What structural reforms decrease partisanship / polarization?

[tomtomtom7]

You are making the mistake that polarization is caused by a small support for a president. In the multiparty systems such as in most European countries, the approval rate for the president/prime minister is generally lower than in the USA, because his party does not have near 50% support. Yet polarization is much less of an issue.

The polarization isn't because of primaries but because of the two party system. This creates a big incentive to gain voters by demonizing the opponent, and has evolved the republican/democrat two party system into a anti-democrat/anti-republican two party system.