Democratic party politics are correlated with higher population size. This is not controversial.
A high population size ought to correlate with more overall murders, but a similar murder rate. Moreover, in many countries, urban areas are less violent.
If you actually checked, then you'll see that Democratic and Republican controlled cities have seen an uptick in murder rate. In some cases, Republicans have a higher increase. So this isn't an issue with any particular party.
> in many countries, urban areas are less violent
This might be because of better mental healthcare, less drug/gang related violence, better gun control, or a variety of factors. If America had the same laws as other countries then this discrepancy would be strange however the US has been very slow at getting up to the current standards that most developed countries have adopted.
What's more likely?
1. High population size implies Democratic party policies implies less police funding implies higher murder rate
OR 2. High population size implies higher murder rate (potentially because of higher poor populations or more population density or many other reasons that aren't because of party).