I think that for most donors this can be disregarded. Even if the marginal use of your additional tax dollars is still pretty good (e.g. 10% as valuable as your best charitable option), you’re still better off donating to the charity. In extremis, it would imply that your best marginal donation option would be to voluntarily pay more tax, rather than donate.
While it seems in theory possible that the marginal dollar that your government spends is more effective than your best charitable donation option, I’d guess that in practice this is almost never the case, largely just because Your Dollar Goes Further Overseas, but also because your contribution to government revenue will be diffused between the many hundreds of programs that the government runs (some of which may be positive, like preventive health or basic research, others which may be pretty harmful, e.g. subsidies for industrial agriculture or maintaining nuclear arsenals).
I think that for most donors this can be disregarded. Even if the marginal use of your additional tax dollars is still pretty good (e.g. 10% as valuable as your best charitable option), you’re still better off donating to the charity. In extremis, it would imply that your best marginal donation option would be to voluntarily pay more tax, rather than donate.
While it seems in theory possible that the marginal dollar that your government spends is more effective than your best charitable donation option, I’d guess that in practice this is almost never the case, largely just because Your Dollar Goes Further Overseas, but also because your contribution to government revenue will be diffused between the many hundreds of programs that the government runs (some of which may be positive, like preventive health or basic research, others which may be pretty harmful, e.g. subsidies for industrial agriculture or maintaining nuclear arsenals).