Suffice to say, observing the data from California or New York allows us to quickly reject this theory.
No we shouldn’t. Dominance of a single party will prevent inter-party fighting/fingerpointing/blame-shifting, but their intra-party equivalents can be just as bad. This is especially the case given the US has very weak party discipline by global standards, the party leadership has rather limited powers to tell the party’s elected officials what to do
In a parliamentary system, the executive needs to actively retain the confidence of the legislature, since a bare majority of the legislature can fire the executive at any time for any reason (and in a bicameral system, generally just a bare majority of the lower house). In a presidential system, the executive is an independent power base, separately elected, and the legislature can only remove it on the basis of serious misconduct (and often needs a supermajority to do so)
> What makes you think that’s the problem?
The US has a presidential system (even at the state level) where the legislative and executive branches are kept independent, which harms policy coherence
Most of the rest of the Western world (outside of Latin America) has either a parliamentary system (in which the executive is fully subordinated to the legislature) or a semi-presidential system (in which the executive is partially subordinated to the legislature and partially independent of it).
This doesn't just happen at the national level, but often gets repeated at the state level – in American states the executive (Governor/etc) and legislature are independent, in Australian states and Canadian provinces the state/provincial legislature controls the state executive
When things go wrong in the US system, it is so easy for the legislature and executive to engage in buck-passing in which each side claims they could solve the problem if only the other side would let them do it. In the rest of the English-speaking world, in most of Europe (and Japan too), that technique doesn't work, since the leaders of the legislature are also the leaders of the executive.
See also Latin America, where the same system leads to even worse problems. The US has other advantages (wealth, power, etc) which lets it overcome some of the downsides of its political system, where countries lacking those advantages can't