The design of election laws can radically affect the results of an election. A top-two system would have had a huge influence on two hotly contested May 18 Senate primary races, in Arkansas and Kentucky. If these two states used this system, with no change in turnout or votes, neither of the Republican victors, Rand Paul and John Boozman, would have made it to the general election ballot. Looking at the election results:
Arkansas Primary (results)
Kentucky Primary (results)
In Arkansas, the top-two vote getters were Democrat Blanche Lincoln and Democrat Bill Halter. In Kentucky, Democrat Jack Conway and Democrat Dan Mongiardo were the top two. Republicans Rand Paul in Kentucky and John Boozman in Arkansas each finished third in the total number of votes. Under the new system, neither would have made it to the general election, even though both are currently favored to win.
The important thing about this analysis is that it assumes the turnout and vote would be the same. Clearly that would not happen under the new rules. For example, Kentucky has a closed-party primary, so independents did not vote on Tuesday. If voters understand how the “primary” took on a radically different role, the turnout would probably change. Also, candidates would run different campaigns.
If California adopts the top-two primary system, eventually candidates and voters will adjust their behavior until they achieve a new equilibrium. But what might happen in those first few elections could be extremely interesting. People get very locked into voting behavior and are probably used to thinking of primary elections as unimportant, unless there is a heated race on their side. Election results in 2012, with the new primary system and new districts, could be very strange if people don't internalize that the June “primary” has become more like the actual general election.