consider a district with 35% Republican voters, and 65% Democratic voters, each party running two primary candidates.

The Republican candidates split the republican vote 60% - 40% (21% and 14% of the at-large total). The Democrat candidates split the democrat vote 65% - 35% (42.25% and 22.75% of the at-large vote).

Now, even though the total Republican base is 35% and larger than the runner-up Democrat's total vote share, the whole Republican party is wiped out from contention.